Time to talk about oil gas mix and velocity.
Time to talk about oil gas mix and velocity.
Here's something I never hear people talk about is RPM and velocity. If you start your bike and it is sitting there idling it loads up and starts smoking and if you open up the throttle it will clean up....what made it clean up you have the same gas oil mix? It is all about rpm and velocity of the air/gas/oil mix coming through the carb into the engine. The gas and air will separate from the oil as a vapor...and will travel through the crank case and up the transfer port to the combustion camber. The oil will stay in the crank case and do it's job of lubing the engine.....till the rpm and velocity starts to rise. As the velocity goes up it starts to pull the oil up the transfer port also.....so as the rpm goes up you have less oil in the crank case to lube your crank, cylinder wall, piston and bearings. This is why you will hear about a air cooled go kart that runs on 16:1 and a low rpm big bore that will run just fine on 40 or 50:1. I myself like to run a ratio higher in oil. One other thing about oil....you never hear of a high rpm kart motor (air cooled) that doesn't run on bean (caster oil) oil or a mix of it with synthetic oil like 927. Thanks DG
Re: Okay it's time to talk about oil again!!!!!
Well the Gang is bad about getting off the subject and I'm particularly guilty. Hmm,--karting, It's been a long time since I did that. I'm not even up to date on what engines they use today. Of course any high demand air cooled two stroke needs the best lubrication possible and the ones you mention probably are. A kart engine probably has very little low RPM use where a street/trail bike that we Forum people ride has more low and mid RPM use and certainly more hours between maintenance. Oils must meet the requirements of our bikes, one of them being low maintenance over long distances and time. Sustained high RPM engines such as karts and road racing motorcycles have a different requirement of lubricants not failing at high RPMs, temperatures and loads. The oils I personally use meet all requirements of my vehicle. If you are having to clean rings and exhaust ports on your two stroke street/trail bike or adding additives to treat problems means your oil doesn't meet the requirements of your vehicle. I do not dispute that the lubricants that you mention are probably better suited for sustained high RPM racing, but don't meet the requirements for a street vehicle.
A two stroke engine TUNED for low RPM use such as a trials bike does not load up at idle. It's all in the mixture and RPM and velocity are certainly factors.
OK--I've talked more than I should but I did stay on the subject.
Max's opinion of course.
PS--What do they use for fast kart engines today?
A two stroke engine TUNED for low RPM use such as a trials bike does not load up at idle. It's all in the mixture and RPM and velocity are certainly factors.
OK--I've talked more than I should but I did stay on the subject.
Max's opinion of course.
PS--What do they use for fast kart engines today?
Re: Time to talk about oil gas mix and velocity.
Max.....the only time that I have had trouble with the bean oil is on the street using a model 97 piston with the ring on top. It would always stick the ring around the exhaust port. I have switched over to the new weisco piston that paul sales and have never had a problem with it sticking. Before that I ran a model 94 cylinder with a 94 piston with out the rings sticking. I was just wondering about the yamaha ring free in the other post if everyone had used it in a 2 stroke. Again with a trials bike it is set up for a lot of low end and would have or should have more velocity in the transfer ports. You would also be able to run a lower gas oil ratio than a high rpm engine. I would love to hear what others think on this subject.....it's always good to hear other options.
Re: Time to talk about oil gas mix and velocity.
While your description is perhaps generally true, I disagree that oil separates from gas/air as a vapor and stays in the crankcase until higher revs are achieved. If this premise were true, why does the engine, as you mention, begin to get more and more smokey as it sits and idles? It may be that SOME oil condenses out in the crank at low rpm, more so than at higher rpm, but not all certainly or the net equation would ultimately lead to a full crankcase and a stalled engine, whereas a two stroke can sit and idle indefinitely as long as there is a fuel source and a spark.
The demands of a two stroke for oil in the gas certainly are different at idle versus high rpm, and of course, with premix as opposed to injection oil, we mix for highest demand. This will mean more oil in the mix at idle than the engine technically demands, but the process of oil condensation and migration occurs at all rpm, not just at higher rpm. That there is a shift in efficiency in the migration process at higher rpm is likely true. It is not, however, an all or nothing proposition. If it were, then by your description, an engine run for a protracted period of time at high rpm would eventually seize crank bearings for lack of oil. This is not the case in reality.
Efficiency of crankcase transfer during the stroke is a function of the efficiency of the engine as a pump. Because it is piston port, reeds notwithstanding, a two stroke engine is a more efficient air pump at higher rpm. To this extent, some of the tendency to load at low rpm is not due to velocity per se, but effective volume of transference attributable to a given rpm. More of the fuel air charge that passes volumetrically through the carb per cycle transfers from crank to top end at higher rpm than at low rpm. I am ignoring ram air effect for purposes of this discussion.
We should aim our premix ratio at the high side of the average rpm at which we run our bikes. Trials bikes and flat trackers have different needs. We should mix accordingly.
As for bean oil, I like its properties inside the engine, but I don't like its properties outside the engine. Bean oil seeks heat, which is great, and generally has a higher film strength when hot compared to petro or synthetic oil. All to the good. However, it is cranky about mixing with and staying mixed with gasoline. That is not good. Petro and synthetic two stroke oils mix very readily with gas and stay mixed indefinitely once mixed. If bean oil did this, it would be terrific. Because it does not do this, it is relegated to special uses, such as road race, motocross, or karting.
The demands of a two stroke for oil in the gas certainly are different at idle versus high rpm, and of course, with premix as opposed to injection oil, we mix for highest demand. This will mean more oil in the mix at idle than the engine technically demands, but the process of oil condensation and migration occurs at all rpm, not just at higher rpm. That there is a shift in efficiency in the migration process at higher rpm is likely true. It is not, however, an all or nothing proposition. If it were, then by your description, an engine run for a protracted period of time at high rpm would eventually seize crank bearings for lack of oil. This is not the case in reality.
Efficiency of crankcase transfer during the stroke is a function of the efficiency of the engine as a pump. Because it is piston port, reeds notwithstanding, a two stroke engine is a more efficient air pump at higher rpm. To this extent, some of the tendency to load at low rpm is not due to velocity per se, but effective volume of transference attributable to a given rpm. More of the fuel air charge that passes volumetrically through the carb per cycle transfers from crank to top end at higher rpm than at low rpm. I am ignoring ram air effect for purposes of this discussion.
We should aim our premix ratio at the high side of the average rpm at which we run our bikes. Trials bikes and flat trackers have different needs. We should mix accordingly.
As for bean oil, I like its properties inside the engine, but I don't like its properties outside the engine. Bean oil seeks heat, which is great, and generally has a higher film strength when hot compared to petro or synthetic oil. All to the good. However, it is cranky about mixing with and staying mixed with gasoline. That is not good. Petro and synthetic two stroke oils mix very readily with gas and stay mixed indefinitely once mixed. If bean oil did this, it would be terrific. Because it does not do this, it is relegated to special uses, such as road race, motocross, or karting.
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taber hodaka
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Re: Time to talk about oil gas mix and velocity.
loading up and smoking at lower rpm I just tune it where I want it. -----------Clarence
Re: Time to talk about oil gas mix and velocity.
This is perhaps the most definitive discussion one is likely to find regarding two stroke oil, lubrication, oil migration, and all the related issues regarding what to use and what ratio works best.
http://www.klemmvintage.com/oils.htm
http://www.klemmvintage.com/oils.htm
Re: Time to talk about oil gas mix and velocity.
As far as the comment of no karter runninng anthing but castor oil not so! I personally have karting customers who use Red line synthetic , And Red line actually sells thier 2 stroke race oils under two labels , one specificly labeled for karts although it is the same product. Also This product surpasses castor oil lubricity as well as thermal capabilities and has none of the dirty burn effects .
Rich
Rich
Re: Time to talk about oil gas mix and velocity.
Oh and back to the low speed load up syndrome. One must take into account that there is a reversion effect going on as the piston proceeds downward in the stroke before the piston skirt closes off the intake port some aie fuel mix heads back through the carb and then gets pulled back in again on the next intake stroke pass over the discharge ports from the pilot circuit as well as the needle jet pullin more fuel in , and walla you have an over rich condition that needs to be cleared out occasionally because YES lack of air speed at low rpm. This also happens on a reed engine but to a lesser degree. I have watched the fuel vapor cloud above the carburetors on jet skis while I was tuning them in a run tank its cool to observe , the reed engine cloud is about 1/2 or less than the piston port engines but still there .
