The Stovebolt Site Forums The Shop Area Engine Shop 750cfm Carburetor for 305sbc?

I wanted to know if a 305sbc could handle an Edelbrock 1407 750cfm carburetor. It also has the Performer RPM intake manifold. So, would me 305sbc be able to handle 750cfm ok?

- Shane

Shane: I'm going to be gentle here, yes if you want to overload, the true answer is No to make that 305 sing I would use 600 cfm elderbrock.

A 750 is more compatable for big block (cubic inch) or super hopped up 350's that has the right pistons, cam, heads, and proper headers. pops

popcorn The Truth is Out There (About as plain as I can put it son)

Have to agree with popcorn. Go with something in the 500-600 cfm range. 600 cfm is good for up to around 325 hp, depending on the manufacturer specs.

Here's a quick way to figure the CFM an engine actually needs, and the results are going to surprise some folks:

Divide the cubic inch displacement by 2. Multiply this figure by the max RPM you plan to run. Divide this figure by 1728. The resulting figure will be the CFM the engine should theoretically be able to use, with 100% volumetric efficiency. For a more practical figure, multiply the theoretical CFM by .9, or 90% Volumetric efficiency, which is about right for a street engine without a radical camshaft and big ports and valves.

Using this formula, your 305 has a theoretical CFM of 353 at 4,000 RPM, and a practical CFM of about 318. I'd say a 750 CFM carb would be WAY over-carbureted! A 500 would be about right. If you want the best possible solution, use the factory Quadrajet calibrated for a 305, with the stock manifold for the carb. The secondary air valve prevents the carb from opening too far and leaning the mixture out, and the jetting and manifold design have had untold hours of factory dyno work built into it.
Jerry

Depends on the carb. The rochester Q-jets had a spring loaded air flap over the secondaries. The carb would only give the engine what it wanted according to vacuum demand. The airflap would open progressively at full throttle on a small motor, to keep the airspeed thru the carb venturis at a high enough speed for a good mixture independant of RPM. The Q-jets were all rated at 750-800 CFM, and were factory equipment on everything from 305s to 455s. They were just factory calibrated for each application.

On the other end of the specturm is the Holley manual secondary models. When you mash the gas, it opens all 4 bbls at once with no airflap. Manifold vacuum goes to squat, resulting in a huge BOG until the motor RPMs catch up to the carb's throttle position.

Some edelbrock carbs operate on the same principle, so see if your carb is a Q-jet replacement or a Holley replacement.

I must respectfully disagree with Mr. Lincoln on one point. The Performer (not RPM) manifolds are equal or superior to many chevy stock intakes in weight and port CFM. They can provide a slight HP increase on a stockish motor with a significant weight savings. The RPM model will hurt your low-end torque below 1500 RPM. But you'll see gains in midrange and top end with proper cam and valvetrain selection.

chip

Ok guys thanks, I was outbid anyway on it and decided best to just get a 1405 (600cfm manual choke) thats either new, or barely used. That one 750cfm only had 300 miles on it to break in a motor. Ok 600 CFM it is, thanks!

- Shane

Is there a reason why you're going with the manual choke? I went with the Holley, vacuum secondaries and electric choke and couldn't be happier.

Well, I figure I have more control over the carb that way, and if I need more RPM,s, like say when running a winch then that would be helpful, or when running a Tranny powered winch (I think those are good examples). If I need the electric choke, then I have a 1406 that has it. But my 1406 is worn out and needs replacing.

- Shane