So if I use this formula for my build it shows I need 242 cfms. The original formula shows 166 cfms, so what would be the target to shoot for?
cfm vs velocity
- Thread starter hustler
- Start date
Gentlemen, the "formulas" you are discussing to determine VE / HP from air flow is predicated on the air the engine "breathes". That's the complete intake tract, carb, even the air filter.
In my humble estimation, an engine that will "breathe" 49 CFM per cylinder will be capable of achieving a maximum of 20HP per cylinder on the proper "gasoline" matched for the CR. That's considering the engine is assembled to a high standard, proper camshaft, cylinder sealed ect. Now this can be increased with a fine tuned exhaust system which causes a greater depression in the intake tract under dynamic conditions. BTW, raising the static compression ratio ONLY,, no other changes, will cause a greater depression in intake manifold vacuum, all RPM. Best, WP
In my humble estimation, an engine that will "breathe" 49 CFM per cylinder will be capable of achieving a maximum of 20HP per cylinder on the proper "gasoline" matched for the CR. That's considering the engine is assembled to a high standard, proper camshaft, cylinder sealed ect. Now this can be increased with a fine tuned exhaust system which causes a greater depression in the intake tract under dynamic conditions. BTW, raising the static compression ratio ONLY,, no other changes, will cause a greater depression in intake manifold vacuum, all RPM. Best, WP
alvin l nunley
Site Supporter
There are so many nuances in explaining the flow of air through a four cycle engine, there's no way you're going to explain it in a paragraph.
The ambient air pressure is pretty much fixed at sea level. A lower pressure is created in the cylinder as the piston travels down. Air flows from the outside of the engine to the inside of the engine. I'm thinking, the trick is to get as much air and fuel into the engine as possible. At the right ratio of course. Volumes have been written in attempts to explain how to do this most efficiently.
The ambient air pressure is pretty much fixed at sea level. A lower pressure is created in the cylinder as the piston travels down. Air flows from the outside of the engine to the inside of the engine. I'm thinking, the trick is to get as much air and fuel into the engine as possible. At the right ratio of course. Volumes have been written in attempts to explain how to do this most efficiently.
Well said Mr. Nunley.
flattop1
Dawg 89
Cfm vs velocity . The intended use would play into this imo .
A Bonneville land speed vehicle would need all the cfm possible while a motocross bike would want good cfm with max velocity .
Everything is a comprimise, not being a flow bench person are they measuring velocity ? I rarely see it mentioned as a parameter?
A Bonneville land speed vehicle would need all the cfm possible while a motocross bike would want good cfm with max velocity .
Everything is a comprimise, not being a flow bench person are they measuring velocity ? I rarely see it mentioned as a parameter?
alvin l nunley
Site Supporter
It's a question far beyond my ability to add or subtract, but I have a few ideas, right or wrong.
There's a tuned length through the intake track, from the face of the carburetor to the valve. Like a musical instrument, changes in diameter and length effect that tuned length. There's a tuned length from the exhaust valve to the end of the pipe. I have an idea that there is a tuned length from the carburetor face to the end of the exhaust pipe. I believe that getting the tuned length just right means the most efficient flow of the air, horsepower is the results.
Assume we have a pipe with an ID of 1 inch. We have a pump, with a fixed capacity to pump air, pumping air through that pipe. The longer that pipe is, the less air we can get through it. Increase the diameter of the pipe, and we can get more air through it, but at any point along that pipe, I think the pressure would be less, along with the velocity of the air. It's far too complicated for me.
There's a tuned length through the intake track, from the face of the carburetor to the valve. Like a musical instrument, changes in diameter and length effect that tuned length. There's a tuned length from the exhaust valve to the end of the pipe. I have an idea that there is a tuned length from the carburetor face to the end of the exhaust pipe. I believe that getting the tuned length just right means the most efficient flow of the air, horsepower is the results.
Assume we have a pipe with an ID of 1 inch. We have a pump, with a fixed capacity to pump air, pumping air through that pipe. The longer that pipe is, the less air we can get through it. Increase the diameter of the pipe, and we can get more air through it, but at any point along that pipe, I think the pressure would be less, along with the velocity of the air. It's far too complicated for me.
flattop1
Dawg 89
Heres a novel idea , left field for sure . Today the wash machince acting up not enough flow or air in the drain pipe .
Why could you not use water as a test for flow ? Easily regulated pressure with a simple volume measurement . The characteristic's of air vs water are dissimilar , yet they use wet flow porting . Obviously its not used , it must not work .
Why could you not use water as a test for flow ? Easily regulated pressure with a simple volume measurement . The characteristic's of air vs water are dissimilar , yet they use wet flow porting . Obviously its not used , it must not work .
paulkish
old fart
It's really simple the piston sucks air in and no matter what it can only suck in so much air and no more.