Rich
Rich
Re: Time to talk about oil gas mix and velocity.
I believe this happens with all engines to some degree. I remember seeing a fuel cloud dancing over the carb on my '79 Mustang Cobra, probably for the same general reason. Wet intake system, overlapping intake and exhaust timing.
Re: Time to talk about oil gas mix and velocity.
Exactly thats why at the drag races they wont let someone race with exposed carbs with out sone sort of barier such as a scoop
Rich
Rich
Re: Time to talk about oil gas mix and velocity.
Hey Rich, Like this? 
(I couldn't resist)
Maxie
(I couldn't resist)Maxie
- Attachments
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Re: Time to talk about oil gas mix and velocity.
Yeh like that but thats injected . I had to improvise years ago at the drags for that , they didnt want furl vapors to gatthero the windshield , Maybe it depends on who is teching 8o) The guy in the picture could become flamable ha ha
Rich
Rich
Re: Time to talk about oil gas mix and velocity.
http://www.tlr-online.com/yahoo_site_ad ... 80056.html
http://www.maximausa.com/pdf/Oil%20Migr ... 0Sheet.pdf
Found this article about oil ratio and tuning.....read the part called (say what) that is what I was trying to say about rpm and velocity.
http://www.maximausa.com/pdf/Oil%20Migr ... 0Sheet.pdf
Found this article about oil ratio and tuning.....read the part called (say what) that is what I was trying to say about rpm and velocity.
Last edited by DGardner on Sat Nov 21, 2015 1:35 am, edited 1 time in total.
Re: Time to talk about oil gas mix and velocity.
Spooge... (Here's some more good reading)
Any of you that believe that spooge is caused by too much oil in the mix are flat out wrong. If you know how to jet, you can run any amount of oil you choose, and have absolutely zero spooge.
Looks like it's time for a little pre-mix 101. I don't usually get into ratio discussions, because mix ratios are like religions to most people, and they tend to be closed-minded and hard-headed on the subject, but I'll put in my $.02 here anyway.
There is a prevailing myth that less oil is better, and that the oil in the fuel is what lubricates the engine. And there is also a very common belief that spooge is caused by too much oil in the fuel mix. Both are wrong. The engine is lubricated by the residual oil that builds up in the crankcase. All the oil in the fuel does is replenish this oil. And spooge is caused by rich jetting.
When an engine is jetted too rich, the excess fuel leeches heat from the combustion process, causing the combustion chamber temperatures to be too low to effectively burn the oil, or even completely burn all of the fuel. The result is spooge and deposits. The spooge is nothing more than unburned fuel and oil passing out the exhaust.
If you have a spooge problem, you have a jetting problem. You don't get rid of the spooge by reducing the oil, you get rid of it by fixing the jetting. Correct jetting will produce an air/fuel ratio of about 14:1, which will produce combustion temperatures in the 1200 degree range. This will provide sufficient heat to consume the premix oil.
You don't choose a mix ratio based on "spooge", you choose the ratio based on the amount of oil your engine needs to provide sufficient protection and adequate ring seal. The common misconception is that mix ratios are "one-size-fits-all", when in fact nothing could be frther from the truth.The amount of oil that is correct for one rider on his bike may not be enough oil for another rider/bike, or it may be too much oil. It all depends on engine displacement, riding style, and how hard you push the engine.
The best way to determine if you are running enough oil is to check the level of the residual oil in the crankcase. If the ratio you run leaves enough residual oil in the crankcase to cover about 1/8" of the bottom of the crank wheels, then you are fine. If you don't have that much residual oil in your crankcase when you pull the top-end off, you aren't running enough oil for your riding style and conditions.
With that said, to have that amount of residual oil in the crankcase at 50:1 (a ratio made popular by magazines and oil bottles), you can't be riding very hard, or your bike is jetted richer than necessary simply to deliver enough oil. I arrived at 26:1 for my bike with my riding style because that is the amount that gives me the proper amount of residual build-up. Small-bore engines require greater oil concentrations than larger engines to achieve the proper amount of residual build-up, because they rev higher and have higher intake velocities. Along the same lines, someone that pushes the engine harder, and keeps the revs higher, also needs to use higher oil concentrations to achieve the proper residual build-up.
To understand why the residual oil is so important, you have to understand what happens to the oil in your fuel when it goes into the engine. While the oil is still suspended in the liquid gasoline, it can not lubricate anything. It has about as much lubricity at that point as straight gasoline. When the gasoline enters the engine, it evaporates, dropping the oil out of suspension. Now that the oil is free, it can lubricate the engine, but it must get to the parts to lubricate them. The way it gets to the bearings and onto the cylinder is by being thrown around by the spinning crankshaft, and being distributed through the engine by the air currents moving through the crankcase. The main bearings are lubed by some of this oil dripping down through tiny "drip passages" in the cases above the bearing pockets.
People believe that the oil just rushes right through a two-stroke along with the fuel, but that just isn't so. It can take 90 minutes or more for the oil migration through a two-stroke to result in a complete oil exchange.
The oil eventually makes it into the combustion chamber, where it is either burned, or passes out the exhaust. If the combustion chamber temps are too low, such as in an engine that is jetted too rich, the oil doesn't burn completely. Instead, some of it hardens into deposits in the combustion chamber, on the piston, and on the power valve assembly. The rest becomes the dreaded "spooge". The key to all of this working in harmony is to jet the bike lean enough to achieve a high enough combustion chamber temperature to burn the oil, but also still be able to supply enough oil to protect the engine. If you use enough oil, you can jet the bike at it's optimum without starving the engine of oil, and have excellent power, with minimal deposits and spooge. At 50:1, you simply can't jet very lean without risking a seized engine due to oil starvation.
With the high oil concentrations that I use, I tend to get far more life from my cranks and rings than most of my friends that run leaner oil ratios. The high oil content also produces better ring sealing, so more of the combustion pressure is retained.
One small point. No one ever broke an engine by using too much oil.
Now we come to the issue of ring seal. Simply put, the rings alone can not effectively seal the cylinder. They also need oil to provide a complete seal against the bore surface. And up to a point, more oil will provide a better seal.
I have run Dyno tests on this subject, as a school project in Tech School. We used a Dynojet dynamometer, and used a fresh, broken in top-end for each test. We used specially calibrated jets to ensure the fuel flow was identical with each different ratio, and warmed the engine at 3000 rpm for 3 minutes before each run. Our tests were performed in the rpm range of 2500 to 9000 rpm, with the power peak of our test bike (an '86 YZ 250) occuring at 8750 rpm. We tested at 76 degrees F, at 65% relative humidity. We started at 10:1, and went to 100:1. Our results showed that a two-stroke engine makes its best power at 18:1. Any more oil than that, and the engine ran poorly, because we didn't have any jets rich enough to compensate for that much oil in the fuel. The power loss from 18:1 to 32:1 was approximately 2 percent. The loss from 18:1 to 50:1 was nearly 9 percent. On a modern 250, that can be as much as 4 horsepower. The loss from 18:1 to 100:1 was nearly 18 percent. The reason for the difference in output is simple. More oil provides a better seal between the ring and the cylinder wall.
Now, I realize that 18:1 is impractical unless you ride your engine all-out, keeping it pinned at all times. But running reasonable ratios no less than 32:1 will produce more power, and give your engine better protection, thus making it perform better for longer.
Any of you that believe that spooge is caused by too much oil in the mix are flat out wrong. If you know how to jet, you can run any amount of oil you choose, and have absolutely zero spooge.
Looks like it's time for a little pre-mix 101. I don't usually get into ratio discussions, because mix ratios are like religions to most people, and they tend to be closed-minded and hard-headed on the subject, but I'll put in my $.02 here anyway.
There is a prevailing myth that less oil is better, and that the oil in the fuel is what lubricates the engine. And there is also a very common belief that spooge is caused by too much oil in the fuel mix. Both are wrong. The engine is lubricated by the residual oil that builds up in the crankcase. All the oil in the fuel does is replenish this oil. And spooge is caused by rich jetting.
When an engine is jetted too rich, the excess fuel leeches heat from the combustion process, causing the combustion chamber temperatures to be too low to effectively burn the oil, or even completely burn all of the fuel. The result is spooge and deposits. The spooge is nothing more than unburned fuel and oil passing out the exhaust.
