What is the optimum jetting in an air cooled two stroke? First thoughts would be getting the maximum torque and/or horsepower for our needs. If we start with this idea a problem occurs in that the engine seizes. Some of us have experienced this burst of power just prior to disaster. When we tune for maximum horsepower our engine temperatures exceed the capability of our lubricants. It's obvious what caused the seizure when we inspect the engine. Aluminum in the piston has reached the temperature of about 1221 degrees F. and is melting. The melting piston sticks to the cylinder bore bringing every thing to a halt. The oil lost its lubrication properties long before that which caused the heat of friction causing the disaster. Jetting for maximum horsepower is not going work because combustion temperatures are causing the oil to fail.
Optimum jetting which we should define as the perfect ratio of fuel and air to produce the most power, combustion temperatures are too high and must be reduced somehow. Since we don't have liquid cooling as an option which would carry away combustion temperatures much more efficiently we go with increasing the cooling area of the cylinder and head with fins. Atomization of fuel as it enters the engine dramatically increases the surface area of fuel and allows it to evaporate causing a major cooling effect. This cooling by evaporation is the primary cooling factor in an air cooled engine.
We now probably come to the realization that we are never going to reach optimum jetting with this design. Since the failure temperature of the lubricating oil is our limiting factor we can approach the problem with designing or choosing lubricants with higher temperature requirements. This will allow us to lean the engine closer to optimum jetting.
Since evaporation is doing most of the cooling we are always caught in the dilemma that as we lean the engine toward the maximum power output the operating temperature exceeds the capability of our lubricants. One of our best indicators of heat is the spark plug. Generally speaking the color and/or shade darkens with rich conditions (cooler) and lightens with lean conditions (hotter). Later technology provided the engine with sensors and metered fuel injection to provide a monitored mix for the conditions but we have to do this manually. As ambient temperatures, pressures and demands change so must the jetting. We are not jetting for performance, we are jetting for engine cooling to prevent the lubricants from failing.
When all mechanical requirements are met (proper timing, oil, clearances, compression, timing etc.) the cause of an engine seizure is almost certainly caused by the failure of the lubricating oil due to heat caused from improper jetting. Jetting is not going to cure mechanical issues, only temperatures. Until we can remove that extra heat on our approach to optimum jetting which is beyond the capabilities of this design, we are stuck with jetting requirements that only involve preventing seizure.
When an engine seizes, its easy to blame it on oils, clearances, pipes and such but the real blame after excluding these mechanical causes probably should go to the tuner who allowed the engine to get too hot for the conditions. It is the cause of most engine seizures. The dilemma can be improved on with liquid cooling, porting designs, better oils, more volatile fuels, fuel injection and its location and spray patterns, exhaust designs and more, most but not all, beyond the design limit of our subject.
In summary, we will probably never reach the optimum mixture in the air cooled two stroke engine due to operating temperatures above the limits of the available lubricants. We can however with tuning, improved oils, and improvements in cooling, approach the performance limit of our engine.
Max
The Air Cooled Two stroke Jetting Dilemma.
The Air Cooled Two stroke Jetting Dilemma.
Last edited by hodakamax on Wed May 04, 2016 12:47 am, edited 1 time in total.
Re: The Air Cooled Two stroke Jetting Dilemma.
Max,
I think it is necessary to first identify some of your basic terms before delving into discussion. "Optimum" for example is a concept, not a fact. Optimum for a 40 year old design could be construed as simply the best all around setting you can achieve. Whether that approaches a theoretical ideal isn't necessarily the point, depending on the thrust of your premise.
You first pose a question: what is the optimum jetting in an air cooled two stroke? The question can be answered variously depending on the definition you choose for optimum. I would say, for example, that optimum is the jetting that provides the best balance of performance and longevity, but that would be based on my own definition of optimum.
So are you asking in a practical sense, or a theoretical sense?
I understand the idea of cooling being in some part aided by fuel. True enough. But it isn't the only cooling function going on inside the engine, or even, necessarily, on the piston crown. Thermal transfer occurs through the fuel, through dumping excess heat out the exhaust, and by transferring heat to the cylinder and to the head through contact of gas to metal, and by contact of metal to metal (in proximity if not actual contact allowing for an intact oil film.) Which of these is the principal source of heat loss, I don't know, but I am guessing thermal shedding through piston ring to cylinder and proximal exposure of head and cylinder are the largest component of heat transfer, or cooling, considering only those pathways that are internal to the engine, as opposed to exhaust.
Even the term "cool" or cooling is subject to definition for purposes of the discussion. Cool is a relative thing. Ambient temperature could be 75* but by the time the fuel charge gets to the combustion chamber, what is its temperature? It has been passed over and through a hot intake manifold, a hot crankcase, and hot transfer ports. It has also been compressed at somewhere around seven or eight to one compared to atmospheric pressure, which itself creates heat. Admittedly the fuel charge is well below spontaneous ignition, but it isn't exactly cold either. So does it cool the piston for example? Yes. But does a significant amount of the heat present in the equation transfer to the fuel charge? I don't think so. It is all part of the equation, true, but how much so? I don't have a means of testing it. That richer mixtures run cooler than lean ones is only part of the question of cooling. Does the richer mixture offer more or better heat transfer than lean mixtures, or does it simply generate less heat during each cycle? Or both?
Since I am not sure where your premise is going, adding definition to core terms will help us see your point.
I think it is necessary to first identify some of your basic terms before delving into discussion. "Optimum" for example is a concept, not a fact. Optimum for a 40 year old design could be construed as simply the best all around setting you can achieve. Whether that approaches a theoretical ideal isn't necessarily the point, depending on the thrust of your premise.
You first pose a question: what is the optimum jetting in an air cooled two stroke? The question can be answered variously depending on the definition you choose for optimum. I would say, for example, that optimum is the jetting that provides the best balance of performance and longevity, but that would be based on my own definition of optimum.
So are you asking in a practical sense, or a theoretical sense?
