How to determine what size carb you need

4x4 Rock CrawlerI’ve been trying to figure what I need in a new carburetor for my Bronco and have been struck dumb-founded by the multitude of possibilities in make, and size. Okay, so what the heck do I need? Well, out of the blue the other day my Dad calls, he was reading in Hot Rod magazine about doing just that.

My Dad, being a Corvette guy (I think he’s had close to six of them over the years now – in fact he has two right now), was at his mechanics a while back, and was looking at the carb on his vette. With a few quick keystrokes it came up that his carb was bigger than what is needed. Fast forward to this week, he comes across an article that gives him this equation:

(cubic inches) x Max RPM (shift point) / 3,456

Pretty straight forward. Using the cu. in. of your block, in my case 302. Then you take your max RPMs at your shift point and multiply it by the cu. in. of your block. This is sort of the tricky one, if your engine can red line at 7,000 RPMs but the best place to shift is at 5,500 RPMs, then use 5,500. I can’t find a good calculator or chart for this, but it’s safe to assume that for my application 5,000 – 5,500 is a good point.

The take your total of those two numbers; 1,661,000 and divide it by 3,456. And I get 480 for the CFM of my engine. Viola! Now, I have no idea what the significance of the 3,456 is – so don’t ask. I tried to find and couldn’t. The next time I am at the mechanics I will ask him if he knows.

I could easily get a 500 or 550 CFM carb and be good to go, and this would still give me an acceptable carb for a 350 if I wanted to step up to a larger engine – which I will most likely not do. It’s a Bronco, not a Mustang after all.

Here’s a little calculator for figuring out what your carburetor CFM should be; have fun!