If you have a spooge problem, you have a jetting problem. You don't get rid of the spooge by reducing the oil, you get rid of it by fixing the jetting. Correct jetting will produce an air/fuel ratio of about 14:1, which will produce combustion temperatures in the 1200 degree range. This will provide sufficient heat to consume the premix oil.
You don't choose a mix ratio based on "spooge", you choose the ratio based on the amount of oil your engine needs to provide sufficient protection and adequate ring seal. The common misconception is that mix ratios are "one-size-fits-all", when in fact nothing could be frther from the truth.The amount of oil that is correct for one rider on his bike may not be enough oil for another rider/bike, or it may be too much oil. It all depends on engine displacement, riding style, and how hard you push the engine.
The best way to determine if you are running enough oil is to check the level of the residual oil in the crankcase. If the ratio you run leaves enough residual oil in the crankcase to cover about 1/8" of the bottom of the crank wheels, then you are fine. If you don't have that much residual oil in your crankcase when you pull the top-end off, you aren't running enough oil for your riding style and conditions.
With that said, to have that amount of residual oil in the crankcase at 50:1 (a ratio made popular by magazines and oil bottles), you can't be riding very hard, or your bike is jetted richer than necessary simply to deliver enough oil. I arrived at 26:1 for my bike with my riding style because that is the amount that gives me the proper amount of residual build-up. Small-bore engines require greater oil concentrations than larger engines to achieve the proper amount of residual build-up, because they rev higher and have higher intake velocities. Along the same lines, someone that pushes the engine harder, and keeps the revs higher, also needs to use higher oil concentrations to achieve the proper residual build-up.
To understand why the residual oil is so important, you have to understand what happens to the oil in your fuel when it goes into the engine. While the oil is still suspended in the liquid gasoline, it can not lubricate anything. It has about as much lubricity at that point as straight gasoline. When the gasoline enters the engine, it evaporates, dropping the oil out of suspension. Now that the oil is free, it can lubricate the engine, but it must get to the parts to lubricate them. The way it gets to the bearings and onto the cylinder is by being thrown around by the spinning crankshaft, and being distributed through the engine by the air currents moving through the crankcase. The main bearings are lubed by some of this oil dripping down through tiny "drip passages" in the cases above the bearing pockets.
People believe that the oil just rushes right through a two-stroke along with the fuel, but that just isn't so. It can take 90 minutes or more for the oil migration through a two-stroke to result in a complete oil exchange.
The oil eventually makes it into the combustion chamber, where it is either burned, or passes out the exhaust. If the combustion chamber temps are too low, such as in an engine that is jetted too rich, the oil doesn't burn completely. Instead, some of it hardens into deposits in the combustion chamber, on the piston, and on the power valve assembly. The rest becomes the dreaded "spooge". The key to all of this working in harmony is to jet the bike lean enough to achieve a high enough combustion chamber temperature to burn the oil, but also still be able to supply enough oil to protect the engine. If you use enough oil, you can jet the bike at it's optimum without starving the engine of oil, and have excellent power, with minimal deposits and spooge. At 50:1, you simply can't jet very lean without risking a seized engine due to oil starvation.
With the high oil concentrations that I use, I tend to get far more life from my cranks and rings than most of my friends that run leaner oil ratios. The high oil content also produces better ring sealing, so more of the combustion pressure is retained.
One small point. No one ever broke an engine by using too much oil.
Now we come to the issue of ring seal. Simply put, the rings alone can not effectively seal the cylinder. They also need oil to provide a complete seal against the bore surface. And up to a point, more oil will provide a better seal.
I have run Dyno tests on this subject, as a school project in Tech School. We used a Dynojet dynamometer, and used a fresh, broken in top-end for each test. We used specially calibrated jets to ensure the fuel flow was identical with each different ratio, and warmed the engine at 3000 rpm for 3 minutes before each run. Our tests were performed in the rpm range of 2500 to 9000 rpm, with the power peak of our test bike (an '86 YZ 250) occuring at 8750 rpm. We tested at 76 degrees F, at 65% relative humidity. We started at 10:1, and went to 100:1. Our results showed that a two-stroke engine makes its best power at 18:1. Any more oil than that, and the engine ran poorly, because we didn't have any jets rich enough to compensate for that much oil in the fuel. The power loss from 18:1 to 32:1 was approximately 2 percent. The loss from 18:1 to 50:1 was nearly 9 percent. On a modern 250, that can be as much as 4 horsepower. The loss from 18:1 to 100:1 was nearly 18 percent. The reason for the difference in output is simple. More oil provides a better seal between the ring and the cylinder wall.
Now, I realize that 18:1 is impractical unless you ride your engine all-out, keeping it pinned at all times. But running reasonable ratios no less than 32:1 will produce more power, and give your engine better protection, thus making it perform better for longer.
Re: Time to talk about oil gas mix and velocity.
A good read from Tim Harral on 2 stroke karts. ( Read the part about 3/4 of the way down about Lubrication related seizures )
Since Birth
It's a common sight. You see a kart in the pit with the cylinder head off. A group of technical "guess
men" are assembled in a circle passing around a seized piston as if touching it can give them greater
insight as to the reason for it's failure. Unfortunately, when this whole mess of parts gets dragged own to the shop, the engine builder may not be able to provide much more insight unless he is very
familiar with that particular engine's "sources of seizure". It's a lot to ask.
Even among engine builders, there's plenty of confusion about what causes a particular piston to seize. The following information will help to dispel some myths, and shed some light on the understanding of piston seizures. The objective of this article is to make piston seizures a part of your past.
Some fundamentals:
Many people believe that piston seizures occur when engine heat causes the piston to expand larger
than the size of the cylinder bore .... This is not true. If you could freeze your engine "in motion" in the middle of a long full throttle pass, and disassemble it for micrometer measurement, you would find
the piston skirt to measure at a 0.0000 to 0.0005" or so press fit into the bore. That's right, a slight
press fit! The reason that it doesn't seize is because the premix oil has such a terrific film strength that it acts as an unremovable buffer between the piston and the cylinder. That is, the bare metal surface
of the piston never actually touches the bare metal surface of the cylinder because the oil stays
between them. Many mechanics have experienced this phenomenon while cleaning a freshly bored
cylinder. Completely dry without cleaning solvent, the piston moves through the bore with difficulty.
After rinsing the piston glides all the way through with no resistance at all. This is because the solvent
acts as a film between the piston and cylinder.
A piston seizure can only occur when something burns or scrapes away the oil film that exists
between the piston and the cylinder wall. Understanding this, it's not hard to see why oils with
exceptionally high film strengths are very desirable. Good quality oils can provide a film that stands
up to the most intense heat and the pressure loads of a modern high output engine.
The difference between seizure and scoring:
Seizure and scoring are two different stages of the same problem. When the oil film on a cylinder is
momentarily burned or brushed away, the metal surfaces of the piston and the cylinder wall will
actually touch. When this happens, there is a sort of scraping that takes place between them. When
the oil film is resumed, the marks from this scraping will often remain on the piston and (or) the
cylinder wall. This momentary scraping or "scoring" seldom causes any permanent or performance
robbing damage. No significant damage takes place because the oil film is resumed before the piston and cylinder have a chance to start exchanging material onto one another.
Scoring is commonly seen on the piston face directly below the piston ring end gaps. The blast of
combustion can get between the large end gap of a worn out ring and burn the oil off the piston and
cylinder in that area...Hence the surface scoring. In most cases, score marks can simply be sanded
off of the piston and cylinder. However when ever you see scoring, it's a good idea to find the
source so that it doesn't develop into a full blown seizure.
Seizure is a case of scoring where the oil film does not immediately return. After a few moments of
constant scoring, the piston and cylinder will scratch each other hard enough to remove material from each other. This floating material grinds itself into the piston and the cylinder as it continues to grow
in size. As this snowballing material grows, it will drive the opposite side of the piston against the
cylinder wall with a pressure so terrific that scoring begins to take place. While all this is going on,
your engine is still running wonderfully at full throttle.
