I think we did hijack his thread. AZ, you are right (shudder) about taking two engines and just reed valving one with no porting mods. Not much difference. My argument is based on the whole treatment. We modify a race engine to be more efficient and increase fuel/air flow. In the reed valve "treatment" we make more passages to the combustion chamber via holes and grooves. We put in a one-way valve to stop the inherent problem of reverse flow out the carb.
My testbed started with a 90cc race modified piston port engine with a 32mm Amal. The next step was the reed valve "treatment". No dyno testing in those days but we did have another bike to compare it to. There was a remarkable increase of power in the reed engine.
My conclusion was that we were moving more fuel/air mix through the engine. It all enters through the carb so it must of had an increase in velocity. That is why we installed a larger carb in the first place; It was the constriction point.
All too long ago to remember if there was a jetting change. (Which is what the original question was all about!)
Back to you Rich! Thanks for showing your shop and cool toys and tools. Seems as though you do have everything needed to do your good work. I really liked your right angle grinder in the last shot. Bill and I really could have used that "back in the day"!
Max
PS--any comments on AZ/Max theory? Don't want to put you on the spot but you probably are the current expert! (It's all for fun).
Larger carbs reduce velocity. I don't have much difference in opinion regarding jetting and reeds, just that the increases you mention are not attributable to increased velocity. Everything you do to increase cross sectional dimension of a passage decreases flow all other things equal. What you notice in increased performance is attributable to increased volumetric efficiency, not velocity.
Last edited by Zyx on Sun Apr 19, 2015 9:15 am, edited 2 times in total.
Here is an example away from reeds and two strokes that might be familiar. Take a normal old school car engine. Modify nothing but the carb. Gasoline internal combustion engines make vacuum, which we all know. We use it to power brakes and so on. That vacuum is made by the Venturi effect of a low pressure zone in the carb and intake system as a result of increased velocity through the venturi. Bernoulli principle.
Add a bigger carb and what happens? Vacuum falls off. Add a really big carb, and there may not be enough vacuum to power brakes, but the visible effect is that if you stomp on the gas with a big carb, there is a hesitation due to dropped vacuum when the throttle opens, dropping velocity momentarily as a result of making the opening a lot larger all of a sudden. Larger passage, less vacuum due to less velocity, even though the air pump this is all attached to is exactly the same. Same engine, no mods, different pumping efficiency due to different intake size.
Same with the two stroke. Make the carb bigger and that is all you do, and you will have a boggy engine on your hands. Add a big carb and modify the ports, and you have a really boggy engine on your hands, sometimes to the point of stalling the engine. Add reeds to that equation and suddenly things start working again, not because of increased velocity, but because of the quantitative control on volume intake that the reeds provide versus the tendency to revert or reverse pulse direction due to piston pressure on overlapped ports.
A piston port engine pumps the same volume for a given bore stroke combo as does the reed engine. The reed engine retains a bit more of the volume but interferes with flow, so make the reed intake bigger to compensate, so that the amount of flow through the reed assembly is comparable to the smaller but uninhibited non-reed system, and you now have the flow you used to have, but with more retention of charge. You get a bit more efficient intake and transfer because the engine does not need to wait for the piston port to close before making pressure. So it isn't getting more air or fuel/air, it is just keeping more of it per stroke, and it isn't moving faster, it just can't escape as much as it used to.
I totally agree with Greg , velocity is not increased theough the carb , if it was going to larger jets would not be necessary as the needle jet would sence it and supply fuel to supply the additional air passing by it. also streamlining the air bleed at the back of the carb will decrease the jet size requirement since the some of the incoming air is passing over the bleed rather than smacking against the 90degree stock casting , its like adding a smaller air bleed but increases air flow at the same time. What will increase velocity though is the addition of venture dividers in the carb , which makes a large carb behave like a small one through out the slide opening range then at wot the engine has had time to catch up to the carbs potential. Harry Taylor got me on to this principle and have since went beyond What HARRY was doing , I went to 4 dividers 1/4 & 1/2 throttle positions on both sides of the slide then to 3 on each side and everyone loves the way they work , I have national champions in Road racing , motocross and snow mobile racing useing them. It make using a 39.2 mm carb a reality on a 100 ot 125 a doable reality with massive performance when used withV force reeds , And regardless of what theory anyone believes it works . We could talk about this stuff for ever , some will never grasp it and some will totally understand it . But we keep on learning no one is beyond learning . As Harry Taylor would often tell me ( no one knows everything ) and he was VERY smart and done more R and D than most will ever do .
