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How to determine cfm requirement for engine/carburetor

Root Pass

Root Pass

Well-known member
Take this for what it is worth, I read it in a book and I have never seen a formula for figuring out what cfm carburetor to use or at least get in the ball park.

cubic inches X Maximum rpm
_____________________________ = Carburetor cfm X .85
3456

(85% volumetric efficiency)
A BB Chevy 454 operating @6500rpm the cfm would be 853.9 @100% volumetric efficiency. At 85% volumetric efficiency would be 725.8. Round up and you get a 750 cfm carburetor. Like I said for what it's worth I know that there are people on here that are a whole lot smarter than me.
 
That is the basic formula for bare minimum CFM requirement. It works very well for people to not overcarb. It will put you in a ballpark that will have good throttle responce, and decent manners. But it will leave you down on power, even factoring for a 85% VE engine. 85% is a good number for a street engine, but NA engines see WELL over 100%, and I'd say a fair number for a performance engine is between 90-95%. And a problem that occurs when a carb is too small..At upper RPM the air velocity gets too high, and the fuel tends to want to seperate, as well as go rich. (Any circle track guys usin the 4412 500 2 barrell know all about this)

For max power based on a Holley's booster venturis...I try to factor the formula based on minimum operationg range. Holleys with straight boosters usually need approx 20% of their peak CFM rating before they will start to signal the main circuit. So if you have an engine that doesn't see a heavy load till 3500 (The accelerator pump circuit is very important it getting it there BTW), calculate the airflow of the engine at that RPM and multiply the number by 5 and you would see how much carb it could ultimately still use. For max HP you want the least restriction. and as long as it signals, it can't be too big. That's why injection works well...It doesn't have to be signaled..So they have insane airflow through their throttle bodies.

I'm hopefully not missing too much cuz I'm in a rush right now, So I'll be happy to go more into details later.

But some food for thought. About 15 years ago I built a 415" small block that was in a drag application turning 7500RPM. It had DUAL 1050 Dominators on it. The car ran 9.02 in a 2500# car and ran a powerglide trans. The customer wanted 8's so bad he could taste it, and I thought maybe it was overcarbed, so I built two 750s for it..It slowed down 2 tenths and lost several MPH...Bear in mind, the car had a 5000RPM stall converter, so it had that luxury. Like I said..Food for thought.

Sorry for any typos or errors....but I better get..
 
Boblee, I wouldn't pretend to add anything to what Mike Felber just posted. He's the best of the rest IMHO.

When in doubt, go bigger ..... to a point of course. You can always re-jet, but when you hit the wall on airflow you're done.

Also my opinion, most A/C engines out there today are begging for a more modern carburetor when run on an airboat. I have nothing to substantiate this with but I honestly believe that the guys who are running the 520 Connies and the 540 Lycomings are leaving 30-40 hp laying on the table with their intake setups.

olf
 
So on a gpu 220, at 470cu.in it works out to 350 cfm ????? WOW! I had a 650 on my last boat. 13.5 ', 70x36 prop and it ran great. :pirate:
 
I think it all matters how many beers you have had, X(times) how hard you press on the gas?? Maybe it's just me....CFM = cold fermented Malt...
 
Hey Rich,

Sometimes how hard you press on the gas is dependent on how many beers are left in the cooler and what time the store closes. Your analogy is in the right direction! There is, of course, those times when you leave your wife at the bus and someone else supplies the beer in the interim which allows you to run a smaller cfm carb. I still owe you some Bud Lights! :drunken:
 
Excellent reading for the antis' Just my humble opinion but helps to justify the statement that airboaters are just a bunch of drunks.
 
dont worry all the antis at the 19th hole drinking chevets heck even they have a sense of humor :joker:
 
Just as Felber stated all I can add is I have never seen an aplication that should be over carberated more then an airboat. I run 830cfm carbs on most my 347cubic inch LS1's and 850CFM and bigger on my 383's. My opinion with an airboat motor is over jet it and loose the 8HP but keep it from burning holes in thing's. Here is a fact if you correctly jet an airboat to make max power or get the correct A/F ratio you will burn a hole in your piston when running it hard and dry.
 
So on my direct drive 383 will I gain power to go up from a 500cfm?
 
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