The rule of thumb is 2.056HP per cfm gained in a perfect world. So the 20 potential horsepower is correct as has been posted. The .500" lift is a red herring and has no bearing on the answer.

He is asking how much horsepower is to be gained by going with heads that flow 250cfm versus heads with 240cfm. Not knowing the efficiency of the motor we are assuming it is 100% efficient. It could be more it could be less.

4% is not an answer in horsepower as the original poster asked.

 

So you would submit that flow at max lift is really what matters.

Also, that 2.05 HP per CFM is with a 100% efficient engine. Since an engine is about 45% efficient AT BEST, the real HP would be about .9 HP per CFM.

Do I have that right?

 

Regarding head flow. When a head is flowed the valves are opened in either .100" or .050" increments and the measurement is taken. For street use they usually go to .600" lift or more if a real high lift cam.

Sometimes max flow is not at the highest lift point but usually is or close to it. Sometimes a head will go into choke at a higher lift and the flow will drop. If that is the case you do not want to open the valve that high.

The efficiency I'm talking about is called volumetric effiency or commonly called VE. You can have over 100% VE in a well tuned intake system. Your standard motors are well below that. Maybe 80% VE. The newer performance motors are getting pretty good and are in the 90% VE range.

 

You can exceed 100% VE with the right parts. This is where intake/exhaust runner lengths, and cross sections, with matching components come into play. Off hand I don't remember the VE of an Formula-1 engine, but it's considerably above 100%.

 

Ok sorry I am going to have to bite on this one. There is no way a N/A car is going to run 100% VE. IT does not happen. The best matched engines running at 7000 rpm only push 70-80 percent. That is also at a sustanied 7000 rpm. A street car is very rarely going to see 60%. The only way to get 100%, plus a cooler charge which is what I assume you are refering to, is in a FI application. In FI you are not really increasing the VE you are just increasing the atmospheres to make it seem like you are.

 

You CAN Exceed 100% VE with a street driven motor and most do in a given RPM range - again this is where velocity comes in and the ability for the incoming air charge to "pack" the cylinders.

With good intake and heads that you have on todays racing motors you can get over 120% VE.

Here is a comment from an engine builder:

Most of our single carb ,single plane manifold, race type engines ve from 105 to 115%,a few have been around 116-118%,and a couple have been over 120%

 

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By efficiency I thought you meant the efficiency of a motor when burning gas. I looked it up and about 55% at best is lost to heat.

 

Ok, so what is typical VE at .500? Is it typical to get your VE at that lift? If not, where is it...

What does that extra 10CFM mean to VE and how does it translate to HP?

 

You are asking question that can't readily be answered without some type of analysis software or actual flow readings on YOUR specific motor.

Each engine is different and results will vary. Motors that make over 2hp per CI are well into the 120+% efficiency range.

Think of it or try to imagine how a motor works. Intake valve opens and allows the in-rush of air as the piston makes it way to bottom dead center. On properly cammed and built motors with optimized heads/intake the piston will actually be coming back up before the intake valve closes (believe it or not) this is to allow the incoming air that has momentum to further fill the cylinder - I saw a video a while back and it was wild to see this happening - but it does. In other words if you were to pull the piston down to the bottom and stop it, while still watching the incoming air, you would see air STILL flow into the cylinder well after the piston stops moving. Thus you get greater than 100% VE.

That is why you have to talk about velocity of the incoming charge....its a very dynamic system and one that you can't just look at in a static or "assumed" state. If you do, then you are not realistically making accurate correlations to what is really happening.

 

Won't an additional 10 CFM get you to 100% VE faster in that scenario, therefore getting you to above 100% faster? And isn't there an advantage to that? Or is there a specific, static number that once reached, you can't possibly get more air into the cyl? I guess this is the balance between port velocity, manifold pressure, and CFM. Right? All three play a role in getting to VE, and as one is increased, the only element in those three that can throw off the equation is CFM. Because if either one of the other two are increased, VE is increased to the point of diminishing returns. Right?