I understand the idea of cooling being in some part aided by fuel. True enough. But it isn't the only cooling function going on inside the engine, or even, necessarily, on the piston crown. Thermal transfer occurs through the fuel, through dumping excess heat out the exhaust, and by transferring heat to the cylinder and to the head through contact of gas to metal, and by contact of metal to metal (in proximity if not actual contact allowing for an intact oil film.) Which of these is the principal source of heat loss, I don't know, but I am guessing thermal shedding through piston ring to cylinder and proximal exposure of head and cylinder are the largest component of heat transfer, or cooling, considering only those pathways that are internal to the engine, as opposed to exhaust.
Even the term "cool" or cooling is subject to definition for purposes of the discussion. Cool is a relative thing. Ambient temperature could be 75* but by the time the fuel charge gets to the combustion chamber, what is its temperature? It has been passed over and through a hot intake manifold, a hot crankcase, and hot transfer ports. It has also been compressed at somewhere around seven or eight to one compared to atmospheric pressure, which itself creates heat. Admittedly the fuel charge is well below spontaneous ignition, but it isn't exactly cold either. So does it cool the piston for example? Yes. But does a significant amount of the heat present in the equation transfer to the fuel charge? I don't think so. It is all part of the equation, true, but how much so? I don't have a means of testing it. That richer mixtures run cooler than lean ones is only part of the question of cooling. Does the richer mixture offer more or better heat transfer than lean mixtures, or does it simply generate less heat during each cycle? Or both?
Since I am not sure where your premise is going, adding definition to core terms will help us see your point.
GMc
Re: The Air Cooled Two stroke Jetting Dilemma.
Ah, all good questions Greg. It's almost time for a great meal and wine from my wonderful chef and wife so I'll start with optimum jetting and probably not get to reach other good points until later. As you can tell this is a generalization to stir discussion. In my mind optimum jetting would be the correct air to fuel ratio that produces the most power from the two ingredients regardless of the engine or even outside the engine. Whatever that ratio is, combustion temps are above what our engine can handle. Compression might figure into all of this but that's probably another discussion. The optimum or ideal mix would produce the most possible energy from fuel and air. The heat does leave through the exhaust but even though passing through rings, crowns and oil is either balanced by the cooling effect in the incoming charge or transferred to the cylinder fins and exhaust. It appears to me that incoming charge is very cold due to atomization and evaporation. Remember carburetor icing? Hmm, the mixture not only gets compressed but is subjected to low pressures near vacuum during intake also. I would think that all would be cool in the transfers but you are right in that its just been compressed as the piston comes down. All complicated for sure. I'm guessing the intake charge actually falls way below the freezing point of water (as observed frosty reed manifolds) and would substantially add to the cooling process. This to me only opens more questions than answers without a working experiment with sensors. Only guessing but I think probably 1/2 to 2/3 of the cooling comes through the intake and the other half or third leaves through the exhaust and fins. (not very scientific but my guess.) Again, all good questions and interesting for sure. My most important evidence is when it gets too lean, it gets too hot--seizure hot. Fun discussion, I'm open to all input! Let me sleep on it. 
Later,
Max
Later,
Max
Re: The Air Cooled Two stroke Jetting Dilemma.
As a small plane pilot, I am familiar with carb icing. What I would point out is where it happens and why. It happens at the Venturi of the carb, and happens because of the rapid drop in pressure that occurs there. After air passes the Venturi into the carb, it rapidly warms up again. Also, icing per se only occurs at certain atmospheric conditions including relative humidity. The drop in temperature, however, is fairly constant for a given velocity across the Venturi.
Icing, or rapid drop in temperature attendant to drop in pressure, doesn't occur anywhere else in the intake stream. So, while I would agree that intake charge gets a cooling bump going in, it doesn't last.
Personally, I think detonation is more often the culprit in seizures due to oil failure because of the huge difference in flame temperature of detonating fuel mix versus a controlled flame kernel. That sudden spike in flame temp, and the pounding things get, and the spike in pressure, overcome oil in places most exposed to the heat.
Icing, or rapid drop in temperature attendant to drop in pressure, doesn't occur anywhere else in the intake stream. So, while I would agree that intake charge gets a cooling bump going in, it doesn't last.
Personally, I think detonation is more often the culprit in seizures due to oil failure because of the huge difference in flame temperature of detonating fuel mix versus a controlled flame kernel. That sudden spike in flame temp, and the pounding things get, and the spike in pressure, overcome oil in places most exposed to the heat.
GMc
Re: The Air Cooled Two stroke Jetting Dilemma.
Regarding icing, setting aside the need for water in the air to produce ice at all, the temperature drop to a point below freezing doesn't happen across the entire carburetor bore. It happens in a narrow band of laminar flow right at the Venturi surface where pressure differential is at its highest. A Venturi has the general effect of speeding air flow across the narrow part of the carb throat. It is very much the same as a wing, but bent into a circle. What would produce lift due to pressure drop in a wing produces the same rapid increase in surface flow in a Venturi. But the speed and pressure drop are not constant across the entire throttle bore. Speeds at the dead center of the carb throat don't increase as much as or as quickly as the outer part of the bore does. I am inclined to think that, depending on the bore size of the carb and the displacement of the engine, intake charge acceleration at the dead center of the carb is not a lot more than it would be if the carb bore was a pipe instead of a Venturi.
So while pressure drop can and does cause cooling, and even icing under the right circumstances, the effect isn't as substantial as you are suggesting. Certainly it is not enough to defeat the heat of a high performance engine. The best way to determine the degree of cooling due to the Venturi would perhaps be with one or more thermal probes in the intake manifold to verify temperatures in the intake tract.