Air being sucked in has a velocity or a speed which can be used to also suck fuel in along with the air. If Velocity can't suck enough fuel in then you have to help put the fuel in by squirting it in. There ain't no other way to do it and all will work so long as you get the right amount of both fuel and air sucked and/or squirted in. All that's left is to provide the correct amount of time to burn the air/fuel per the speed the piston is moving. It nets out to if you don't have enough velocity to get enough fuel in the you need to mechanically pump extra fuel. Too much fuel sucked or squirted in and it blubbers. Not enough fuel and you burn a little hole in your piston. simple huh? ...
The biggest problem is because with most any carb your trying to squirt or flow fuel to be use at different piston speeds with just one pump setting. Bias the pump flow either for low or high speed and you have to make trade off's. The solution is so obvious it's kind of dumb to have a velocity verses flow problem to begin with.
Air being sucked in has a velocity or a speed which can be used to also suck fuel in along with the air. If Velocity can't suck enough fuel in then you have to help put the fuel in by squirting it in. There ain't no other way to do it and all will work so long as you get the right amount of both fuel and air sucked and/or squirted in. All that's left is to provide the correct amount of time to burn the air/fuel per the speed the piston is moving. It nets out to if you don't have enough velocity to get enough fuel in the you need to mechanically pump extra fuel. Too much fuel sucked or squirted in and it blubbers. Not enough fuel and you burn a little hole in your piston. simple huh? ...
The biggest problem is because with most any carb your trying to squirt or flow fuel to be use at different piston speeds with just one pump setting. Bias the pump flow either for low or high speed and you have to make trade off's. The solution is so obvious it's kind of dumb to have a velocity verses flow problem to begin with.
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paulkish
old fart
Hint on the above real dumbest post ever posted. Old time antiquated logic when mechanical fuel pumps were used with mechanical fuel injection was and dumbly is to make sure you provide enough low speed fuel and then bleed off excess fuel when the engine rev's up the mechanical fuel pump beyond your needed fuel supply. The only difference in results between the carbs we use on karts and mechanical fuel injection is one tends to limit trade off needs towards too much fuel supplied and the other towards not being able to have enough fuel. Today sensors and electronically controlled valving of air and fuel now let you set air/fuel mixture for your engine application. Even with the best ability to set air/fuel mixture with racing it's still about making an educated, experienced guess about your application.
The application and how you need to use your racing equipment is as it always has been what separates the fast from the slower. Today as it always has been especially in the UAS racing is about "Brute Force verses Finesse".
Velocity verses flow or volume is critical on the non electronic stuff we use on karts and should be looked at in terms of each's limitations because in most cases your not going to be able to satisfy needs between low and high speeds. Who most closely matches one or the other or both to your specific racing needs on a given race day will be the most successful. The art of racing is recognizing what you have to do on the track to get around it efficiently and then adjusting your equipment to be able to accomplish it and having the ability to drive as needed to accomplish it. That and knowing how to race because this is all just IMHO and ain't necessairly right anyway. ...
The application and how you need to use your racing equipment is as it always has been what separates the fast from the slower. Today as it always has been especially in the UAS racing is about "Brute Force verses Finesse".
Velocity verses flow or volume is critical on the non electronic stuff we use on karts and should be looked at in terms of each's limitations because in most cases your not going to be able to satisfy needs between low and high speeds. Who most closely matches one or the other or both to your specific racing needs on a given race day will be the most successful. The art of racing is recognizing what you have to do on the track to get around it efficiently and then adjusting your equipment to be able to accomplish it and having the ability to drive as needed to accomplish it. That and knowing how to race because this is all just IMHO and ain't necessairly right anyway. ...
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flattop1
Dawg 89
Interesting , I have a mechanical two seperate feed / butterflys , for a twin , mechanical injection setup . Never had it on an engine . So the pill is for bleeding fuel off at high rpm ?
Looking at the limitations of the current air driven carburetor, its basically a siphon tube , bowl and butterfly .
Looking at the limitations of the current air driven carburetor, its basically a siphon tube , bowl and butterfly .
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I've read, there's no substitute for air flow. With that said I've seen intake ports that were smaller, CC wise, than could otherwise be obtained on a given engine, flow a greater bulk volume,CFM. Now it goes without saying a port like this would have a higher average air velocity too.
We have to keep in mind, the karting 4 cycle engines are industrial engines. Their ports, intake and exhaust, are not designed for maximum high RPM performance. Most are 3600 RPM designs. So any port modification we do to increase bulk flow is an improvement and probably increases average velocity too. How many racing motorcycle 200 CC engines only make 6.5HP ?
