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500 cfm carb, will it harm my mileage?

6858 Views 2 Replies 2 Participants Last post by  70Mach03
I've been to the preformance sites and I noticed a intake that was single plane. It recommended using a larger 2 barrel holley carburator. Holley offers the 350 and 500 cfm carbs. Will the 500 harm my wanted revving capabilities and gas mileage? Will the 350 work fine for the single plane?
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I had a Holley 500 CFM on a 1970 302-2V auto fastback Mustang many years ago. The car was stock with the only mod being dual exhaust. I received a very nice sit-of-the-pants boost. I never dyno'ed it as we didn't have access to them in the late 70's, but it felt like probably a 15 to 20 hp boost. I was really surprised but my mpg actually increased slightly. Using a stopwatch, my 0-60 mph time dropped about a second and a half, and I gained about 10 mpg on the top speed. It worked great for me. I played around with the jets a bit and ended up with either 71's or 73's.

Looking at old car magazines from the early 70's showed healthy increases over the standard 2bbl carbs and some were even nearly as good as 4bbl's. For me back then it was a great mod. I always thought if I ever picked up a 351-2V powered vehicle it would be a great mod. Of course back then they only cost around $59 to $79 through the JC Whitney catalog.
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