As for atomization and pressure drop once inside the engine, I think the changes don't include cooling due to rapid depressurization. Rather, the engine is an air pump. It transfers charge (once inside the crankcase) from bottom to top with positive pressure, not negative pressure, and even if there is a negative shift, it is small, covers very little time, and doesn't result in cooling the charge. The amount of heat from the surrounding environment is very substantial; the amount of intake charge is relatively small. The thermal mass of 100cc's of air/fuel is nothing like the thermal mass of 100cc's of steel. The thermal equation is very much shifted in favor of the large amount of hot dense metal in the crankcase, and a small amount of somewhat cooler intake charge isn't going to significantly offset the heat balance of such a large thermal mass as the crankshaft. Nor is air in its gaseous state very efficient at transferring heat. That's why air cooling isn't up to par with water cooling.
It is a fact of life that two stroke motors can and do run for years without failure when built well and operated sanely. Yet they do get hot to the touch even when idling. So I think it is possible to achieve some degree of optimal state of tune, no matter what you are building. On the other end of the spectrum is an engine built for maximum, as opposed to optimal, output. Even in liquid cooled engines, pushing the limits causes early failure. I see no reason that an air cooled two stroke could not produce the same power as a liquid cooled two stroke keeping other build factors equal, other than air versus water cooling. But to achieve the same thermal efficiency in an air cooled engine's ability to absorb heat as that of a water cooled engine would require an engine you probably could not get your legs around. It would be impractical. It takes a lot of aluminum to represent the same thermal mass and rate of transfer as water, which is very efficient at absorbing and shedding heat. Without equivalent heat processing ability, however, an air cooled engine heat soaks quicker, stays hot longer, and fails sooner when pushed to the limit.
There are examples of manufacturers trying to increase an air cooled engines ability to take and shed heat. The engines on the Sachs/Pentons and Maicos are exercises in increasing heat endurance by increasing surface area and metal in the cooling surfaces of an engine. Done right it does work, but I expect not as well as a good water jacket and radiator would.
Perhaps one day someone with more time and money than we have will take a Hodaka and convert it to water cooling, case induction, and so on, and we will see what an old design can do with enough money.
So while pressure drop can and does cause cooling, and even icing under the right circumstances, the effect isn't as substantial as you are suggesting. Certainly it is not enough to defeat the heat of a high performance engine. The best way to determine the degree of cooling due to the Venturi would perhaps be with one or more thermal probes in the intake manifold to verify temperatures in the intake tract.
As for atomization and pressure drop once inside the engine, I think the changes don't include cooling due to rapid depressurization. Rather, the engine is an air pump. It transfers charge (once inside the crankcase) from bottom to top with positive pressure, not negative pressure, and even if there is a negative shift, it is small, covers very little time, and doesn't result in cooling the charge. The amount of heat from the surrounding environment is very substantial; the amount of intake charge is relatively small. The thermal mass of 100cc's of air/fuel is nothing like the thermal mass of 100cc's of steel. The thermal equation is very much shifted in favor of the large amount of hot dense metal in the crankcase, and a small amount of somewhat cooler intake charge isn't going to significantly offset the heat balance of such a large thermal mass as the crankshaft. Nor is air in its gaseous state very efficient at transferring heat. That's why air cooling isn't up to par with water cooling.
It is a fact of life that two stroke motors can and do run for years without failure when built well and operated sanely. Yet they do get hot to the touch even when idling. So I think it is possible to achieve some degree of optimal state of tune, no matter what you are building. On the other end of the spectrum is an engine built for maximum, as opposed to optimal, output. Even in liquid cooled engines, pushing the limits causes early failure. I see no reason that an air cooled two stroke could not produce the same power as a liquid cooled two stroke keeping other build factors equal, other than air versus water cooling. But to achieve the same thermal efficiency in an air cooled engine's ability to absorb heat as that of a water cooled engine would require an engine you probably could not get your legs around. It would be impractical. It takes a lot of aluminum to represent the same thermal mass and rate of transfer as water, which is very efficient at absorbing and shedding heat. Without equivalent heat processing ability, however, an air cooled engine heat soaks quicker, stays hot longer, and fails sooner when pushed to the limit.
There are examples of manufacturers trying to increase an air cooled engines ability to take and shed heat. The engines on the Sachs/Pentons and Maicos are exercises in increasing heat endurance by increasing surface area and metal in the cooling surfaces of an engine. Done right it does work, but I expect not as well as a good water jacket and radiator would.
Perhaps one day someone with more time and money than we have will take a Hodaka and convert it to water cooling, case induction, and so on, and we will see what an old design can do with enough money.
GMc
Re: The Air Cooled Two stroke Jetting Dilemma.
Doug posted a mention an of oil made by Powermist. Not having heard of it, I went looking. While I don't necessarily buy into the brand hype part of the website, there is at least a generally usable tech discussion on various engine efficiency parameters. Most of the discussion there sounds familiar, and although they don't cite sources (who does) I don't have a problem with their assertions or conclusions, except for the part about chemical oxygenating agents. I don't and won't buy into that part without data, simply because in pump gas, ethanol, which no one here likes, is used as an oxygenating agent, and it's use, instead of increasing combustion efficiency, decreases it significantly over straight gasoline. So whether their products for sale benefit or don't benefit performance, still their general discussion on efficiency is based on conventional wisdom. At least it doesn't hurt to know some of the terms.
http://www.nowearracing.com/tech1.HTML
http://www.nowearracing.com/tech1.HTML
GMc
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Bullfrog
- Posts: 2784
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- Location: Oregon, 12 miles from the center of the Hodaka Universe(Athena)
Re: The Air Cooled Two stroke Jetting Dilemma.
Short post here.
1. While I agree that the incoming air/fuel mix provides cooling for the engine. I'm on a way different plane than Max regarding just how "cold" that air/fuel mix might be. Even if the air/fuel mix is 100 degrees (or 212 degrees) F when it is traveling past the crank cheeks or the bottom side of the piston crown . . . it is providing cooling because it is not as hot as all the metal surrounding it.
2. Why do all Hodies up thru the Combat Wombat have "heat blocks" (non-metal heat insulators) between the intake manifold and the carb?
3. Has anyone ever touched the intake manifold on their Hodaka immediately after running down the trail or around the race track and jerked their hand back because the intake manifold was so cold it was a shock to the senses?
