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JAH's 1968 GTO

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Holley 770 carburetor

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I got a Pontiac 400 with a lumpy cam otherwise stock I have a Holley 770 street avenger and was wondering if I have too much carburetor for the engine it's still on the engine stand looking for an after market intake manifold 

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Hello. I commend you for asking this question. I can and will give you the correct answer based on my experience with multiple carbs and numerous Pontiac engines. No, it's not too big.  The question really is, is it too small? Since nearly every 67-79 Pontiac came with a Rochester carb, and they were rated between 700 to 800 cfm, a 770 is plenty close enough to never cause a problem from being too big. However, the real answer needs more data. For a Pontiac V8, we essentially need 4 things to know for sure what's best, since we know displacement already.

1. Heads, stock or aftermarket aluminum? Ie do they have improved flow rate and if so how much?

2. Camshaft duration at .050". If you open the valves longer, it sucks harder on the carb, hence bigger can help, to a point.

3. Compression: If you are running aluminum heads, 10.5 to 1 compression, it has a totally different need than a 7 to 1 smog motor.

4. Rear gear ratio- the higher numerically the rear ratio, the faster it revs, the faster it needs air.

Many would say RPM operating range. However, when you tell me the above, I will know the operational range of the engine. I ran a 700 Holley 4150, a quadraject, a 650, and a 750 holley, both vacuum and mechanical secondaries on my 1976 Pontiac 400. They all ran about the same. There is no real hp to be gained here. Power is in cam and heads.

I think if you are looking for an aftermarket intake you are preparing to waste $400. It will run slower with any aftermarket intake up to at least 500hp than a factory 1968-1972 IRON intake even considering the difference in weight! I spent the $ on the dyno to prove it. My 469 made 500 hp, 577tq with the factory intake, 10 hp more than a performer rpm, performer, torker, etc...

Give me more engine details like the cam specs. A lumpy cam could be a worn out factory lp cam or a hot solid roller with 250 at .050 duration. What are your performance goals of the car and how much money would you spend to get there? Ive been thru a lot of combos, and can likely give yo some insight of the things you really need to get there. Teaser> intake changes, ignition changes, rocker arm changes, carb changes, and chrome do not produce results. Heads, Cam, Cubes, Compression.


Oh, I have numerous pontiac aftermarket manifolds for sale if the above sound like BS.


Edited by Stripes
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