The death blow comes when the mass of material between the piston and the cylinder wall finds it's
way to the piston ring. This nearly molten mixture of aluminum and iron will instantly lock the ring in
it's groove. This ring locking, not the piston surface seizure, is what actually causes your engine to
quit. When the piston ring becomes locked back in it's groove, it's incapable of providing
compression sealing against the cylinder wall. This instant loss of compression, while the engine is at
speed, causes a dramatic loss of power. That power loss, along with the added drag of the badly
seized piston, makes the engine quit or lock up in a nanosecond. In fact this entire seizure process,
from the first scoring scratch to the piston locked solid, takes less than a second at full rpm. That's
not even enough time for you to utter the first syllable of your favorite profanity.
THERE ARE MANY CAUSES OR SOURCES OF PISTON SEIZURE.
Each cause has it's own symptoms and it's own visual results. The following is a description of
several very common types of seizures, and the most common problem source for each one. It
should be understood that diagnosing piston seizures is not done with any precision by even the best
engine builders. However this information may allow many of you to make a more educated and
accurate guess.
Four corner seizure: Both sides of the piston will show heavy scoring and seizure marks on each
side of the wrist pin hole. The pattern of these four seizure points often appears to be a perfect
square, hence the slang term "four corner". The scoring takes place in this pattern because those
areas of the piston casting are the thickest. When the piston is seriously overheated, the thick areas
will expand and distort the most. High output kart engines usually experience this type of seizure
pattern when a piston has been fitted with too little clearance. Most experienced , and well meaning,
kart mechanics would take one look and immediately say that insufficient piston clearance is the
cause. However that diagnosis, on watercooled engines, would be wrong about 99% of the time.
Four corner seizures in watercooled engines are almost always a result of the engine creating more
heat than the cooling system can exchange away. That is not to say that most cooling systems are
under built, but rather that it's easy to make a modification that creates too much internal heat for
even the most beefed up cooling systems. Even though a constant feed of cool water is being moved
through the cooling system, the cooling system must be capable of exchanging the engine heat away
at a rate quicker than the engine is creating it.
The engine factors that have the greatest seizure related effect are operating temperature, excessive
compression ratio, ignition advance, high rpm, insufficient fuel octane level, insufficient cooling, or any combination of these. Properly adjusting these same factors will have the greatest effect on total
power output. The job of the engine builder is to find the right combination, or "blend", of these
factors that will result in strong overall power output at a pace that your cooling system can keep up
with. There are many engine builders who have mastered their own combination "blend" that can get
you all the power your after without risking a seizure.
A group of mismatched modifications is a first class ticket to "seizure-land". Any inexperienced
individual who sets up your engine with over 200 psi of compression and advanced timing, is also
guaranteeing your arrival. If your big mouthed racing buddy down the street tells you that he can
make any engine "roost".... You should think twice. You could be in for a very expensive lesson.
Lean seizures: The high speed circuit on almost all kart engine carbs are responsible for delivering fuel in the 30%-100% throttle range. If the high speed circuit is lean enough to cause piston seizure,
it will also tend to cause a laziness in mid-range throttle response. Dangerously lean high rpm racing
motors can sometimes offer acceptable mid-range, however they will accelerate to peak speed very
slowly.
The classic lean seizure exhibits heavy scoring and seizure along the entire width of the exhaust port with only light scoring on the opposite piston faces. In lean mixture conditions, the exhaust gas
temperatures escalate quickly into the meltdown stage. Those high temperature gases can
compromise or completely burn off the oil film on the exhaust piston face as the exhaust port is being covered up. With the oil film weakened or gone, scoring quickly turns into seizure and ring locking.
Air leak seizures: If you could pressure check every engine that showed up at a local racing event, you would find some of them to have an airleak. Because of the varying degree of these leaks, some will result in seizure, others will only cause poor carbureton or slight overheating. The varying effects of these air leaks makes this a difficult diagnosis.
In any situation where an engine has seized for no apparent reason, the motor should be pressure
tested before any other teardown work is performed. If a mechanic does not have the equipment to
pressure test your seized engine, it's very unlikely that he will have the finesse to accurately diagnose
your problem either. In fact, pressure testing should be a standard finishing procedure for any major
engine reassembly work. Race engines should be pressure tested every re-build.
The air leak piston seizure, depending on the severity of the leak, can look like a four corner type or a lean mixture type of scoring pattern. If an engine is operating on the ragged edge of overheating, a small air leak can easily cause the extra overheating that will result in a four corner seizure. On the
other hand, a huge air leak will draw in so much additional air that even an over rich engine can
experience a lean type seizure at full rpm's.
Detonation seizures: If a modified engine has been prepared with too much compression or spark advance, or if it's run on unacceptably low octane fuel, it will begin to "ping" or detonate. Detonation is a big subject the merits another article of it's own. For now we only need to understand that it causes a terrific amount of internal heat in a very short time, as well as physical damage to the combustion chamber. If you have ever seen the outer diameter of a cylinder head dome that looks as if it's been eaten by termites, you have seen the results of detonation. It packs a physical force that is roughly equivalent to hitting the edge of the piston crown with a full arm swing of a ball peen hammer. In a short amount of time, this detonation pounding will collapse the ring land and lock the ring in place (usually on the hotter running exhaust side of the piston). As soon as the ring is locked, the flames of combustion burn the oil film off of the cylinder wall, and the scoring/ seizure process begins. Because of the exhaust side scoring and the swift overheating caused by detonation, you'll have a 50/50 chance of a four corner seizure or a "lean mixture" appearing seizure. Only an experienced engine builder will be able to accurately diagnose this seizure source.
Cooling system seizures: This cause cuts into the gray area of piston seizure. A clogged cooling
system on any machine can cause swift and serious temperature problems. However, no engine will
ever experience a seizure purely as a result of inadequate cooling from the stock system. I have seen
several karts run an entire trouble free season with a bone stock cooling system. These engines are
not a statement of the effectiveness of the stock cooling system, but rather a statement to the benefits of having a professionally prepared high performance combination. The larger line and dual line
cooling kits certainly have their merits on high output race engines that will be run at full throttle for
extended periods of time. Their ability to more rapidly exchange away engine heat is a great asset on modified engines that are run at full throttle only. However, none of them can exchange away the
excess heat created by a poorly prepared engine package. If you are experiencing chronic piston
seizures of any kind, increased cooling may temporarily stave off the problem, however it will almost
never cure it.
Piston clearances As mentioned earlier, too little piston clearance is one of the most common "
wrong " diagnoses made on seized watercooled engines. Most of today's engines come brand new
with cylinder clearances that are .001"-.002" over the recommended factory setup clearance. This
extra clearance is an added protection against drivers who don't follow the proper break-in
procedure. If the clearance of a bored cylinder has been set at the factory recommended clearance,
the close piston clearance by itself will not cause seizure. There is usually an added factor such as
excessive compression or an air leak. If a piston if fitted with too little clearance, it will usually
experience a four corner type seizure pattern. In most cases the ring will experience little or no
damage. If this is the case, it's entirely safe to sand the score marks off the pistons and re-use them in the freshly honed cylinder.
Too much piston clearance can also result in piston scoring and seizure. A piston ring, in an
excessively large cylinder bore, will have a very wide end gap not to mention very weak ring tension
against the cylinder wall. The flame of combustion can easily burn past this weak ring seal as well as
down the end gap opening itself. If this flame burns off a significant amount of the oil film on the
cylinder wall, the scoring seizure process begins.
Break in seizures: The most common break in related seizure is usually caused by the ring not the piston. Some new piston rings come with a coating on their outer sealing surfaces. This coating seals
to the cylinder wall in just a few operating minutes, which provides better power during the break in
period. As the engine is breaking in, the Teflon eventually wears away and lets the hard surface of
the ring come in from behind to provide the long term seal. The down side of this coating is that it
makes for a dangerously small end gap during the first hours of operation. If the engine is run too
hard too soon, the heat will cause the ring to expand in diameter which may drive the ring ends
together and drive the ring surface hard against the cylinder wall. A piston ring that is being
overheated in this fashion will easily have enough tension against the cylinder wall to scratch off the
oil film which will begin the scoring/seizure process. A piston seized in this way will have heavy
scoring around the entire diameter of the piston, with the ring usually locked into the groove all the
way around as well.
Lubrication related seizures By now it's apparent how important the oil film strength can be. Equally important is the amount of oil that is present in the engine. Most kart engines carry factory
recommendations for oil premix ratios between 15:1 and 20:1.