4. Do any of our guesses regarding the temperature of the intake flow at any point in the intake process, change the fact that while I'm out tuning the jetting I'm still going to "find rich" and, generally speaking, "lean down" until "rich" just barely goes away? (At every throttle position)
Ed
1. While I agree that the incoming air/fuel mix provides cooling for the engine. I'm on a way different plane than Max regarding just how "cold" that air/fuel mix might be. Even if the air/fuel mix is 100 degrees (or 212 degrees) F when it is traveling past the crank cheeks or the bottom side of the piston crown . . . it is providing cooling because it is not as hot as all the metal surrounding it.
2. Why do all Hodies up thru the Combat Wombat have "heat blocks" (non-metal heat insulators) between the intake manifold and the carb?
3. Has anyone ever touched the intake manifold on their Hodaka immediately after running down the trail or around the race track and jerked their hand back because the intake manifold was so cold it was a shock to the senses?
4. Do any of our guesses regarding the temperature of the intake flow at any point in the intake process, change the fact that while I'm out tuning the jetting I'm still going to "find rich" and, generally speaking, "lean down" until "rich" just barely goes away? (At every throttle position)
Ed
Keep the rubber side down!
Re: The Air Cooled Two stroke Jetting Dilemma.
OK Greg, I did go back and define optimum jetting in the original post. It was a little vague. I think we are all over analyzing this process of the simplest of engines and its cooling. All big fun of course and it does make us think. I'll try to shorten the story. We have a simple air cooled two stroke engine that at one throttle setting (full throttle to further simplify it.) that continues to put out more horsepower as the jetting gets leaner but overheats and seizes. It appears that the engine cannot shed the heat caused by lean conditions. Proof of this concept is to add liquid cooling where the engine can run at leaner setting producing more horsepower. Let us work from here.
When racing, we should start rich and work down close to the limit of overheating. It continues too run better the leaner we go but we know the consequences. Our options are few. It's the oil that fails first when overheating and we can run oils with higher heat limits allowing us to go leaner. The other option is to carry the heat away faster with things like liquid cooling.
Since one of the cooling factors is ambient temperature air entering the engine and atomizing fuel which cools the engine by evaporation, we cannot keep making the engine leaner to get more power at least in this design. Also we cannot ignore changing conditions such as ambient temperature which effects jetting and could put us on the wrong side of the line.
The most important observation we have is that the engine gets hotter when we reduce the amount of fuel entering the engine suggesting that more fuel equals more cooling.
How's that for now?
Max
When racing, we should start rich and work down close to the limit of overheating. It continues too run better the leaner we go but we know the consequences. Our options are few. It's the oil that fails first when overheating and we can run oils with higher heat limits allowing us to go leaner. The other option is to carry the heat away faster with things like liquid cooling.
Since one of the cooling factors is ambient temperature air entering the engine and atomizing fuel which cools the engine by evaporation, we cannot keep making the engine leaner to get more power at least in this design. Also we cannot ignore changing conditions such as ambient temperature which effects jetting and could put us on the wrong side of the line.
The most important observation we have is that the engine gets hotter when we reduce the amount of fuel entering the engine suggesting that more fuel equals more cooling.
How's that for now?
Max
Last edited by hodakamax on Wed May 04, 2016 4:04 am, edited 1 time in total.
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taber hodaka
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Re: The Air Cooled Two stroke Jetting Dilemma.
I like the discussions. You tune or jet according to what you have. A certain pipe exhaust, oil type , fuel ect. If you never run flat out you probably don't need to be tuned at the top, but you need to be tuned just right in the range that you do ride. In reference to power mist I had to go look in the shop and there is the old can of powermist. A good looking old red can of 2 cycle oil fortified castor oil, left over from the days of old the late 60's or about. I was not impressed with it back then and it did not compare to the oil I was using. Back then I would have to rejet or downsize the main jet with my better oil because it ran so much cooler and ended up with much better fuel economy. ----------Clarence
Re: The Air Cooled Two stroke Jetting Dilemma.
Max,
You make two critical assertions for which there is no proof, as yet.
1) Proof of this concept is to add liquid cooling where the engine can run at leaner setting producing more power.
2) The most important observation we have is that the engine gets hotter when we reduce the amount of fuel entering the engine suggesting that more fuel equals more cooling.
Modern water cooled two strokes have far more going for them than just water cooling. Comparing the recently mentioned KTM 125SX to whichever Hodaka you choose is not a comparison from which conclusions can be drawn regarding water cooling. In any scientific experiment, you must run a control test, and reduce to one variable only the parameter you which to test in the comparison. Here, we do not have a control base (two bikes unmodified) and we do not have a test group with only one variable. I think the differences between modern water cooled bikes and vintage air cooled bikes far outweigh the similarities. So we can't draw any conclusions from a simple comparison of any one water cooled bike to any other air cooled bike. Too many variables, and no control over the exercise. It may be that modern bikes of similar displacement run leaner main jets. That observation alone, even if true, can not be attributed to water cooling. So your first offer of proof is not persuasive.
Second point, leaner bikes run hotter, is a generality, not a proof. Leaner combustion mixtures, all other things equal, may or may not produce more heat. I think perhaps they do, but even if true, it is not then true that the reverse is also true, that rich mixtures always run cool. I believe there is evidence that overly fuel rich mixtures can and often do run hot. But again, we don't have a proper basis for comparison because we don't have a means of testing only one variable at a time. Observing generally that leaning out a mixture to the point of failure may produce heat isn't proof that it is the leaning itself, and only the leaning, that causes a rise in temperature. It may be that over leaning sets up a condition in which detonation occurs, and the detonation may cause the rise in temperature. Without a means of control, we are largely guessing at what goes on inside the combustion chamber, and what causes failure. The recent Wombat seizure is an example of that. So far, there is no real consensus of what caused the failure, much less a proof. So your second observation is also not persuasive without much more evidence.