Hey AZ, Let's try this to arrive at common ground. In my test engine the reed version was obviously pumping more air/fuel mixture which I see as velocity increase. I'm measuring it over time as a sum of velocities. In (positive), out (negative). The reed is canceling most of the negative velocities. A measure of a instant might reveal a limited speed (as you say) but with positive and negative values (as I say). Net velocity is what I'm measuring. I would really hate to admit it but we both might be right!
Waddaya think--
Maxie
PS--Rich, your message just popped in. The message is, as you say, about learning, not about me and my so-called expertise or any one else's for that matter. This is how I also run my life, not as an expert, but with an open mind. Does my explanation above make any sense? Mine was but a theory which can be supported (or proven false) by a test that I have already talked about. Of course the test would be impractical but interesting. Science is about proofs not opinions.---Just being the Devil's Advocate here!--
Well, Max, the devil is in the details. Velocity is a measurable phenomenon same as speed on a speedometer. Increased velocity is increased speed. While increased speed of an intake charge can have a dramatic effect on a piston port engine, it is not an increase in speed that results from reed systems and bigger carbs and jets
Velocity aside, you can think of your intake charge as a mathematical number. Without addressing nuance, let's say a motor has an intake displacement of 10. Doesn't matter 10 what, just 10. Engine cycles through it's intake stroke and draws in 10, but then the piston starts back down so as to begin compressing the charge drawn into the crankcase but the intake port is not yet closed by the piston skirt, so of the original 10 drawn in, 2 slip back out. That two creates the fog in front of the carb you see on a running engine. Add reeds with a same flow rate potential ( not speed potential, volumetric rate of flow potential) and do it again. Intake draws in 10, piston starts back down but now the reeds slam shut as soon as intake draw stops and reversion starts, and instead of pushing 2 back out the intake, it only loses maybe 0.75 or 0.50. If the reeds retain all but 0.5 of the original 10, you now have a retained intake charge of 9.5 as compared to the original 8, an increase of nearly 19% in volume retained. That's where your power is coming from. Speed didn't change because we traded an intake with a flow rate of X for a reed intake with a flow rate of X, same rate. Same speed. It is volume doing the work through increased efficiency, not speed.
The only way I know to improve on a reed system for outright efficiency would by ram induction of some type, turbo, blower, or static ram to increase atmospheric pressure on the intake horn. Such systems might also increase intake charge speed, but only because of the increase in pressure pushing inward. When all of the impetus is from the inside of the system through basically suction, the speed in response is increased by reducing diameter of the inlet, or decreased by making the inlet bigger. The relationships of volume, speed, and pressure are a fixed relationship expressable by a formula which I do not claim to understand well enough to claim it with a straight face. What I do understand is that there is a fixed relationship, and what that relationship is. Look this all up on the Internet by searching for velocity versus volume or any combination of terms dealing with fluid speed, pressure, cross sectional area and so on. There are tons of high tech discussions out there on how this works, and they all have the same thing in common. The relationships are fixed.
Ok if velocity was all that there is to it then we should all put 20mm carbs on our racers. Yhen venturi velocity would be extremley High Right? well the point if diminishing returns comes in whrn that small Hi velicity carb is preventing atmospheric pressure from filling the void in the crankcase or the intake plenum in a v 8 engine , when that void cant be filled then air speed between the venturi andt the cylinder slows down and there goes the potential for power. Even in a trials bike Like Jay Laels 125 Hodaka we upped the size from 20mm to 26 with 4 venturi dividers and received great gains i response and power . Also Not been mentioned here a large portion if the reed valve performance improvements is directly related to the addition of boost ports at the rear of the cylinder greatly improving cylinder scavenging . And beyond that performance can pe improved more bt changing the existing transfere ports rear exits form sweeping the rear of the cylinder to aim more to the center leaving tha scavenging to the new boost ports, I do this on all reed engines I port . This is something that was not done or known about bac in the 70s. Th venturi divided carb gives both velicity as well as volume.