The total amount of time that it takes for a drop of oil to get from the carburetor, to the lower end
bearings, to the top end, and out the exhaust port is called "oil migration time". As peak rpms
increase, the amount of time that a drop of oil remains inside the engine is drastically shorter. In other words, a 17000 rpm race engine would need a mix ratio of about 20:1 to maintain the same internal
lubrication presence that a 12000 rpm engine would have with a 30:1 mix. There are several oil
brands that claim that their oil can provide equal lubrication at a leaner mix ratio (40:1 or 50:1)
because of a claimed better lubrication quality. I have never experienced this to be true nor has any
oil manufacturer, to my knowledge, proven it to be true. It's like running your truck on two quarts of
a special oil instead of four quarts of a standard type oil. The quality cannot make up for the
quantity.....Ever.
Seizure by running out of gas: - as many people already know, a larger size needle and seat must
often be installed into a carburetor to contend with the increased fuel demands of a moderately
modified engine. If a modified engine is operated at full throttle with a stock size needle and seat, it
will usually carry full rpm for about 2 or 3 seconds and then shut off as if someone hit the kill button.
When the machine comes to a stop, the driver re-starts to see what the problem is. The engine, no
longer in fuel deficit caused by the undersized needle and seat, unexpectedly starts right up.
This instant high speed shut off is caused by the carburetor literally running out of gas. It is sometimes possible that during this shut down moment of fuel starvation, the engine is also starved of the oil that is pre-mixed in. This oil starvation may cause subsequent piston scoring or seizure.
At the moment that the fuel starved engine shuts down, combustion and all the heat associated with it "ceases". A karts rear wheel traction continues to move the pistons in the bores at a very high
"friction causing" speed,. This same concept applies to any machine that simply runs it's tank dry.
It is possible for a driver, whose carb has an under sized needle and seat, to induce a piston seizure. However this would require a great deal of combined skill and stupidity. Once the driver has established that extended full throttle operation causes his engine to quit, he might make the very
poor choice of only applying enough high speed throttle to avoid starving the engine. When he does
this, he will be capable of maintaining about 90% throttle which will hold the engine endlessly on the
lean thresh hold of fuel starvation. As this driver eventually masters this throttle position, he will be
able to maintain a very high rpm with the carb feeding a horrificly lean mixture. Ultimately his finesse
will be rewarded by one of the most abrupt and destructive lean mixture seizures that his mechanic
has ever seen
FABLES AND UNTRUTHS
Cold seizure: - this is by far the most over used "seizure scape goat". It some how implies that the
driver ran his engine in a way that caused the failure. At least 95% of the "so called" cold seized
engines have had a very apparent problem elsewhere in the engine that the builder failed to see.
Telling a customer that he cold seized the engine is an easy way for a mechanic to immediately
reverse the guilt and the responsibility.
If a freshly bored engine or a high performance engine were started from stone cold, and then run
hard at high rpm within 30 seconds of the start up, it could very likely experience a true cold seizure.
This happens because the aluminum piston would experience a radically faster rate of expansion in
that 30 seconds than the cylinder does. The reason for this difference in expansion rate is two fold.
First and foremost, the internal temperatures that the piston crown is exposed to at full load are on
the order of 1500'C-2500'C. The gases passing through the exhaust manifold ports is also in this
temperature range. The expansion rate caused by these temperatures is usually not a problem when
the water entering the water jacket is preheated. During the first 30 operating seconds, cold
incoming water will maintain the water jacket around the cylinder at "stone cold diameter" while the
piston is becoming "full temperature diameter". On engines with properly sized pistons, the difference in these diameters becomes much more than even the best oils can withstand. Any engine that has
been warmed up for 60 seconds or longer, would be virtually incapable of a "cold seizure"
Hot water seizure: - the hottest water operated in high output race machines is about 92' C.
Machines equipped with a single cooling line system show no signs of any piston scoring or seizure.
There is evidence however that the warmer water causes engines to lose peak power ability during
longer races.
Leaning out: - this is a term for a phenomenon that doesn't really exist. It implies that a carburetor, whose needle/seat and high speed metering screw is properly set, will suddenly begin to meter
slightly less fuel to the engine for no apparent reason. This does not happen .... Ever. In most cases
what a driver is actually referring to is the way his machine begins to slow down noticeably during a
long full throttle pass. In most cases this slowing is the result of a serious overheating problem caused by excessive compression, ignition advance or poor quality gasoline.
Since Birth
It's a common sight. You see a kart in the pit with the cylinder head off. A group of technical "guess
men" are assembled in a circle passing around a seized piston as if touching it can give them greater
insight as to the reason for it's failure. Unfortunately, when this whole mess of parts gets dragged own to the shop, the engine builder may not be able to provide much more insight unless he is very
familiar with that particular engine's "sources of seizure". It's a lot to ask.
Even among engine builders, there's plenty of confusion about what causes a particular piston to seize. The following information will help to dispel some myths, and shed some light on the understanding of piston seizures. The objective of this article is to make piston seizures a part of your past.
Some fundamentals:
Many people believe that piston seizures occur when engine heat causes the piston to expand larger
than the size of the cylinder bore .... This is not true. If you could freeze your engine "in motion" in the middle of a long full throttle pass, and disassemble it for micrometer measurement, you would find
the piston skirt to measure at a 0.0000 to 0.0005" or so press fit into the bore. That's right, a slight
press fit! The reason that it doesn't seize is because the premix oil has such a terrific film strength that it acts as an unremovable buffer between the piston and the cylinder. That is, the bare metal surface
of the piston never actually touches the bare metal surface of the cylinder because the oil stays
between them. Many mechanics have experienced this phenomenon while cleaning a freshly bored
cylinder. Completely dry without cleaning solvent, the piston moves through the bore with difficulty.
After rinsing the piston glides all the way through with no resistance at all. This is because the solvent
acts as a film between the piston and cylinder.
A piston seizure can only occur when something burns or scrapes away the oil film that exists
between the piston and the cylinder wall. Understanding this, it's not hard to see why oils with
exceptionally high film strengths are very desirable. Good quality oils can provide a film that stands
up to the most intense heat and the pressure loads of a modern high output engine.
The difference between seizure and scoring:
Seizure and scoring are two different stages of the same problem. When the oil film on a cylinder is
momentarily burned or brushed away, the metal surfaces of the piston and the cylinder wall will
actually touch. When this happens, there is a sort of scraping that takes place between them. When
the oil film is resumed, the marks from this scraping will often remain on the piston and (or) the
cylinder wall. This momentary scraping or "scoring" seldom causes any permanent or performance
robbing damage. No significant damage takes place because the oil film is resumed before the piston and cylinder have a chance to start exchanging material onto one another.
Scoring is commonly seen on the piston face directly below the piston ring end gaps. The blast of
combustion can get between the large end gap of a worn out ring and burn the oil off the piston and
cylinder in that area...Hence the surface scoring. In most cases, score marks can simply be sanded
off of the piston and cylinder. However when ever you see scoring, it's a good idea to find the
source so that it doesn't develop into a full blown seizure.
Seizure is a case of scoring where the oil film does not immediately return. After a few moments of
constant scoring, the piston and cylinder will scratch each other hard enough to remove material from each other. This floating material grinds itself into the piston and the cylinder as it continues to grow
in size. As this snowballing material grows, it will drive the opposite side of the piston against the
cylinder wall with a pressure so terrific that scoring begins to take place. While all this is going on,
your engine is still running wonderfully at full throttle.
The death blow comes when the mass of material between the piston and the cylinder wall finds it's
way to the piston ring. This nearly molten mixture of aluminum and iron will instantly lock the ring in
it's groove. This ring locking, not the piston surface seizure, is what actually causes your engine to
quit. When the piston ring becomes locked back in it's groove, it's incapable of providing
compression sealing against the cylinder wall. This instant loss of compression, while the engine is at
speed, causes a dramatic loss of power. That power loss, along with the added drag of the badly
seized piston, makes the engine quit or lock up in a nanosecond. In fact this entire seizure process,
from the first scoring scratch to the piston locked solid, takes less than a second at full rpm. That's
not even enough time for you to utter the first syllable of your favorite profanity.
THERE ARE MANY CAUSES OR SOURCES OF PISTON SEIZURE.
Each cause has it's own symptoms and it's own visual results. The following is a description of
several very common types of seizures, and the most common problem source for each one. It
should be understood that diagnosing piston seizures is not done with any precision by even the best
engine builders. However this information may allow many of you to make a more educated and
accurate guess.
