CFM question..

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Jan 5, 2008 | 07:05 PM

TheNomad

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Joined: Apr 2002

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From: Texas

CFM question..

This may sounds really dumb but the other day I was looking at my Jegs mag and started to think what gives a carb its CFM rating? I mean the Holley 500 CFM looks the same as the 750... Is it the jets, air bleeds or what if thats the case why would I pay a ton more for a bigger CFM rated carb then geting a smaller one and bumping up the jets and bleeds, I have a 600 cfm holley is there any way to make it a 500 cfm?.

Jan 6, 2008 | 12:35 AM

fmc400

MSEE

Joined: Apr 2004

Posts: 10,386

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From: Austin, TX

I might be wrong on this, but I believe the bore size of the venturis is the primary factor. The CFM is basically how much air it takes in - so it makes sense that the larger the throat of the carb is, the more volume of air it can take. What's important with CFM is that you don't "drown" your engine or starve it. For a given engine displacement, there is an ideal range of CFM. When you're jetting and rodding the carb, bigger isn't necessarily better - you start with what's given, and dial it in until it runs right, whether that means jetting bigger or smaller.

Jan 6, 2008 | 06:46 AM

LxMan1

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Joined: Dec 2001

Posts: 22,436

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From: Louisville,Ky.

The bore size is correct. Also, some (Edelbrock I think) use the same size venturies, but had different size boosters in them. The 600's have thicker ones than the 750's to restrict airflow that way. It's cheaper to use the same main body and restrict the airflow with the different sized boosters.