I take it that you are trying to say that air cooled bikes can't be expected to make the power of a water cooled bike based only on the means of cooling. I don't think that is true. I would agree that water cooling gives an engine an edge in controlling temperatures up to a point, but I would also point out that detonation and failures do occur in water cooled engines. Fuel mix adds cool to the net heat equation, but the heat of combustion far outweighs the cool provided by the same fuel charge that makes the heat. And, whether air cooled or water cooled, engines are still limited in their thermal efficiency to a range between 25% and 30% or so. That is a very small margin. Far too small to attribute the ability to withstand a bit more heat due to water cooling as the reason a modern bike makes more power.
You make two critical assertions for which there is no proof, as yet.
1) Proof of this concept is to add liquid cooling where the engine can run at leaner setting producing more power.
2) The most important observation we have is that the engine gets hotter when we reduce the amount of fuel entering the engine suggesting that more fuel equals more cooling.
Modern water cooled two strokes have far more going for them than just water cooling. Comparing the recently mentioned KTM 125SX to whichever Hodaka you choose is not a comparison from which conclusions can be drawn regarding water cooling. In any scientific experiment, you must run a control test, and reduce to one variable only the parameter you which to test in the comparison. Here, we do not have a control base (two bikes unmodified) and we do not have a test group with only one variable. I think the differences between modern water cooled bikes and vintage air cooled bikes far outweigh the similarities. So we can't draw any conclusions from a simple comparison of any one water cooled bike to any other air cooled bike. Too many variables, and no control over the exercise. It may be that modern bikes of similar displacement run leaner main jets. That observation alone, even if true, can not be attributed to water cooling. So your first offer of proof is not persuasive.
Second point, leaner bikes run hotter, is a generality, not a proof. Leaner combustion mixtures, all other things equal, may or may not produce more heat. I think perhaps they do, but even if true, it is not then true that the reverse is also true, that rich mixtures always run cool. I believe there is evidence that overly fuel rich mixtures can and often do run hot. But again, we don't have a proper basis for comparison because we don't have a means of testing only one variable at a time. Observing generally that leaning out a mixture to the point of failure may produce heat isn't proof that it is the leaning itself, and only the leaning, that causes a rise in temperature. It may be that over leaning sets up a condition in which detonation occurs, and the detonation may cause the rise in temperature. Without a means of control, we are largely guessing at what goes on inside the combustion chamber, and what causes failure. The recent Wombat seizure is an example of that. So far, there is no real consensus of what caused the failure, much less a proof. So your second observation is also not persuasive without much more evidence.
I take it that you are trying to say that air cooled bikes can't be expected to make the power of a water cooled bike based only on the means of cooling. I don't think that is true. I would agree that water cooling gives an engine an edge in controlling temperatures up to a point, but I would also point out that detonation and failures do occur in water cooled engines. Fuel mix adds cool to the net heat equation, but the heat of combustion far outweighs the cool provided by the same fuel charge that makes the heat. And, whether air cooled or water cooled, engines are still limited in their thermal efficiency to a range between 25% and 30% or so. That is a very small margin. Far too small to attribute the ability to withstand a bit more heat due to water cooling as the reason a modern bike makes more power.
GMc
Re: The Air Cooled Two stroke Jetting Dilemma.
[quote="Arizona Shorty"]Max,
You make two critical assertions for which there is no proof, as yet.
1) Proof of this concept is to add liquid cooling where the engine can run at leaner setting producing more power.
I take it that you are trying to say that air cooled bikes can't be expected to make the power of a water cooled bike based only on the means of cooling. I don't think that is true. I would agree that water cooling gives an engine an edge in controlling temperatures up to a point, but I would also point out that detonation and failures do occur in water cooled engines.
--------------------------------------
Ah, com'n Greg, its a duck, it quacks like one. The water cooled engine can also detonate and fail because it's too lean.
I'll bet that liquid cooled engines with all the surface of a radiator out in the air stream and with a pump and possibly a fan are many times more efficient. Twice as efficient at the minimum, just what we need to pull off that extra heat from being too lean. Of course this is not a fact or proof, just a better (meaning one to bet on).
We really do need a firm scientific test bed for our discussions but the article was written based on my experiences. All in fun though but they do seize when they are too lean.
I'm painting the house--more when I've gotten tired of that.
Maxie
You make two critical assertions for which there is no proof, as yet.
1) Proof of this concept is to add liquid cooling where the engine can run at leaner setting producing more power.
I take it that you are trying to say that air cooled bikes can't be expected to make the power of a water cooled bike based only on the means of cooling. I don't think that is true. I would agree that water cooling gives an engine an edge in controlling temperatures up to a point, but I would also point out that detonation and failures do occur in water cooled engines.
--------------------------------------
Ah, com'n Greg, its a duck, it quacks like one. The water cooled engine can also detonate and fail because it's too lean.
I'll bet that liquid cooled engines with all the surface of a radiator out in the air stream and with a pump and possibly a fan are many times more efficient. Twice as efficient at the minimum, just what we need to pull off that extra heat from being too lean. Of course this is not a fact or proof, just a better (meaning one to bet on).
We really do need a firm scientific test bed for our discussions but the article was written based on my experiences. All in fun though but they do seize when they are too lean.
I'm painting the house--more when I've gotten tired of that.
Maxie
Re: The Air Cooled Two stroke Jetting Dilemma.
I get tired of painting about the time I get the can open.
Of course water cooled bikes can fail if too lean. That's more or less my point, but also that there is more to take into consideration than just water cooling and lean condition in figuring out how newer bikes get away making more power. Water cooling is one such feature, but there are lots of others to consider, making the comparison too complex to say the new bikes can make more power because of water cooling alone.