Take a small cubic inch V8 engine (347) like in my late model race car. 600 CFM carb, 6300RPM limit, 10 to1 compression. Runs great in this class with 195 CC intake port heads. Put a set of well prepared 275 CC heads on and it's a dud.
NOW, we have the subject, "Port Velocity". Will the 275's show more Bulk Air Flow on a flow bench at 28"? Of course. But the air velocity is down and if we mention just one of the problems,, they "dampen the signal" to the small carb. So the old term, "It's a combination" comes into play. Keep the 275's, add a large volume intake manifold, remove the rev-limit, install a higher RPM cam, bigger carb,,etc and it makes more power at a higher RPM. But now you have a different class engine.
Look at this like dinking through a soda straw. A straw way too small,,, lots of velocity and vacuum no bulk flow. Try to drink through a 2" pipe,,, you can't generate enough vacuum to even raise your drink to your lips.
Back to engine analogies,, any of you who have a flow bench knows the largest ID exhaust pipe that will fit on a given engine will flow the most air, all things being equal. Straight pipes ect. BUT...... on the dyno or more important the race track, a smaller pipe (the right size) makes the most power and torque. Think about the soda straw. The exhaust pipe communicates the depression from the exhaust action to draw the next intake charge into the chamber. Like the soda straw communicates the suction from your lips to the drink in the glass.
(In reality the depression assist the atmosphere at 14.5 psi to push the next charge in. But that's a long discussion for another time)
When we were tuning the Briggs FH Stockers, they all were the same cubic inch displacement give or take a few over bores.Built to the same spec's. But we noticed the ones that liked the larger exhaust pipes,,,, just happened to have the most total intake air flow. I liked to say,, "that one takes a deep breath" LOL Same spec, WKA legal. And when tuned properly an engine like that made the most "Full Pull Average HP & Torque".
But lets keep this in perspective. It's 212CC and a little over 10HP,, on ALKY ! So it's not a high flow, high RPM, high efficiency engine.
Back to, "it's a combination" again,,,,,, Sorry. I used to always hate to hear that.
So IMHO I think the original title of this thread should be "A Lesson In Air Velocity". Because Cubic Feet Per Minute (CFM) has a velocity component .
Like Mr. Nunley said above,,, there's WAY more to this than can be conveyed in one paragraph. And I like to think I'm still learning too.
Best, WP
We have to keep in mind, the karting 4 cycle engines are industrial engines. Their ports, intake and exhaust, are not designed for maximum high RPM performance. Most are 3600 RPM designs. So any port modification we do to increase bulk flow is an improvement and probably increases average velocity too. How many racing motorcycle 200 CC engines only make 6.5HP ?
Take a small cubic inch V8 engine (347) like in my late model race car. 600 CFM carb, 6300RPM limit, 10 to1 compression. Runs great in this class with 195 CC intake port heads. Put a set of well prepared 275 CC heads on and it's a dud.
NOW, we have the subject, "Port Velocity". Will the 275's show more Bulk Air Flow on a flow bench at 28"? Of course. But the air velocity is down and if we mention just one of the problems,, they "dampen the signal" to the small carb. So the old term, "It's a combination" comes into play. Keep the 275's, add a large volume intake manifold, remove the rev-limit, install a higher RPM cam, bigger carb,,etc and it makes more power at a higher RPM. But now you have a different class engine.
Look at this like dinking through a soda straw. A straw way too small,,, lots of velocity and vacuum no bulk flow. Try to drink through a 2" pipe,,, you can't generate enough vacuum to even raise your drink to your lips.
Back to engine analogies,, any of you who have a flow bench knows the largest ID exhaust pipe that will fit on a given engine will flow the most air, all things being equal. Straight pipes ect. BUT...... on the dyno or more important the race track, a smaller pipe (the right size) makes the most power and torque. Think about the soda straw. The exhaust pipe communicates the depression from the exhaust action to draw the next intake charge into the chamber. Like the soda straw communicates the suction from your lips to the drink in the glass.
(In reality the depression assist the atmosphere at 14.5 psi to push the next charge in. But that's a long discussion for another time)
When we were tuning the Briggs FH Stockers, they all were the same cubic inch displacement give or take a few over bores.Built to the same spec's. But we noticed the ones that liked the larger exhaust pipes,,,, just happened to have the most total intake air flow. I liked to say,, "that one takes a deep breath" LOL Same spec, WKA legal. And when tuned properly an engine like that made the most "Full Pull Average HP & Torque".
But lets keep this in perspective. It's 212CC and a little over 10HP,, on ALKY ! So it's not a high flow, high RPM, high efficiency engine.
