CFM formula

Is there a formula for figuring out the CFM on a carb that you don't know the size of? I have collected a few old 2 barrel Rodchesters I want to put on my 6. I have a 500 Edelbrock on it know, I would like it to have around 350 CFM's. Maybe I'm SOL on this, I would think there's some way to get close by measuring it though.

 

Well dont know if this will help you but this is the formula for air flow into an engine for optimum combustion. You can use this to find the rate of air your engine needs then find specs on the 2 bl you want to use and see if two will work.

Ma= (D*N*f*r)/1728
D= the displacement of the engine
N= the rpm that you are using or optimum hp rpm ( the rpm you get the most power)
f= 1/2 for a 4 cycle engine
r= density of the air (.0763 lb/ft^3)
Ma= the answer in pounds of air per hour used by the engine so divid by 60 to get lb/min and then divid by 0.0763 lb/ft^3
and that will give you the CFM your engine needs.

 

WTF is all that, I did a formula kind of like that on another site, and I got the info on what I need, so does any body know how big those 2 barrel Rodcesters are. I have a few of them, I figured I would use the biggest??? Maybe I should use one of the smaller ones??? To get my around 300 CFM???

 

Here's one - there are many - search Carburetor CFM Calculator
http://www.csgnetwork.com/cfmcalc.html

Personally, I've found that some engines respond better with smaller carbs and many respond better with larger carbs than any of these CFM formulae come up with.

One myth is that too large a carb will dump to much fuel to your engine - simply isn't so. An engine will only draw in as many CFM as it needs. What happens is venturi velocity goes down with too large a carb.

A venturi requires flow past it to create the low-pressure area that draws the fuel in & atomizes it. If you've got too large a venturi, the velocity goes down and the pressure drop across the venturi isn't quite as high. The fuel that is drawn in isn't atomized as well. That's why plopping a dominator on your V6 is probably going to result in a lot of bog and sluggish response...


EDIT: Just noticed you're asking how to figure out how big the carb is, not how big a carb you need. Short of a flow bench, I don't know any good way. Why not post up what the carb is & perhaps someone will know.

EDIT #2: OK, I see you're using the bigger Rochester 2-bbl. Just do a google search for Rochester 2 barrel CFM or something like that. Also, I believe 2-bbl carbs are flowed at a higher pressure, so comparing CFM to 4-bbl isn't quite apples-to-apples, but then even the 4-bbl guys use different techniques (wet vs dry)

 

Highest RPMs X CID divided by 3456 This is a ball park to what is an average size but as some people have posted , some engines like bigger and some like smaller. I run a 400 Pontiac that is real strong that I race with non ported heads and am running an 800 CFM double pumper and it is very responsive. I had a 750 that ran slower. The formula comes up at around 731 cfm for this engine.

 

I believe the actual cfm flow will vary with different atmospheric pressure and how much negative pressure is drawing on the carb. Sooo..... they have a rating that is not exactly calculable by just measuring the venturi size.

 

They are flowed at a set pressure differential - I believe 4-bbl carbs use 1.5"Hg & 2-bbl carbs use 3"Hg.

So ratings are static. However, there are different ways to flow them using the same pressure drop - dry or wet. Dry numbers (Edelbrock) are typically higher than wet numbers (Holley) - this is why a Holley 390 & a 500 Edelbrock are actually closer in flow characteristics than the numbers would indicate.

In application, atmospheric pressure will vary somewhat and this will affect actual flow for a given throttle setting. That's why most performance flows or dynos are corrected to standard day - 29.92"Hg atmospheric pressure & 59*F.

Thus the reason a set standard is used to flow them - 1.5". This way you're comparing apples to apples (except for the wet vs dry issue) & actual atmospheric conditions don't matter - it's a relative comparison, not an actual comparison.

 

This has realy turned out to be a lot of B.S. I had not counted on. Thanks for all the replys. I supose I will just continue with my plan, and not fret about the CFM factor.

 

Here's how I think about it in simple terms. If a Chevy 216 had 1 rochester, a 235 could handle 2, but it may be slightly over carbed if it's stock. I think of it in terms of cubic inch vs. original carbs. I'm a mechancal engineer, but this is the simple way I like to think about it sometimes. Also, somewhere on the internet there is probably cfm vs. carb number=good luck

 

Look at the two edits in post #4 - if not answers, leads to get your answers.

Thread creep - always happens. Just folks trading ideas - relax.