How to figure CFM?

Question

Does anyone Know how to figure the right cfm rating for a motor?

Answer 1

CFM - (Cubic Feet Per Minute) As It Relates To Your Engine
Air Capacity (cfm): Since an engine is actually little more than an elaborate air pump, it’s ability to perform work—measured in horsepower and torque—is a product of its capacity to inhale and exhale air. An engine’s theoretical air capacity is a product of its rpm and displacement, divided by two (since only half of the engine’s cubic capacity is being displaced during each stroke). For purposes of rating airflow (i.e. via a carburetor), this formula is converted to a quotient reflecting cubic feet per minute (cfm) by dividing both sides of the equation by 1,728, the number of cubic inches in a cubic foot. The reduced formula for cfm:

Rpm x displacement /3,456


3000 rpm x 350 cubic inches displacement divided by 3,456 = 303.82 CFM

Answer 2

John M got pretty close but you have to figure in volumetric efficiency. Most street cars average about 85%. Also, you should figure the max rpm that you will be running. Let's assume ( (6'500 rpm x 350 cu in) / 3456 ) x .85 ( 85 % efficient ) =560 cfm carb. Now a good rule of thumb on a street engine is to go down a size. Holley, Edelbrock both make pretty good 500 cfm carbs. Remember, a 4 bbl carb does atomize better than a 2 bbl, and normal driving, you hardly ever go beyond the idle signal in your carb unless you mash the petal. So, a 303 cfm carb would starve out when you 'get on it' and you could never reach the rpm range if you needed to pass someone. But if you really want to get technical, air filter design, cam design, etc....I guess that will have to wait till you ask. You do the math, you know what you need and now you know how to figure it. Doing John M's figures the right way would result in a carb rated at 258 cfm. I think they sell them at the lawn-mower repair shop. I am only kidding!!!

Answer 3

It depends a lot on what else you have in the engine
cam
turbo or super charger
and how much power are you trying to make
Back when I was a teenager we used a lot of small block Chevy,Ford, and Chrysler's. and would generally put up to 2 CFM per cubic inch. A 650 holly was generally plenty for a 350-360 cubic inch motor with a big cam. A 750 CFM carb on a 350 could be used but would burn a lot of gas if you like having your foot in it. A stock 2 barrel would usually be about 1 CFM per inch.

Answer 4

CFM (cubic feet per minute) has to do with fuel or air flow. Carburators have cfm adjustments. Most new cars air and fuel flow adjustments are regulated by a computer so you can't mess with it. All engines are different.

Answer 5

You need to realize that a cubic foot of steam has a lot more energy in it than a cubic foot of air. Where do you think the torque comes from if it isn't the motor? There are several other things you need to think about like how are how are you going to handle 100 psi steam to get it to the motor and where does the exhaust go. Do you reclaim it or just let it go down the drain.