sorry for insulting the carburetor gods with such a dumb question, but what does cfm mean for carburetors, is a carburetor with more cfm's better than one with less. What kind should i get for my car? 650 cfm? 700 cfm?

 

CFM = cubic feet per minute
it's a measure of the "volume" (not weight) of air that the carburetor will flow @ the standard pressure drop of 1.5" hg (mercury).

The amount of air that a carb can flow is directly dependant on the pressure drop.... or pressure differential between the top of the carb (air cleaner area), and the bottom of the carb ( intake manifold).
On a bigblock engine a small 600cfm carb can flow 1000cfm if the engine creates enough pressure differential.
The downside is that pressure differential creates vacuum in the intake manifold at full throttle which decreases the air density and costs you power. This is why bigger carbs are matched to the larger engines.
A 600cfm carburetor is a pretty good match for a 305 engine.

 

And, the beauty of the factory quadrajet, which came on thousands and thousands of GM cars, is that it adjusts itself to the flow needs of the engine (up to its maximum capacity, of course, which is somewhere between 750 & 800 cfm). If your 305 can only use 500 cfm, it will adjust its "size" to 500 cfm. If it can use 650, it makes itself a 650.

That's why it's typically a waste of money to put an aftermarket carb on a 3rd gen engine, from a performance standpoint. Fuel economy typically suffers as well, since the q-jet has small primaries, which increases the flow velocity and therefore the vacuum signal and therefore effeciency. Oversizing an aftermarket carb to the engine is as bad for performance as undersizing, and will definately reduce economy.

 

I've read that as a rule of thumb.. 2 CFM per Engine Cubic inch is a decent guideline to follow.

350 = 700 CFM's
305 = 600 CFM's

BC