Carb question

[patmckinneyracing]

The BG carb I have isn't a demon. I think its one of BG's original carbs before holley sued him for producing a "holley look-alike".

I have never ran holleys and have had great success with my edelbrocks. I always hear holley guys complaining about how their holley's won't stay in tune long enough to take the car around the block.

 

[jrm81bu]

well actually to a certain extent, yes you could reduce or increase the cfm of a carburetor.No you can't. not without changing the veturi's. Lets say you have a rochester qjet, or a holley 750 dp, you could install leaner jets, different accelerator pumps, secondary metering hanger and rods and basically make it run leaner. And like you said all you did there is make it leaner, not flow less air.Although qjets scondaries flow as much cfm as needed on the engine, so if you have a stock 305, 307 it will only open almost half way, but if you have an engine that makes twice or three times more power than a stock 5.0 litre than those secondaries will flow more cfm, about as much as the engine needs at wot. With holleys, you could always reduce the size of the main bodyMain body yes, since it has the venturi in it. Although when you change the main body you are rally changing carb size anyhow., jet sizes, accelerator pumps and such. These are petty modifications that only increase or decrease the cfm ratings by very little, so getting a proper carb for your application is best. No those changes only alter the amount of fuel for how much air flow there is. To really change the cfm, you need to change the amount of airflow, only the venturi in the carb will change that.

All that said there are some carbs out there with removable venturies. And then you got the Predator carbs that are a variable venturi design.

 

[joe_padavano]

well actually to a certain extent, yes you could reduce or increase the cfm of a carburetor. Lets say you have a rochester qjet, or a holley 750 dp, you could install leaner jets, different accelerator pumps, secondary metering hanger and rods and basically make it run leaner.

All of which makes it run leaner. None of this changes the CFM. The CFM is ONLY governed by the dimensions of the venturies, period. As noted, air valve carbs like the Qjet and the AFB will reduce secondary airflow based on mass air flow into the carb.

More to the point, making it leaner is not necessarily what you want to do. In fact, a too-large carb may result in a lower signal across the venturi, which would result in less fuel being metered than desired. Bottom line is that unless you have a carb with replaceable venturies or other method of varying airflow, get the correct size carb.