 

Why would you care about this scenario on a 421?

 

From the breaking your ***** handbook.

All in fun.

Oh, BTW, did you read the thread, it's actually interesting.

 

No I didnt. I know its about port velocity/cfm, and quality of springs etc....Its not rocket science man. This is why I sold the TFS heads and bought the AFR's. Switching from the TFS heads to the AFR heads on a bigger cubic inch motor will yield HUGE GAINS!!!!!!!

 

Why start dragging this down the sewer? I'm trying to learn something. This has the potential of ending up in a apology to you. Why drag it down the sewer unless you are concerned that it won't play out in your favor. Science doesn't take sides, nor do I. I'm just trying to learn here from people who have more knowledge to me. "Because" is not an answer. This thread has been going just fine until you interjected.

 

Since you changed it, I'm just trying to figure out the science as to why that statement is true.

 

You have to remember the basics in that these things are "air-pumps"... Air in-air out. If you can get additional air into the combustion chamber; with fuel in the proper ratio, It's gonna make a bigger BOOM. What do you think superchargers/turbos do? I think maximum VE is often found around peak TQ, on NA motors... someone correct me if i'm wrong on this. What are your thoughts on the 427 LSX article?

Thanks for dropping in Jessie; I was originally going to give figures of 110-120% VE, but i've been getting enough "static" around here as it is. I'm sure you know about such things yourself.

 

I understand the air pump position pretty well. I am trying to understand leaps in performance from really good to super good and more importantly WHY and HOW MUCH.

Thanks to this thread I understand, or at least I'm digesting the theory.

I also understand that there is a point of diminishing returns.

A turbo/supercharger keeps manifold pressure up to keep port velocity up. So as I understand it CFM, manifold pressure, and port velocity, although related, are three different things.

CFM is basically a measure of volume.

As CFI points out, my example would be a 4% increase in volume. Based on the cam and other factors that 4% can result in up to 120% VE. However, if that 4% is on the cusp of the point of diminishing returns, it can hurt VE. Right?

I'm just trying to understand. Then I'd like to apply the knowledge to the OP to determine how much is 10 CFM worth in terms of HP on the CI engines mentioned.

I am convinced from 170 to 180 the returns are great. At 240-250 are they still as great?

I don't know. I don't know if there is an answer with the information given. Perhaps we should specify cams and intakes for this example?

Let's say we have a Mini Ram, or a Super Ram, Pick one and a CC 503 or a Lingenfelter 74219 for this example. Assume 1.6 rockers for either.

Pick a setup and see if we can't make more sense of this. or perhaps pick both examples and let's see the difference.

There has been a lot of views on this thread I am sure people are interested. I know I am.

 

Then you asked the wrong question. All the comments on VE, velocity, port size, head brand, is interesting, and related, but irrelevant to the question, asked. "if your flow is say 240CFM vs. say 250CFM. How much power does that translate to at the wheels"? And "What does that 10CFM buy you?". That's all that was asked. Since you don't state a hypothetic horse power at the "240CFM" beginning point, it is impossible to speculate on a specific horse power output only knowing that the air input was increase by a rate of "10CFM" (~4%).



NO! A CF (cubic foot) is a measure of volume. A CFM is a measure of a RATE of flow. Saying that a CFM is a measure of volume is like saying that a mile per hour, is a measure of distance.



Ten is ten. Ten is 5.88% of 170. Ten is 4.1 2/3% if 240. Are the returns greater? How about if you increase the flow of air at 240 by the same percentage as at 170?

I am beginning to think that the question asked was not what was intended.

RACE ON!!!

 

I've re-read this 3 times and it still doesn't make any since to me. What is your question? (condensed version) From your statements you're still not getting everything that has been said.
Forced induction does more than "increase pressure" BTW. It takes huge quanties of air and stuffs it into the chamber. NOS is a gas with lots of oxygen contained within its molecules.