Back to, "it's a combination" again,,,,,, Sorry. I used to always hate to hear that.
So IMHO I think the original title of this thread should be "A Lesson In Air Velocity". Because Cubic Feet Per Minute (CFM) has a velocity component .
Like Mr. Nunley said above,,, there's WAY more to this than can be conveyed in one paragraph. And I like to think I'm still learning too.
Best, WP
paulkish
old fart
Here's what I think is the limiting problem in all of the above velocity verses air flow limitations and compromises being discussed. I'm not yet ready to provide the solution on here even though it's a duhhh why didn't I think of it thing.
I think what's being talked about here is maximizing small bore engines where a float carburetor with a high and low speed circuit is able to satisfy 'most' needs because it has cylinder air suck able to make fuel supplies using high and low speed circuits work. It's fine when rpm's are in a lower range, lower range being defined by low and high seed circuits able to supply enough fuel.
The problems occur only because what your racing either goes beyond high speed fuel supply abilities when you increase your rpm or you cylinder volume or size goes beyond what a float carb can supply because of engine suck created when the piston goes down(or a combination of both reasons).
Prior to the now common use of fuel injection on our daily drivers ALL carburetors used on passenger cars had a similar problem but in reality the same problem.
Those carbs had an idle circuit, an intermediate circuit and a high speed circuit built into them the same as the small bore carbs being thought about and written to on here. Butt the passenger cars of old still had one problem which was fixed or over come by the 4th fuel supply circuit, so to speak. EVERY passenger car carb used the 4th circuit.
What is the 4th circuit they used?
I ask because the result of it's function can be used to "FIX" all the velocity/air flow problems being addressed and finely tuned on here. Without it you can't fix it unless your particular compromise use of a float carb is able to match up to your particular tracks fuel supply needs.
answer: ...
I think what's being talked about here is maximizing small bore engines where a float carburetor with a high and low speed circuit is able to satisfy 'most' needs because it has cylinder air suck able to make fuel supplies using high and low speed circuits work. It's fine when rpm's are in a lower range, lower range being defined by low and high seed circuits able to supply enough fuel.
The problems occur only because what your racing either goes beyond high speed fuel supply abilities when you increase your rpm or you cylinder volume or size goes beyond what a float carb can supply because of engine suck created when the piston goes down(or a combination of both reasons).
Prior to the now common use of fuel injection on our daily drivers ALL carburetors used on passenger cars had a similar problem but in reality the same problem.
Those carbs had an idle circuit, an intermediate circuit and a high speed circuit built into them the same as the small bore carbs being thought about and written to on here. Butt the passenger cars of old still had one problem which was fixed or over come by the 4th fuel supply circuit, so to speak. EVERY passenger car carb used the 4th circuit.
What is the 4th circuit they used?
I ask because the result of it's function can be used to "FIX" all the velocity/air flow problems being addressed and finely tuned on here. Without it you can't fix it unless your particular compromise use of a float carb is able to match up to your particular tracks fuel supply needs.
answer: ...
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paulkish
old fart
...
It was written that way by me because i'm also on here for my own enjoyment being retired with nothing better to do. ...
There's no need to get into a debate of weather atmospheric pressure is explicitly correct or not.
An argument can be made for either way and debating which is cut in stone correct will not find an answer suiting everyone. The only solution is to converse with someone without getting into a semantic argument when you already know and understand the differences of argument which will be presented. If you already know the differences and don't have the ability to be civil understanding others you are the one creating the argument.
In this case if your smart enough to know the difference then you should be smart enough to see the humor and not demand everyone tows the line and conforms to your thinking. ...
paulkish
old fart
I was not presenting humor when I related solving the velocity/air flow with something relating to the 4th circuit in an older passenger car carburetor.
I'm totally open to have explained to me where i'm wrong about there being a 4th fuel supply circuit in older passenger car carbs.
That to can be a symatic thing but before we would or could get into the semantics of it we would need to all understand what the heck I mean by a 4th fuel flow circuit beyond idle, intermediate and high speed. And how you might apply a 4th circuit to solve velocity/air flow problems which prevent you from having to make compromises in the racing engines we use.
The solution is simple but not available to be used per all rules sets.
I'm totally open to have explained to me where i'm wrong about there being a 4th fuel supply circuit in older passenger car carbs.
That to can be a symatic thing but before we would or could get into the semantics of it we would need to all understand what the heck I mean by a 4th fuel flow circuit beyond idle, intermediate and high speed. And how you might apply a 4th circuit to solve velocity/air flow problems which prevent you from having to make compromises in the racing engines we use.
The solution is simple but not available to be used per all rules sets.
.