One reason the Hodaka, as the example, made no more power than it did was conservative engineering and marketing on the part of the Japanese. Pabatco was asking for more and better all the time, and rarely got it. I don't think Hodaka or Pabatco had the money to put into a skunk works division whose sole purpose was to squeeze all there was out of a Hodaka. Harry tried valiantly, and succeeded I think to a degree Japan didn't expect. Hodaka was aimed at a trail bike market, low buck entry level stuff. Making one off pro class racers wasn't their intention and I don't think they were even interested. A lot of owners immediately started pushing the envelope and doubling the output of an Ace 100 wasn't unreasonable. It was part of the charm of the Hodaka that guys like us, with almost no money to spend, could still easily improve our rides with hardly more than an investment in time and sweat. To say that a new KTM makes three times the power as the 70's functional equivalent of a two wheeled lawn mower is hardly saying much. The engineering time and costs that went into the final 2016 KTM 125 product would have bought and sold Hodaka completely.
If you want to see what a Hodaka might have been capable of, look back through the threads to the guys in Australia who converted a Hodaka 125 to case induction. That bike is a screamer. I don't know if it could keep up with a new KTM, but I wouldn't bet money that it could not.
Of course water cooled bikes can fail if too lean. That's more or less my point, but also that there is more to take into consideration than just water cooling and lean condition in figuring out how newer bikes get away making more power. Water cooling is one such feature, but there are lots of others to consider, making the comparison too complex to say the new bikes can make more power because of water cooling alone.
One reason the Hodaka, as the example, made no more power than it did was conservative engineering and marketing on the part of the Japanese. Pabatco was asking for more and better all the time, and rarely got it. I don't think Hodaka or Pabatco had the money to put into a skunk works division whose sole purpose was to squeeze all there was out of a Hodaka. Harry tried valiantly, and succeeded I think to a degree Japan didn't expect. Hodaka was aimed at a trail bike market, low buck entry level stuff. Making one off pro class racers wasn't their intention and I don't think they were even interested. A lot of owners immediately started pushing the envelope and doubling the output of an Ace 100 wasn't unreasonable. It was part of the charm of the Hodaka that guys like us, with almost no money to spend, could still easily improve our rides with hardly more than an investment in time and sweat. To say that a new KTM makes three times the power as the 70's functional equivalent of a two wheeled lawn mower is hardly saying much. The engineering time and costs that went into the final 2016 KTM 125 product would have bought and sold Hodaka completely.
If you want to see what a Hodaka might have been capable of, look back through the threads to the guys in Australia who converted a Hodaka 125 to case induction. That bike is a screamer. I don't know if it could keep up with a new KTM, but I wouldn't bet money that it could not.
GMc
Re: The Air Cooled Two stroke Jetting Dilemma.
And speaking of the KTM, I note that the SX has a bore/stroke of 54 x 54.5, as opposed to Hodakas 56 x 50. This might not seem like much, but in a two stroke where blow down time is critical to making power, the longer stroke and smaller bore of the KTM make more power possible, whether water or air cooled. I also have to note that the KTM makes its power between 9,000 and 12,000 rpm. A Super Combat pushed hard will turn maybe 10,000, which is not quite but pretty much what you can get with the bore/stroke configuration. Water cooling such an engine would make it live longer, but wouldn't help it make a lot more power.
GMc
Re: The Air Cooled Two stroke Jetting Dilemma.
Greg , my super combat turns 12,000 Rpm and Harry Taylor questimated that it would be in the area of 35 hp and Is waay faster than I can ride it , it has 195 psi compression runs on 110 octane fuel and never detonated or gave a hint of over heating , I agree that liquid cool don't make HP but will allow a great rider to ride great and harder longer .
Rich
Rich
Re: The Air Cooled Two stroke Jetting Dilemma.
I could use some of that 35hp. I find I adapt rapidly to excess hp. I enjoy my Audi S4, which is faster than anyone needs, but easy to get used to. No matter what I did to my 5.0 Ford powered off road racer, I always wanted more power. Same with the dirt bike. 12,000 is pushing beyond the speed of sound at the peak of the piston speed with a 50mm stroke, and it does it twice per cycle.
GMc
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taber hodaka
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Re: The Air Cooled Two stroke Jetting Dilemma.
A cooler running engine does make more horsepower. A lean running engine burns up and the over rich engine runs crappy. Does the lean running engine produce more horsepower, no it runs hotter and seizes. What was the question?? ----------Clarence
Re: The Air Cooled Two stroke Jetting Dilemma.
What could be more fun than than whole gang discussing mysterious things like jetting and heat transfer. All big fun for sure. I actually do set around thinking up stuff to set someone off and get us all thinking about things all motorcycle. It's good for the brain when someone asks why is this happening? Someone always has some information worth filing in the brain.
Back to the liquid cooling issue. Liquid cooling does not increase horsepower but opens other avenues of performance. Us air cooled guys are always trying to get one main jet leaner to try to approach ideal jetting which is always beyond reach because we can't shed the heat that causes oil failure and finally seizure. If we have a cooling system in place than can carry off some of this excess heat such as liquid and radiators we might be able to approach the optimum jetting which is the ideal mixture of fuel and air that makes the most power. I really don't know if this is achievable but it certainly isn't with an air cooled engine that I know about. Liquid cooling could get you a few (or even more) jets closer to the magic number where it is not to rich or lean to produce maximum power. Food for thought.
All for the night, Cheers.
Max
Back to the liquid cooling issue. Liquid cooling does not increase horsepower but opens other avenues of performance. Us air cooled guys are always trying to get one main jet leaner to try to approach ideal jetting which is always beyond reach because we can't shed the heat that causes oil failure and finally seizure. If we have a cooling system in place than can carry off some of this excess heat such as liquid and radiators we might be able to approach the optimum jetting which is the ideal mixture of fuel and air that makes the most power. I really don't know if this is achievable but it certainly isn't with an air cooled engine that I know about. Liquid cooling could get you a few (or even more) jets closer to the magic number where it is not to rich or lean to produce maximum power. Food for thought.
All for the night, Cheers.
Max
Re: The Air Cooled Two stroke Jetting Dilemma.