Ok if velocity was all that there is to it then we should all put 20mm carbs on our racers. Yhen venturi velocity would be extremley High Right? well the point if diminishing returns comes in whrn that small Hi velicity carb is preventing atmospheric pressure from filling the void in the crankcase or the intake plenum in a v 8 engine , when that void cant be filled then air speed between the venturi andt the cylinder slows down and there goes the potential for power. Even in a trials bike Like Jay Laels 125 Hodaka we upped the size from 20mm to 26 with 4 venturi dividers and received great gains i response and power . Also Not been mentioned here a large portion if the reed valve performance improvements is directly related to the addition of boost ports at the rear of the cylinder greatly improving cylinder scavenging . And beyond that performance can pe improved more bt changing the existing transfere ports rear exits form sweeping the rear of the cylinder to aim more to the center leaving tha scavenging to the new boost ports, I do this on all reed engines I port . This is something that was not done or known about bac in the 70s. Th venturi divided carb gives both velicity as well as volume.
It seems to me that Arizona is talking about comparing apples to apples . . . and you are comparing apples to oranges. Your message above from today at 5:43 mentions your old 90 and how much more power it made with a 32mm Amal.
Changing from the stock 20mm carb to a 32mm high flow carb WITH a much larger intake manifold (which happened to have reeds) results in a situation where the bike's nostrils got wayeeee larger. Those larger breathing parts could deliver the SAME AMOUNT of air/fuel at considerably lower velocity than the stock carb/manifold - and those larger breathing parts could deliver LOTS MORE air/fuel at the same velocity as the stock nostrils. That's an apples and oranges comparison. Yes, the 90 made much more horsepower after the mods -- so it is true that more air/fuel was being delivered (through bigger nostrils), but that doesn't specifically indicate that intake velocities increased. It only shows that a bigger carb AND manifold will be capable of delivering more air/fuel in a given time.
Ed, the test was done with the same carb, a 32mm Amal. My test showed that the engine with the reed modifications was making significantly more horsepower than the piston port engine. It was the same apple. I concluded that it was moving more air/fuel over a certain length of time creating more power. Maybe air velocity does have a speed limit but more air went through the carb over a certain length of time. The reason was that the velocity in the piston port engine had both forward AND backward velocities and the SUM of the velocities was less. The reason for the reed was to cancel the backward velocities.
This is perfectly clear to me. Everybody should note that Max started with a real test. Max's case ended with a proposed test to measure air velocity with two engines with the same carb.
What I LEARNED from this session is that the velocity probably hasn't increased but the SUM of the velocities has.
Another thing I learned is people aren't reading what I say before they start telling me how smart they are. I'm here to help the Forum and share what I remember about a remarkable time in motorcycle history. It's not about me. I am not happy but I'm trying to be.
What a great discussion. Here is what I think I have learned. Velocity, volume and flow. All important, all necessary for a carburetor to work but for different reasons. The flow of a given volume of air over time through a venturi causes an increase in velocity which results in a reduction in air pressure necessary to draw fuel up through the jets. In order to get an engine to run efficiently velocity should be somewhat static through the venturi through the available rpm range.
If velocity were the one determining factor in power, our engines would make the most power when we closed the throttle. Too much velocity can result in a rich condition because of the extreme pressure drop. This is why a bike will load up going down a hill with the throttle closed. Volume is necessary to make more power, but not if it is delivered at too little velocity to create the drop in pressure in the venturi. This could result in a lean condition which explains why we can clean out a loaded up engine by opening the throttle wide open at low engine speeds. Without the proper velocity the engine becomes difficult to jet properly.
Volume and flow is where the increased power comes from. The engine can only pump as much as the displacement will allow over time, but the availability of increased volume at the proper velocity, greater engine speeds can be achieved which can pump more volume to be converted to power generation.
Apparently it's the management of volume, velocity, and flow that determine the efficiency of the engine at different speeds. That's where the discussion becomes interesting! Thanks for the entertainment.
I think it's all positive Max. The mind boggling part of it all to me is that this part of the discussion only pertains to the carburetor part of the flow. Velocity and flow become exponentially more complex when applied to the rest of its journey through the reeds, crank, ports, combustion chamber, and exhaust system... and when is it best to have laminar vs. turbulent flow. It is all interrelated! You guys have all contributed a lot to the discussion and it seems there may be many different ways to achieve positive results in performance. ...The thread has been hijacked though.
about taking two engines and just reed valving one with no porting mods. Not much difference. My argument is based on the whole treatment. We modify a race engine to be more efficient and increase fuel/air flow. In the reed valve "treatment" we make more passages to the combustion chamber via holes and grooves. We put in a one-way valve to stop the inherent problem of reverse flow out the carb.
, and mentioning that the engine can only pump its displacement. Sometimes we forget the obvious.