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Carburetors Carb discussion and questions. Upgrading your Third Gen's carburetor, swapping TBI to carburetor, or TPI to carburetor? Need LG4 or H.O. info? Post it here.

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Old 08-27-2004, 10:04 AM   #1
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Join Date: Sep 2003
Location: Bucks County Pa
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Car: 85 Firebird
Engine: 355 Vortec TPI
Transmission: 700r4 with Pro-Built goodies
Axle/Gears: LS1 3.42 Posi Disc

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750 on 355 sbc, overkill?

I bought a carb that was off of a coworkers drag car (351C Maverick) he had since blown that motor and now has a 302 and says that the carb is too much for it. The carb is a Holley 750 Dual Feed mech choke/secondaries with (i believe 68 and 72 jets in it). Will this carb be over kill for my 355 with vortec heads, flattop pistons, crane .454/.454 lift intake/exhaust, and edelbrock performer rpm manifold?
I want to have good power but i dont want totally absurd gas mileage figures. I really dont know much at all about carbs but what i know is that the bigger the carb the less velocity you have on the intake sidein turn, the less manifold vaccum you have. With low manifold vaccum there are other negative effects. Can someone with more experience in this area shed some light on it for me? Thanks. -Bob
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Old 08-27-2004, 10:26 AM   #2
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It's probably OK. 750 is usually a good size for a mdoerately warmed-up 350. Sounds like it could use some significant tuning though. Usually the requirements of a strip-only car are quite different from a car that's driven on the street.

Carb size does not affect manifold vacuum. The one has nothing to do with the other. It does however affect venturi vacuum; which is the pressure drop created in the carb bores by the air velocity as it passes through them, basically the same effect as an airplane wing except inside the bores instead of on the outside of the wing.

You'll probably also find that you'll have linkage issues, especially if you have an auto trans.
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Old 08-27-2004, 10:52 AM   #3
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Join Date: Sep 2003
Location: Bucks County Pa
Posts: 183
Car: 85 Firebird
Engine: 355 Vortec TPI
Transmission: 700r4 with Pro-Built goodies
Axle/Gears: LS1 3.42 Posi Disc

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Yeah i have a 700r4. It has the ford style throttle linkage. He told me that at any auto parts store would carry a gm throttle/detent linkage conversion package for like 15$ or so.
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Old 08-28-2004, 07:07 PM   #4
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Car: 86 firebird with 98 firebird interi
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The 750 will work great on your combo once you get it tuned properly. Like RB said just about every and any 350cid engine will easily take a 750cfm carb.

Now if this is a daily driver with which you wont be at the track with, a better choice would probably be a 650 vacuum secondary. You will first off notice a big mpg increase over the 750DP, and secondly with the faster velocity you'll have better throttle response off idle, but once your up and running the smaller carb will definately fall victim of being too small and you'll be losing out on higher rpm WOT power.

Goes back to honestly knowing what the cars intent is going to be. If it's just a toy on nice days, stay with the 750DP. Otherwise if it's more your daily driver, use the 750DP and keep your eyes open for a either a 650 or 750VS.

A 750VS would be anthoer good choice, but will feel a wee bit lazier off idle than the 650 but will make up for it once your up and running, so either more torque or more hp, with the VS sizes. If you want the best of both worlds power wise stick with what you got and dont expect to set mpg records.
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Old 08-28-2004, 07:07 PM
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