Carborator selection chart

I found this chart while researching my intake manifold. Thought I would share. What do you guy think? Accurate?

 

What are the units for the plotted #s?

 

I'd assume CFM.

 

I will have to print that out when I get home. It really tells a fella just how little carb is needed. It always makes me laugh when I see a 300 ci or so V8 with a big ol' honkin' Holley 750 stuck on it!

My little 2bbl is just fine.

-Hill

 

whatt ya guys think about the new holley truck avenger 470 cfm on the 225?

 

rejet the 450cfm. 2-barrel is enough carb for my taste.

 

For the folks not in-the-know, wtf is cfm?? :?

 

cfm = air flow thru the carb. cubic feet per minute. Other factors are needed in the formula. Cam lift and duration, port restriction in and out, exhaust scavenging, ignition duration, advance and timing, as well as compression ratio.
Bottom line is ya cant burn a given fuel air ratio twice. As fast as ya stuff it in, ya gotta push it out.

 

Thanks Maxx. So....air is measured in feet per minute?? How the heck do you measure air feet? :? Is density figgered in? (I believe you, it just doesn't make sense to me--not a gearhead )

 

Alan: It's "cubic feet per minute". A little easier to "picture" that way.

 

Hope I don't make this more confusing - I assume it actually is a cubic foot of air at atmospheric pressure. Gases increase in volume with a decrease in pressure, and the change in air pressure between the atmosphere and the manifold is what makes the air flow from out to in.

So, the use of volume of air isn't very accurate unless you specify the pressure. The pressure changes as the air goes down the airhorn, past the venturi and into the manifold. Fortunately the relationship between pressure and volume is simple (P1 * V1 = P2 * V2) as long as the temperature stays constant. If you needed to, you could calculate the velocity of gas in the venturi if you knew what the volume is in (standard) CFM and what the pressure is at the venturi. Not that you would ever need to unless you were using a flow bench for a new carburetor design or such.

I expect this is where the "volumetric efficiency" part of the CFM rating comes in. Holley says that the VE of street engines rarely gets above, say, 70%. This means that the pressure in the cylinder before ignition rarely gets above 70% atmospheric becasue of the flow restrictions imposed by the intake tract. Race engines flow more freely, so the VE is higher. For a street engine then, you only need 70% of the calculated CFM (displacement * RPM) in order to fill the cylinders.

 

Tim: You're absolutely correct. I thought about that as I was typing it but didn't want to muddy the water.

By reducing your CFM at the carb 70%, you "might" also reduce your overall VE--sort of a "slipery slope" kind of thing.

 

Yup. If you sized the carb expecting 70% VE, you might actually get, say, 60 or 65% peak VE. I'd look at it from the other direction though as a diminishing returns effect. The larger you make the input tract, the higher the volumetric efficiency is, but the more slowly it approaches the ideal 100%. Importantly, as the diameter of the intake tract gets larger, the velocity of the air decreases. Atomization depends on velocity (Bernoulli effect), so at some point your control over the mixture starts to fall off.