cfm vs velocity

hustler

New member
I'm trying to further my education in the world of engine building and need guidance from wise sages. From what I think I know, an engine is an air pump. Bore and stroke dictate the displacement which is a fixed or constant volume of air (cu.ft.). When you bring rpms to the mix, volume over time, cu.ft. per min. or cfm. As rpms increase so does the cfms of air needed. A formula I found that is supposed to give a starting reference goes as (cu.in x rpm)/3456 = peak cfm for peak power rpm. Granted this is a base or reference figure and im sure dyno results will prove more reliable. My question is this, I have seen some of the drag bike videos, the mini bikes with some wicked gx200 with 3" bores and monster strokes guessing 3" +/- . So 3" bore and 3" stroke would give you 21.21 cu.in. I'll call 22 cu.in. and I've seen cfm claims of over 100 cfm on the flowbench. But using the equation above shows 66 cfm to support 10,500 rpm, 89 to support 14,000. So am I missing something or what. I mean I know an engine will run no matter the size port but was always told to get the port big enough to just flow the needed cfms so port velocity stays high to aid in cylinder filling. An engine can only take in so much air per intake stroke based on the displacement so seems like going bigger wouldn't do much good. But then again my formula may be way off. Also these bikes are built for top end power in straight line acceleration drag racing, much different than what goes on a dirt oval. Also I know airflow has a direct correlation with hp, the more air fuel in, the more power out. But with that said, I don't think if you port a head to support 60 hp and put it on a 200cc engine, that engine will put out 60 hp so there has to be a point at where the port just gets too big. Anyway if someone could straighten me out on this i would appreciate it. Not trying to ruffle feathers or call anyone out, just trying to learn. Thanks
 

flattop1

Dawg 89
Good question.
I just left the holiday inn soo.
A couple things come too mind as you said claimed cfm.
Mathematical calculations vs real world situations don't always match up 100%.
I'm wondering if the head alone flows 100 cm add the rest of the intake tract and mount it on the engine . Will the engine be able to pump 100% of the heads ability to flow 100 cfm?
 
A flow bench is a comparator, it only tells you if you have higher or lower flow at various valve lifts but has no meaning to how an engine really operates. (compared to a 'stock' head) A flow bench uses steady state flow through a port, the engine operates on a pulse flow.
It is useful tool , But like you said in your description, Air speed is critcal also.
For a perfect setup it needs to be balanced. Cam timing, rpms, overlap play a huge role also.
 

hustler

New member
They run a lot of compression on some of these motors, 15-18:1. Someone told me that compression ratio will actually help a big port head work. Now tell me if this makes sense, if you figure a 9:1 chamber, the cylinder volume from TDC increases 9 times to BDC. Now if the same engine is used and the chamber is changed to 13:1 CR, the cylinder volume will change 13 time the original volume from TDC to BDC. So therefore a greater negative pressure is created in the cylinder (theoretically?) and an equal and opposite positive pressure is created in the port and induction system. I guess that would make a huge port on a small engine flow better. I'm just spit balling, but if that were the case, I guess I could see why the engine could make use of a larger port. Does any of this make sense or am I star gazing during the day again?
 

Bob Evans

Grumpy Old Admin
Staff member
This is a pretty good read http://www.candsspecialties.com/ratings.html

They point out that many flow systems flow at much lower vacuum than real world situations, then use calculations for the flow readings, either electronically or manually. While pretty good, they usually result in higher readings.

OneFastCat makes an interesting observation about the pulsing. The actual demand for air/fuel is only during 1/4 (180 deg) of the two revolutions (720 deg) used during the 4 cycles. So there's very little time to fill the cylinder.

My 2 cents thrown out there. :)
 

95 shaw

Premium User
More static compression ratio is a bandaid for incomplete cylinder filling.
Changing compression ratio does not change the displacement of the cylinder.

At one time Nascar restrictor plate engines used compression ratios in the 20:1 plus range. Care had to be taken not to fully load the engine below peak torque. This is where the engine reaches the maximum fill of the cylinder with the existing ports/flow.
 

Kart43

Member
They run a lot of compression on some of these motors, 15-18:1. Someone told me that compression ratio will actually help a big port head work. Now tell me if this makes sense, if you figure a 9:1 chamber, the cylinder volume from TDC increases 9 times to BDC. Now if the same engine is used and the chamber is changed to 13:1 CR, the cylinder volume will change 13 time the original volume from TDC to BDC. So therefore a greater negative pressure is created in the cylinder (theoretically?) and an equal and opposite positive pressure is created in the port and induction system. I guess that would make a huge port on a small engine flow better. I'm just spit balling, but if that were the case, I guess I could see why the engine could make use of a larger port. Does any of this make sense or am I star gazing during the day again?
The pressure outside the cylinder Atmospheric does not change because you changed the compression ratio, Dynamic compression ratio is actually calculated from the swept volume of the piston from the point the intake valve closes not BDC. The negative space in the cylinder is determined by much more than the simple volume, temp, and residual exhaust pressure, so now you see the exhaust system is tied to the intake.
 