Four corner seizure: Both sides of the piston will show heavy scoring and seizure marks on each
side of the wrist pin hole. The pattern of these four seizure points often appears to be a perfect
square, hence the slang term "four corner". The scoring takes place in this pattern because those
areas of the piston casting are the thickest. When the piston is seriously overheated, the thick areas
will expand and distort the most. High output kart engines usually experience this type of seizure
pattern when a piston has been fitted with too little clearance. Most experienced , and well meaning,
kart mechanics would take one look and immediately say that insufficient piston clearance is the
cause. However that diagnosis, on watercooled engines, would be wrong about 99% of the time.
Four corner seizures in watercooled engines are almost always a result of the engine creating more
heat than the cooling system can exchange away. That is not to say that most cooling systems are
under built, but rather that it's easy to make a modification that creates too much internal heat for
even the most beefed up cooling systems. Even though a constant feed of cool water is being moved
through the cooling system, the cooling system must be capable of exchanging the engine heat away
at a rate quicker than the engine is creating it.
The engine factors that have the greatest seizure related effect are operating temperature, excessive
compression ratio, ignition advance, high rpm, insufficient fuel octane level, insufficient cooling, or any combination of these. Properly adjusting these same factors will have the greatest effect on total
power output. The job of the engine builder is to find the right combination, or "blend", of these
factors that will result in strong overall power output at a pace that your cooling system can keep up
with. There are many engine builders who have mastered their own combination "blend" that can get
you all the power your after without risking a seizure.
A group of mismatched modifications is a first class ticket to "seizure-land". Any inexperienced
individual who sets up your engine with over 200 psi of compression and advanced timing, is also
guaranteeing your arrival. If your big mouthed racing buddy down the street tells you that he can
make any engine "roost".... You should think twice. You could be in for a very expensive lesson.
Lean seizures: The high speed circuit on almost all kart engine carbs are responsible for delivering fuel in the 30%-100% throttle range. If the high speed circuit is lean enough to cause piston seizure,
it will also tend to cause a laziness in mid-range throttle response. Dangerously lean high rpm racing
motors can sometimes offer acceptable mid-range, however they will accelerate to peak speed very
slowly.
The classic lean seizure exhibits heavy scoring and seizure along the entire width of the exhaust port with only light scoring on the opposite piston faces. In lean mixture conditions, the exhaust gas
temperatures escalate quickly into the meltdown stage. Those high temperature gases can
compromise or completely burn off the oil film on the exhaust piston face as the exhaust port is being covered up. With the oil film weakened or gone, scoring quickly turns into seizure and ring locking.
Air leak seizures: If you could pressure check every engine that showed up at a local racing event, you would find some of them to have an airleak. Because of the varying degree of these leaks, some will result in seizure, others will only cause poor carbureton or slight overheating. The varying effects of these air leaks makes this a difficult diagnosis.
In any situation where an engine has seized for no apparent reason, the motor should be pressure
tested before any other teardown work is performed. If a mechanic does not have the equipment to
pressure test your seized engine, it's very unlikely that he will have the finesse to accurately diagnose
your problem either. In fact, pressure testing should be a standard finishing procedure for any major
engine reassembly work. Race engines should be pressure tested every re-build.
The air leak piston seizure, depending on the severity of the leak, can look like a four corner type or a lean mixture type of scoring pattern. If an engine is operating on the ragged edge of overheating, a small air leak can easily cause the extra overheating that will result in a four corner seizure. On the
other hand, a huge air leak will draw in so much additional air that even an over rich engine can
experience a lean type seizure at full rpm's.
Detonation seizures: If a modified engine has been prepared with too much compression or spark advance, or if it's run on unacceptably low octane fuel, it will begin to "ping" or detonate. Detonation is a big subject the merits another article of it's own. For now we only need to understand that it causes a terrific amount of internal heat in a very short time, as well as physical damage to the combustion chamber. If you have ever seen the outer diameter of a cylinder head dome that looks as if it's been eaten by termites, you have seen the results of detonation. It packs a physical force that is roughly equivalent to hitting the edge of the piston crown with a full arm swing of a ball peen hammer. In a short amount of time, this detonation pounding will collapse the ring land and lock the ring in place (usually on the hotter running exhaust side of the piston). As soon as the ring is locked, the flames of combustion burn the oil film off of the cylinder wall, and the scoring/ seizure process begins. Because of the exhaust side scoring and the swift overheating caused by detonation, you'll have a 50/50 chance of a four corner seizure or a "lean mixture" appearing seizure. Only an experienced engine builder will be able to accurately diagnose this seizure source.
Cooling system seizures: This cause cuts into the gray area of piston seizure. A clogged cooling
system on any machine can cause swift and serious temperature problems. However, no engine will
ever experience a seizure purely as a result of inadequate cooling from the stock system. I have seen
several karts run an entire trouble free season with a bone stock cooling system. These engines are
not a statement of the effectiveness of the stock cooling system, but rather a statement to the benefits of having a professionally prepared high performance combination. The larger line and dual line
cooling kits certainly have their merits on high output race engines that will be run at full throttle for
extended periods of time. Their ability to more rapidly exchange away engine heat is a great asset on modified engines that are run at full throttle only. However, none of them can exchange away the
excess heat created by a poorly prepared engine package. If you are experiencing chronic piston
seizures of any kind, increased cooling may temporarily stave off the problem, however it will almost
never cure it.
Piston clearances As mentioned earlier, too little piston clearance is one of the most common "
wrong " diagnoses made on seized watercooled engines. Most of today's engines come brand new
with cylinder clearances that are .001"-.002" over the recommended factory setup clearance. This
extra clearance is an added protection against drivers who don't follow the proper break-in
procedure. If the clearance of a bored cylinder has been set at the factory recommended clearance,
the close piston clearance by itself will not cause seizure. There is usually an added factor such as
excessive compression or an air leak. If a piston if fitted with too little clearance, it will usually
experience a four corner type seizure pattern. In most cases the ring will experience little or no
damage. If this is the case, it's entirely safe to sand the score marks off the pistons and re-use them in the freshly honed cylinder.
Too much piston clearance can also result in piston scoring and seizure. A piston ring, in an
excessively large cylinder bore, will have a very wide end gap not to mention very weak ring tension
against the cylinder wall. The flame of combustion can easily burn past this weak ring seal as well as
down the end gap opening itself. If this flame burns off a significant amount of the oil film on the
cylinder wall, the scoring seizure process begins.
Break in seizures: The most common break in related seizure is usually caused by the ring not the piston. Some new piston rings come with a coating on their outer sealing surfaces. This coating seals
to the cylinder wall in just a few operating minutes, which provides better power during the break in
period. As the engine is breaking in, the Teflon eventually wears away and lets the hard surface of
the ring come in from behind to provide the long term seal. The down side of this coating is that it
makes for a dangerously small end gap during the first hours of operation. If the engine is run too
hard too soon, the heat will cause the ring to expand in diameter which may drive the ring ends
together and drive the ring surface hard against the cylinder wall. A piston ring that is being
overheated in this fashion will easily have enough tension against the cylinder wall to scratch off the
oil film which will begin the scoring/seizure process. A piston seized in this way will have heavy
scoring around the entire diameter of the piston, with the ring usually locked into the groove all the
way around as well.
Lubrication related seizures By now it's apparent how important the oil film strength can be. Equally important is the amount of oil that is present in the engine. Most kart engines carry factory
recommendations for oil premix ratios between 15:1 and 20:1.
The total amount of time that it takes for a drop of oil to get from the carburetor, to the lower end
bearings, to the top end, and out the exhaust port is called "oil migration time". As peak rpms
increase, the amount of time that a drop of oil remains inside the engine is drastically shorter. In other words, a 17000 rpm race engine would need a mix ratio of about 20:1 to maintain the same internal
lubrication presence that a 12000 rpm engine would have with a 30:1 mix. There are several oil
brands that claim that their oil can provide equal lubrication at a leaner mix ratio (40:1 or 50:1)
because of a claimed better lubrication quality. I have never experienced this to be true nor has any
oil manufacturer, to my knowledge, proven it to be true. It's like running your truck on two quarts of
a special oil instead of four quarts of a standard type oil. The quality cannot make up for the
quantity.....Ever.