It's well documented that the optimum air to fuel ratio for power is between 12.5 to 1 and 13 to 1. Any leaner than this and maximum power suffers but emissions are improved. Modern automotive engines are designed to run at 14.7 to 1 which is the point at which emissions are lowest, but power is less that the richer ratio. Leaner does not equal more power above the 13 to 1 ratio. If that were true a logical extension would be that if the ratio was 100 to 1, or all air and no fuel, the power would be greater. We all know that is not true. Generally the way to increase power is to pump more air through the engine and provide enough fuel to maintain the 12.5 - 13 to 1 air to fuel mix.
The easiest way to accomplish this is to increase the size of the pump. There is no substitute for cubic inches. However when larger displacement is not an option or easily accommodated there are a multitude of ways to pump more air through the engine. Larger carburetors, velocity stacks (remember those), less restrictive exhaust systems, and larger ports, along with greater port or cam duration help to accomplish this but ultimately the displacement of the engine as a pump will be the limiting factor. This is why superchargers and turbos are used in the automotive world to increase the ability to force more air through the system.
Moving more air is the primary driver and then supplying enough fuel to maintain the optimum air to fuel ratio is the way to more horsepower. Of course the way that air/fuel charge is managed by the configuration of the engine is way more complicated. Add heat management and lubrication to the equation and one can see why these discourses go on for eternity.
See! Anyone can write an article!
The easiest way to accomplish this is to increase the size of the pump. There is no substitute for cubic inches. However when larger displacement is not an option or easily accommodated there are a multitude of ways to pump more air through the engine. Larger carburetors, velocity stacks (remember those), less restrictive exhaust systems, and larger ports, along with greater port or cam duration help to accomplish this but ultimately the displacement of the engine as a pump will be the limiting factor. This is why superchargers and turbos are used in the automotive world to increase the ability to force more air through the system.
Moving more air is the primary driver and then supplying enough fuel to maintain the optimum air to fuel ratio is the way to more horsepower. Of course the way that air/fuel charge is managed by the configuration of the engine is way more complicated. Add heat management and lubrication to the equation and one can see why these discourses go on for eternity.
See! Anyone can write an article!
Ivan AKA "Pop"
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taber hodaka
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Re: The Air Cooled Two stroke Jetting Dilemma.
Max I agree with you a cooler running engine does produce more horsepower. I don't agree that a tuner tunes for maximum torque and horsepower I believe he tunes for the fine line between rich and lean and I do not believe that it is a three or five jet leeway. Using a poor grade of two cycle oil can be compensated for by a re tune using larger jets, not mixing more oil which would be a guess. The cars we ran in the early days had adjustable main jets, we dialed them for optimum performance and the timing was adjustable from inside the car or truck. Also remember the hot plug, cold plug theory use the coldest plug you can use, without fouling. Max did you see my first sentence where I agree with you? just Clarence Not science to all of you, just my experience. Hope to show you in a video some day. ( how to post a video.) for dummies
Re: The Air Cooled Two stroke Jetting Dilemma.
Hey Ivan, thanks for the numbers, that was going to be my first question of the day. Our air cooled two stroke engines probably can't reach 12.5 to 1 air/fuel ratio due to build up of heat. Liquid cooling has a big advantage in that it can allow us to reach those numbers. We can only improve the efficiency of our pump not its displacement.
It is complicated to me and I'm not an expert, but speaking from lots of hours of observation while racing air cooled two strokes one can not help noticing that the engine runs better the leaner you go but overheats. Once we figured that out seizures were a thing of the past in our racing program. I also road raced a couple of Yamaha RZ-350 liquid cooled two strokes that appeared to reach what I called optimum jetting. I was always spooked at high speeds and even and run the RZs a bit rich (and always kept a finger on the clutch lever on long straights.)
Max
PS--A little digging and I did find a picture of my first liquid cooled two stroke. What a blast that thing was!
It is complicated to me and I'm not an expert, but speaking from lots of hours of observation while racing air cooled two strokes one can not help noticing that the engine runs better the leaner you go but overheats. Once we figured that out seizures were a thing of the past in our racing program. I also road raced a couple of Yamaha RZ-350 liquid cooled two strokes that appeared to reach what I called optimum jetting. I was always spooked at high speeds and even and run the RZs a bit rich (and always kept a finger on the clutch lever on long straights.)
Max
PS--A little digging and I did find a picture of my first liquid cooled two stroke. What a blast that thing was!
- Attachments
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- The liquid cooled RZ-350, a "cool" machine.
Last edited by hodakamax on Thu May 05, 2016 4:13 am, edited 1 time in total.
Re: The Air Cooled Two stroke Jetting Dilemma.
Hey Clarence, Thanks! I think you are the first and only one (due to your experience) to agree with me that we tune for the fine line between rich and lean and that crossing the line results in seizure. That comment about 3-5 jets was not about leeway but how much leaner could we go with some way to remove the heat (such as liquid cooling). 3-5 steps (jets) is probably a little too big a guess, probably 1-3 would be more like it.
We also should mention the obvious in that liquid cooled engines also can be too lean and fail due to heat which would be the center of the piston (the furtherest from the liquid cooling), or at least my guess.
Max
PS--and we should use the most heat tolerant oil which allows us to go closer to the line.
We also should mention the obvious in that liquid cooled engines also can be too lean and fail due to heat which would be the center of the piston (the furtherest from the liquid cooling), or at least my guess.
Max
PS--and we should use the most heat tolerant oil which allows us to go closer to the line.
Re: The Air Cooled Two stroke Jetting Dilemma.
Max,
I think we are all agreeing, but are trying to discuss a topic that has so many variables that it's difficult to see the agreement. 12.5:1 air/fuel ratio was always held to be the standard for maximum power, but until electronic fuel injectors with sensors and exhaust sensors it was very difficult to achieve or measure. With new head design technology in the automotive world the new standard for power might be as high as 13.3:1 but we are talking about 40-50 year old engines here so who knows. The point is that you could probably run a detuned mildly ported engine at the maximum ratio without a heat issue but as soon as you modify the engine configuration for more power, you definitely have to deal with heat issues. I also agree with Ed that for longevity and reliability you jet the carb for one step below blubbering.
