If you ask or look around...or do some reading, you'll find all sorts of whiz-bang "formulas" and such supposedly designed to help you select the theoretically "perfect" carb size (cfm rating) for your engine...but they all make it WAY more complicated than it really has to be!

I've been at this game a long time, folks, and I've developed a feel for what works and what doesn't. so...to save anyone here the trouble of wondering or asking, I'll spell out it out simply and easily for ya!!

To determine what carb size your engine needs, my tip is to first decide if you want an aftermarket four barrel that leans towards economy, or performance. With thta in mind, my simple "formula" is to double the engine's cubic inch rating for an optimal performance carb, or just add half to it for a milder or economy-minded application. As an example, a 302 cubic inch motor slated for performance street/strip driving will do fine with 604cfm...which puts you into a 600cfm carburetor. If you want that same 302 to be a little less "wild" and deliver better fuel economy numbers, then just add half of it's displacement, or 151 inches to the original 302 and you'll get 453cfm. In that scenario, a Holley 450cfm 4v carb, or even their 390cfm carb will do the trick...or you could go for a compromise and run a 500cfm Holley two barrel or a 500cfm Edelbrock 4v carb!

Got a 400M with a decent cam, aftermarket intake, headers and such? Double that and you get 800cfm! Go with an 800cfm Edelbrock, or a 750cfm Holley (or Edelbrock) and you've got your performance! For a 400M that will be more of a daily driver where fuel consumption is a concern, add half the cubic inches (200) and run a 600cfm Edelbrock or Holley carb for a more economical package!

See how easy it can be?!

Nothin' to it but ta DO it...so toss aside all those old, overly complicated "formulas" and wive's tales and use this simple method to figure out what carb you oughtta be running!!!

[8D]

 

I'm not sure where you got your info from and drew your conclusions to. Selecting carb size has a lot to do with volumetric efficientcy also. An engine is basically an air pump that produces power. If you can't move the air thru an engine that an intake system can deliver you are over carbureting. When you make air flow thru the intake ports and out the exhaust ports ten you can utilize the larger inlet system. Along with the efficientcy there are the rpm band the engine will run in to be considered. Another thing is the cam selection. All this and you have to take into consideration the gearing used in the vehicle AND it's weight.

 

All of those are contributing factors, for sure...but I outlined my approach as a general guideline. Using the accepted industry "formulas" usually nets you a carb that is too small, since the mathematical "rules" represent hopelessly optimistic conditions...that is, a 100% efficient engine operating in perfectly ideal atmospheric conditions...something the true "street" car will never see!

Sure, putting an 800cfm "double pumper" on your 2.3 litre "Pinto" engine is going to be far more carb than you need...but then, according to my "rule" such a motor would only require 280cfm...so a 350cfm Holley 2v would be fine!

It's possible to over-carb anything...but if you follow the basic guidelines I stated, you'll be damn close to what is right for your needs. It's not meant to be the Final Word, just an easy-to-figure "rule of thumb", so to speak.