Seizure by running out of gas: - as many people already know, a larger size needle and seat must
often be installed into a carburetor to contend with the increased fuel demands of a moderately
modified engine. If a modified engine is operated at full throttle with a stock size needle and seat, it
will usually carry full rpm for about 2 or 3 seconds and then shut off as if someone hit the kill button.
When the machine comes to a stop, the driver re-starts to see what the problem is. The engine, no
longer in fuel deficit caused by the undersized needle and seat, unexpectedly starts right up.
This instant high speed shut off is caused by the carburetor literally running out of gas. It is sometimes possible that during this shut down moment of fuel starvation, the engine is also starved of the oil that is pre-mixed in. This oil starvation may cause subsequent piston scoring or seizure.
At the moment that the fuel starved engine shuts down, combustion and all the heat associated with it "ceases". A karts rear wheel traction continues to move the pistons in the bores at a very high
"friction causing" speed,. This same concept applies to any machine that simply runs it's tank dry.
It is possible for a driver, whose carb has an under sized needle and seat, to induce a piston seizure. However this would require a great deal of combined skill and stupidity. Once the driver has established that extended full throttle operation causes his engine to quit, he might make the very
poor choice of only applying enough high speed throttle to avoid starving the engine. When he does
this, he will be capable of maintaining about 90% throttle which will hold the engine endlessly on the
lean thresh hold of fuel starvation. As this driver eventually masters this throttle position, he will be
able to maintain a very high rpm with the carb feeding a horrificly lean mixture. Ultimately his finesse
will be rewarded by one of the most abrupt and destructive lean mixture seizures that his mechanic
has ever seen
FABLES AND UNTRUTHS
Cold seizure: - this is by far the most over used "seizure scape goat". It some how implies that the
driver ran his engine in a way that caused the failure. At least 95% of the "so called" cold seized
engines have had a very apparent problem elsewhere in the engine that the builder failed to see.
Telling a customer that he cold seized the engine is an easy way for a mechanic to immediately
reverse the guilt and the responsibility.
If a freshly bored engine or a high performance engine were started from stone cold, and then run
hard at high rpm within 30 seconds of the start up, it could very likely experience a true cold seizure.
This happens because the aluminum piston would experience a radically faster rate of expansion in
that 30 seconds than the cylinder does. The reason for this difference in expansion rate is two fold.
First and foremost, the internal temperatures that the piston crown is exposed to at full load are on
the order of 1500'C-2500'C. The gases passing through the exhaust manifold ports is also in this
temperature range. The expansion rate caused by these temperatures is usually not a problem when
the water entering the water jacket is preheated. During the first 30 operating seconds, cold
incoming water will maintain the water jacket around the cylinder at "stone cold diameter" while the
piston is becoming "full temperature diameter". On engines with properly sized pistons, the difference in these diameters becomes much more than even the best oils can withstand. Any engine that has
been warmed up for 60 seconds or longer, would be virtually incapable of a "cold seizure"
Hot water seizure: - the hottest water operated in high output race machines is about 92' C.
Machines equipped with a single cooling line system show no signs of any piston scoring or seizure.
There is evidence however that the warmer water causes engines to lose peak power ability during
longer races.
Leaning out: - this is a term for a phenomenon that doesn't really exist. It implies that a carburetor, whose needle/seat and high speed metering screw is properly set, will suddenly begin to meter
slightly less fuel to the engine for no apparent reason. This does not happen .... Ever. In most cases
what a driver is actually referring to is the way his machine begins to slow down noticeably during a
long full throttle pass. In most cases this slowing is the result of a serious overheating problem caused by excessive compression, ignition advance or poor quality gasoline.
Re: Time to talk about oil gas mix and velocity.
A few comments on the next to the last article above, again my opinion. I do not agree that residual oil is what lubricates a two-stroke engine. A fuel/oil mix is an emulsion perhaps down to individual molecules suspended in fuel. The molecule chains provide lubrication whether diluted or not. A certain amount is needed to pass through the system at a given time to provide this service. During atomization of the fuel evaporation does take place, but I personally doubt that oil is dropping out to any extent. The atomized fuel still includes the molecules of oil until evaporation. Most of the fuel arrives at the combustion chamber as droplets rather than a gas. Oil droplets are pulled through the engine by the velocity of the air and don't necessarily collect in the crankcase while the engine is running. When the engine is shut off the residual fuel/oil mix settles to the lowest point in the crankcase where the more volatile fuel evaporates. Of course oil can accumulate during reduced velocities such as idling or over rich points in RPM range but probably not in high RPM situations.
"Spooge" as mentioned in the article is mostly carbon products left over from the lubricants. A rich/lower temperature combustion area does hinder the carbon from combining with oxygen for complete combustion (even from the fuel.) Pure castor oil will clog the exhaust port even in near lean conditions. It is a good lubricant but high in carbon or chemically bound carbon. Synthetic lubricants can provide the required lubrication without producing as much build up in the system.
Dyno tests are good in that they collect and compare data but need to be interpreted for more real world conditions. When I buy two-stroke racing oil (say Yamalube), I trust the manufacturers recommendations (32:1) based on real world testing of race engines in real conditions. A static dyno test that concludes that 18:1 at 8750 RPM is best should be taken with a grain of salt.
OK, that's my opinion today.
Max
"Spooge" as mentioned in the article is mostly carbon products left over from the lubricants. A rich/lower temperature combustion area does hinder the carbon from combining with oxygen for complete combustion (even from the fuel.) Pure castor oil will clog the exhaust port even in near lean conditions. It is a good lubricant but high in carbon or chemically bound carbon. Synthetic lubricants can provide the required lubrication without producing as much build up in the system.
Dyno tests are good in that they collect and compare data but need to be interpreted for more real world conditions. When I buy two-stroke racing oil (say Yamalube), I trust the manufacturers recommendations (32:1) based on real world testing of race engines in real conditions. A static dyno test that concludes that 18:1 at 8750 RPM is best should be taken with a grain of salt.
OK, that's my opinion today.
Max
Re: Time to talk about oil gas mix and velocity.
Max, I agree with you. If all of the engines lubrication occurred as condensate in the crankcase and had to migrate upward from there, it would eventually enter the combustion chamber as a condensate not much different from the oil in the bottle before mixing with fuel. At that concentration, and in an essentially liquid state, it wouldn't burn worth a darn, and it wouldn't matter then what ratio you use, the combustion mix would be very oil rich.
Residual oil found in the crankcase is just that, residual, from having shut down the engine. What is in droplet form cools and condenses, and falls out due to gravity. There is always a smear of oil in the crankcase, as well as on piston and cylinder. But to think that there is a constant puddle of oil in the case an eighth inch deep or otherwise is silly.
If someone really wants to know what spooge consists of, send a sample out for testing. Appears to me to be a heavy mixture of fuel ash, oil ash, water, and....unburned fuel mix, both gas and oil. That is pretty much what goes out the pipe. But it is heavy on the unburned oil. Two stroke oil, any oil, makes a poor fuel. Don't expect it all to burn during combustion.
Residual oil found in the crankcase is just that, residual, from having shut down the engine. What is in droplet form cools and condenses, and falls out due to gravity. There is always a smear of oil in the crankcase, as well as on piston and cylinder. But to think that there is a constant puddle of oil in the case an eighth inch deep or otherwise is silly.
If someone really wants to know what spooge consists of, send a sample out for testing. Appears to me to be a heavy mixture of fuel ash, oil ash, water, and....unburned fuel mix, both gas and oil. That is pretty much what goes out the pipe. But it is heavy on the unburned oil. Two stroke oil, any oil, makes a poor fuel. Don't expect it all to burn during combustion.
Re: Time to talk about oil gas mix and velocity.
Thanks AZ, after writing all that I see you posted much the same opinion several posts above that. I think we all agree on this one.
Maxie
Maxie
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taber hodaka
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Re: Time to talk about oil gas mix and velocity.
I am convinced. I am going to try amsoil 100 to 1?? Some times I'm a lap ahead of everybody just before I get passed by the leader. -------------------Clarence
Re: Time to talk about oil gas mix and velocity.