I agree with you, I agree with Clarence, I agree with Greg, I agree with Kelsey, I agree with Ed, I agree with Rich, I agree with Kelly, I agree with Danny, ... and everyone else (I hope I haven't offended anyone by leaving them off the list, my memory is not as good as it once was). However each of us may be focusing on different individual parts of a much bigger equation. If we give consideration to all the comments from all the sources and add them all together it may help us to get a step closer to that perfect engine configuration. However if you focus on just one component without considering it's impact on the other components you are setting yourself up for failure. (And I use the term "you" in its generic sense, not aiming it at one individual.) Most of the variables will have to be compromised in order to accommodate all the other variables. This is where we have to throw some "art" in with the "science". Each component ultimately has to be configured in a way that accommodates all the other components including the big one, jetting, which you have eloquently pointed out. I have learned a lot from these discussions and hope to learn more.
I think we are all agreeing, but are trying to discuss a topic that has so many variables that it's difficult to see the agreement. 12.5:1 air/fuel ratio was always held to be the standard for maximum power, but until electronic fuel injectors with sensors and exhaust sensors it was very difficult to achieve or measure. With new head design technology in the automotive world the new standard for power might be as high as 13.3:1 but we are talking about 40-50 year old engines here so who knows. The point is that you could probably run a detuned mildly ported engine at the maximum ratio without a heat issue but as soon as you modify the engine configuration for more power, you definitely have to deal with heat issues. I also agree with Ed that for longevity and reliability you jet the carb for one step below blubbering.
I agree with you, I agree with Clarence, I agree with Greg, I agree with Kelsey, I agree with Ed, I agree with Rich, I agree with Kelly, I agree with Danny, ... and everyone else (I hope I haven't offended anyone by leaving them off the list, my memory is not as good as it once was). However each of us may be focusing on different individual parts of a much bigger equation. If we give consideration to all the comments from all the sources and add them all together it may help us to get a step closer to that perfect engine configuration. However if you focus on just one component without considering it's impact on the other components you are setting yourself up for failure. (And I use the term "you" in its generic sense, not aiming it at one individual.) Most of the variables will have to be compromised in order to accommodate all the other variables. This is where we have to throw some "art" in with the "science". Each component ultimately has to be configured in a way that accommodates all the other components including the big one, jetting, which you have eloquently pointed out. I have learned a lot from these discussions and hope to learn more.
Ivan AKA "Pop"
Re: The Air Cooled Two stroke Jetting Dilemma.
Hey Ivan, good input. It certainly IS complicated! I really like these discussions in that even though I'm one to start something controversial I usually learn something. To me this is one of the fun parts of the forum. I'm always amazed how far it sometimes leaves the subject. Racing the early two strokes was a fun experience in that it was all new to us. In the shop there were usually several discussions per week after 5 o'clock where over a beer the crew and the riders discussed how to make things go faster. There was usually a piston and cylinder on the bench along with Harry Taylor's racing bulletins and Gorden Jennings book on two stroke theory. One clever guy was also our wheel and pipe man who also did our metal fab for the racers. He was always bringing us new pipes to try. My teammate racer was young and curious and had all kinds of wild ideas some of which worked. We all did get a lot of experience and most of all laughed and had a good time at the shop and track. All good days.
I sometimes just write things to start a discussion just as we did in in the day (ITD.) Books were reference but nothing beats the experience of field work. It requires both and more is better. Us guys from the past bristle when someone who has never ridden a air cooled two stroke tells us about jetting or wasn't even alive until decades after this research. That's why they call us Grumpy Old Men.
OK, just rambling---
Max
I sometimes just write things to start a discussion just as we did in in the day (ITD.) Books were reference but nothing beats the experience of field work. It requires both and more is better. Us guys from the past bristle when someone who has never ridden a air cooled two stroke tells us about jetting or wasn't even alive until decades after this research. That's why they call us Grumpy Old Men.
OK, just rambling---
Max
Re: The Air Cooled Two stroke Jetting Dilemma.
Carburetor are notoriously hard to tune to a single mix ratio. I would say impossible in any real sense. But as long as it is tuned for the throttle position at which the majority of your riding is done, being a bit off otherwise is no crime. Mixtures will run from 10 or 11 to one, up to 15 or so to one, back and forth as throttle position changes, which of course it does frequently unless you race TT or road course. To say that we can't achieve 12.5:1 is to misunderstand what is going on. We can't hit it with the targeted consistency of electronic fuel injected engines with sensors and computers, but we can and do achieve such ratios all the time. It isn't 12.5:1 that burns up engines, nor is such a mix ratio on the verge of too lean. 15:1 is on the verge of too lean. Our problem isn't the ratios we can achieve. It is the inability to control the variables on the fly as can be done with EFI and other computer based tech. Our timing for example is not computer variable. If it were, and if it was synched with knock sensors and fuel delivery, we could live consistently at a ratio closer to the edge, like the 14.7:1 mentioned earlier. A ratio of 12.5:1 isn't some holy grail of mixtures, it is a median ratio that targets burn efficiency, and not necessarily optimum power. I would posit that we achieve 12.5:1 all the time, but we just don't usually hover there for long.
GMc
Re: The Air Cooled Two stroke Jetting Dilemma.
Greg, also good input. I'm always thinking wide-open and I sometimes forget that there are other settings. Too much short track I guess. Electronic Fuel injection was and is quite the invention. Tuners become programers and jetting can be made near perfect. Throw in computer monitored ignition and real gains in overall performance can be had. It's kinda like photography where everything had to be figured out manually ITD. Now anyone can buy a relatively inexpensive camera that solves most of the old problems with a computer. Most pros are all out of business, (not complaining. I'd rather be retired and working on fun stuff), as probably are tuners. F-stops are irrelevant as are main jets to a young person, but we still have emerging "experts" in both fields. (Vintage tuning and Photography).
Anyway, just throwing in some observations from observing!
Max
Anyway, just throwing in some observations from observing!
Max
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