Kart43

Member
Simply because the combustion chamber is smaller that does not cause a corresponding change in negative pressure.
 

hustler

New member
The above post wasn't meant to sound like I was stating truth, just stating what I was told so you fine folks could say different haha. That's what I'm here to do learn.
 

Freezeman

Premium User
Your track example sounds good until you realize that combustion chamber size which makes the 1 part, is much smaller in the 13 to 1 than the camber of the larger 1 of the 9 to 1.
So you are compressing the same volume of area into 1 volume of an area with the only difference is the difference of area it's compressed into.

The 13 to 1 ends up with a smaller yet denser charge while the 9 to one ends up with a larger less dense charge.
 
Hustler. I used to work in an engine shop building racing and street engines for enthusiasts.
You are listening to people at the track. They sometimes have ideas that are brilliant, sometimes not so much.
I remember one customer that brought me an engine, 427 Chevrolet . Wanted it bored .60 over, wildest cam that I had and two four barrel 660 cfm holley carbs. , and 2 1/4 inch pipe headers. I tried to explain very gently that this was a terrible combination for the street. . Arrogantly he reach in his pocket pulled out his wallet , plunked a lot of money on the table and said, I am paying you to build my engine the way I want it. We continued for a few moments and finally I said OK. But there is no guarantee that this engine when finished will be as fast,as it is now. I built the engine,
one week later, after he had the car two days. he came back. Very unhappy, I replaced the carbs with a two 450 cfm carbs and smaller headers, One day later he was ecstatic.


Now back to your original post. Your formula is the generally accepted one.
And your post is correct That is why I posted the above actual event. Just because it is bigger , does not mean it is better.
Now to compression ratio and air flow. No relationship. More compression will build more hp.
but not more volumetric efficiency.
Bigger valves will improve both volumetric efficiency and hp. It has been my experience ,
Getting more air in is the main goal. The air flow from the Air filter to the cylinder must be treated as one unit. It needs to be as straight as possible without anything creating air turbulence. Example if the gasket between the intake and head is not trimmed to fit exactly and protrudes into the air flow or if it is just mis-aligned, it will create turbulence and reversion. That will hurt performance.
That is why I suggest that getting a professional builder to build your engine is well worth it. Especially a stock engine. AND No. I do not build engines for other people now. Just my own and for several friends. So this is not a plug for you to send me your engine.
 
Ok ... here is the real answer to the original question ..
the engine yes is an air pump .. also think of it as a syringe .. if you have a syringe that only holds a certain amount of water ( the syringe is the engine ) and you have a bowl with 5 gallons of water in it ( the bowl is the head flow ) it doesn’t matter how much water is in the bowl if the syringe can only hold so much water .. so it doesn’t matter how much cfm the head flows if the engine can’t use it .. when I build cylinder heads I ask a few questions .. what size engine volume .. what rpm do you want to make power at .. from these two questions I can determine cfm requirements and cam selection that best suites the head I’m building it around .. if you have to much cfm you are sacrificing velocity .. it’s like a garden hose .. you wouldn’t use the same pressure to wash your driveway and water the flowers .. that’s why the nozzle is adjustable .. so the fight velocity and flow for the right job .. you want to be able to dial it in just right for the job .. too much cfm , you lose velocity , to much velocity , you lose cfm .. I use a flow bench with a manometer and velocity probe to dial this in perfectly .. now the cam you select has a lot of more a ton to do with these (cfm,velocity) numbers you go for .. a cam with a lot of duration actually doesn’t produce lots of velocity thru the intake , and a cam with a little duration produces more ... this is a phenomenon that is usually missed or not known about .. so pairing the right engine volume (or body) to the right cam (.or brain) to the right head ( or lungs & cardio vascular system ) creates the olimpiin of an engine .. this is the tuning required to get the engine to perform at its peak level .. intake and exhaust widths and lengths are also extremely important .. there are equations for both but the equations have a + or - 3 factor of the answer to emphasize real experimentation on a dyno .. so to sum it up .. CFM/ VELOCITY need to be dialed in to the engine as a whole’s requirements .. also an engine if tuned properly can operate over 100% of its volumetric efficiency .. this is done by tuning and creating a supercharging effect thru the intake utilizing the exhaust as a vacuum .. if your length is tight and you have the correct timing on your cam to this event and the cam has the appropriate lobe delegation angle (LSA) this event can be created and the exiting sxhaust gasses can create a low pressure behind them creating a vacuum in the cylinder as the intake opens and create a supercharging effect .. this is what I am currently using to get my engines to preform .. I have info on this on my you tube channel ... Paul’s kart’s ... the videos on this are under a playlist called engine optimization... hope this helps .. I also have videos on how to chose the right cam etc ... good luck !!!
 
Yes .. I achieve over 100% ... this is done thru tuning .. if you create a vacuum in the cylinder it is now taking in more dynamic volume than static numbers portray...truely understanding the inner Machinations of the engine is my line of study ..achieving great success .. dynamic numbers !!! Think outside the box !!!
 
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