One last thought: the two strokes we use are loop scavenged. They rely on several things simultaneously to keep as much fresh charge as possible in the combustion chamber every cycle, while expelling as much combustion gas as possible every cycle. These include things such as port timing and transfer angles, crankcase volumetric scavenging, and exhaust resonance. This is not a perfect method. There is always a combined loss both ways, fresh charge lost, burned charge retained, every cycle. Jetting has no effect on this process to the best of my knowledge, but expansion chambers do have. The problem is that tuned exhaust can be tuned to only one target rpm at a time, and even at peak of design, some losses still occur. And, we rarely run at design peaks for more than a few seconds at a time. Everything is a trade off, and both modest trail bikes and wild ride land record machines share similar problems with volumetric inefficiencies.
Since we are constantly pumping fresh fuel mix out the pipe no matter what we do, finding a residue in the pipe can't be a surprise. The only way to keep two stroke oil out of the sludge mixture is to use none, which we can't do. Residue can be reduced through careful tuning and mixing but can't be eliminated regardless of which oil is used or which fuel or which exhaust system.
Personally, I have yet to see a two stroke exhaust pipe that was dry and clean after extended use, and only on rare occasions such as this one do I lose sleep over it.
Since we are constantly pumping fresh fuel mix out the pipe no matter what we do, finding a residue in the pipe can't be a surprise. The only way to keep two stroke oil out of the sludge mixture is to use none, which we can't do. Residue can be reduced through careful tuning and mixing but can't be eliminated regardless of which oil is used or which fuel or which exhaust system.
Personally, I have yet to see a two stroke exhaust pipe that was dry and clean after extended use, and only on rare occasions such as this one do I lose sleep over it.
Re: Time to talk about oil gas mix and velocity.
About Premix Ratios and “Oil Migration Time” - Oil Premix ratios are another subject that some folks tend to get emotional about … and like oil brand choice, our choices are about science and results…not emotion.
The objective of the premix ratio is to maintain a certain level of “oil-presence” in the engine interior during it’s average “operating-use” cycle. But how does one measure or assess the “oil-presence” … The most effective way has been with a radioactive additive. We explain below.
Trying to keep it simple…here is how it works. A test lab sets up an engine on a dyno stand, and begins feeding the engine a premix of an oil that has a specific level of mixed-in radioactive additive. As the engine is run, a Geiger counter at the exhaust exit measures the amount of radioactive material being eliminated. In this way, it is possible to factor the amount of radioactive material being put into the engine, verses the amount being sent out the exhaust. The net result is the amount of “oil-presence” inside the engine. In short, these tests showed that the oil-presence in the engine is a function of the operating rpm. That is, the “oil-presence" inside a two stroke drops significantly as the operating rpms increase. What this means is that an engine being run at 4000 rpm can maintain a very healthy and happy level of oil-presence with a 40:1 premix. However that exact same engine being run at 8000rpm needs to have a 20:1 premix to maintain the exact same level of oil-presence inside. This is why our 350 Bighorn road racers ran happily on the public roads on 40:1, but needed 20:1 for our sustained high rpm racing use. It bears noting that in both the 40:1 street and 20:1 racing situations, our Bighorns made no visible exhaust smoke at all, except when they were held at idle speeds for a long time.
A further example of this is shown in the carbureted two-stroke 951cc SeaDoo watercrafts of the early 2000s. In an effort to reduce the smoking during initial take-offs, SeaDoo engineers setup the oil injection systems to deliver no oil at all at idle speeds (and we mean zero oil). The logic was that at idle speeds there is virtually no oil migration at all. The high oil presence from the previous high speed runs was enough to allow the engine to run happily at idle for 10+ minutes with no oil at all being added…. And it worked great.
The lesson here is that your premix ratio should be a function of the average operating rpm that your vintage two stroke runs at. If you are at peak rpm all the time, 20:1 is a good idea. However for recreational level riders that don’t “scream” their engines constantly, leaner premixes will yield excellent long term wear.
The objective of the premix ratio is to maintain a certain level of “oil-presence” in the engine interior during it’s average “operating-use” cycle. But how does one measure or assess the “oil-presence” … The most effective way has been with a radioactive additive. We explain below.
Trying to keep it simple…here is how it works. A test lab sets up an engine on a dyno stand, and begins feeding the engine a premix of an oil that has a specific level of mixed-in radioactive additive. As the engine is run, a Geiger counter at the exhaust exit measures the amount of radioactive material being eliminated. In this way, it is possible to factor the amount of radioactive material being put into the engine, verses the amount being sent out the exhaust. The net result is the amount of “oil-presence” inside the engine. In short, these tests showed that the oil-presence in the engine is a function of the operating rpm. That is, the “oil-presence" inside a two stroke drops significantly as the operating rpms increase. What this means is that an engine being run at 4000 rpm can maintain a very healthy and happy level of oil-presence with a 40:1 premix. However that exact same engine being run at 8000rpm needs to have a 20:1 premix to maintain the exact same level of oil-presence inside. This is why our 350 Bighorn road racers ran happily on the public roads on 40:1, but needed 20:1 for our sustained high rpm racing use. It bears noting that in both the 40:1 street and 20:1 racing situations, our Bighorns made no visible exhaust smoke at all, except when they were held at idle speeds for a long time.
A further example of this is shown in the carbureted two-stroke 951cc SeaDoo watercrafts of the early 2000s. In an effort to reduce the smoking during initial take-offs, SeaDoo engineers setup the oil injection systems to deliver no oil at all at idle speeds (and we mean zero oil). The logic was that at idle speeds there is virtually no oil migration at all. The high oil presence from the previous high speed runs was enough to allow the engine to run happily at idle for 10+ minutes with no oil at all being added…. And it worked great.
The lesson here is that your premix ratio should be a function of the average operating rpm that your vintage two stroke runs at. If you are at peak rpm all the time, 20:1 is a good idea. However for recreational level riders that don’t “scream” their engines constantly, leaner premixes will yield excellent long term wear.
Re: Time to talk about oil gas mix and velocity.
The article above was posted by you Arizona....so do you believe it or not?
Re: Time to talk about oil gas mix and velocity.
This conversation is getting silly in that written opinions are just that---opinions. We are complicating one of the most basic engine designs possible. We don't need dynos and radioactive isotopes to figure out that with premix fuel (or injection) the oil presence increases as the throttle is opened. Jets open, more fuel, more oil. As the RPMS increase however, total oil presence drops due to burning of the lubricants in the combustion chamber along with the fuel. Some of the lubricants don't burn and coat the interior of the exhaust pipe. I find it hard to believe that oil is disappearing at higher RPMs other than combustion. The isotopes are not combusting. Isotopes enter the engine and leave the engine, where could they go. I'm guessing the exhaust pipe is becoming radioactive with an oil coating. RPM has a function of time (per minute). Try to figure that in and it doesn't work either, it's isotopes in and isotopes out. Either I'm not thinking right or the test is flawed---all possible.
I don't think I'm buying this one.
Maxie
I don't think I'm buying this one.
Maxie
Re: Time to talk about oil gas mix and velocity.
Not necessarily. You need to consider radioactive decay.hodakamax wrote:it's isotopes in and isotopes out.
Brian
Re: Time to talk about oil gas mix and velocity.
Hmm, even though the isotope decayed while passing through making it less radioactive, the products of the decay could register on the geiger counter fooling even the most clever scientist into thinking that radiation had increased rather than decreased. I think. 
---- 
What fun!
Max
PS-- seriously, maybe the test is flawed because the counter is unable to keep up with the particles as the exhaust velocity increases OR the faster the exhaust passes the counters gate in less time at higher RPMs showing less quantum events--maybe that's it! Let me explain, If you were measuring say how many horses are passing through two gates, the count does't change if the horses are running or walking but radioactive decay is an event that happens over period of time. Speed up the velocity and less events happen between the gates suggesting that there are less particles passing by which is not true. This could be the flaw in this test. Hmm, but there are more particles coming through.
This is giving me a headache.
Max again.
---- What fun!
Max
PS-- seriously, maybe the test is flawed because the counter is unable to keep up with the particles as the exhaust velocity increases OR the faster the exhaust passes the counters gate in less time at higher RPMs showing less quantum events--maybe that's it!
Let me explain, If you were measuring say how many horses are passing through two gates, the count does't change if the horses are running or walking but radioactive decay is an event that happens over period of time. Speed up the velocity and less events happen between the gates suggesting that there are less particles passing by which is not true. This could be the flaw in this test. Hmm, but there are more particles coming through.This is giving me a headache.
Max again.
Last edited by hodakamax on Mon Nov 23, 2015 5:57 am, edited 1 time in total.
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