Thread: Carb question

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Everybody seems to recommend a vac secondary carb when you have an automatic transmission. Why is that? What's the difficulty in setting up a mechanical secondary carb with an automatic trans?

Vacuum from the engine determines when the secondaries begin to tip in on a vacuum secondary carb to eliminate engine bog. And yes you can adjust the mechanical linkage on a mechanical secondary carb to accomplish the same thing, but it's a major pain in the butt and you generally end up with the secondaries not completely open at WOT.
With a Holley vacuum secondary carb you can fine tune the secondaries with their spring package.

Originally Posted by NTFDAY

Vacuum from the engine determines when the secondaries begin to tip in on a vacuum secondary carb to eliminate engine bog.

The bog you mention, what's the cause?

Originally Posted by ceh383

The bog you mention, what's the cause?

A carburetor works off Bernoulli's Principle. Airflow velocity is increased at the venturi(s) because they are smaller than the carb inlet at the airhorn. This increase of velocity creates a vacuum that pulls fuel from the reservoir bowl to mix with the incoming air. If there is not enough air flow (thus insufficient vacuum) to pull fuel and atomize it properly, then you will experience a bog. That's what happens when all four barrels open at low rpm's. Too much carburetor or too little motor, however you want to look at it.

Most carburetor manufacturers will recommend a vacuum secondary carb if you are using a converter that stalls at less than 3000 rpm's. Over 3000, there's enough airflow to support all four barrels and get a good mix, assuming you have sized the carburetor properly for the size of the motor. For max power street and strip, a good rule of thumb is cubic inches times 2 = CFM.

Originally Posted by techinspector1

A carburetor works off Bernoulli's Principle. Airflow velocity is increased at the venturi(s) because they are smaller than the carb inlet at the airhorn. This increase of velocity creates a vacuum that pulls fuel from the reservoir bowl to mix with the incoming air. If there is not enough air flow (thus insufficient vacuum) to pull fuel and atomize it properly, then you will experience a bog. That's what happens when all four barrels open at low rpm's. Too much carburetor or too little motor, however you want to look at it.

Most carburetor manufacturers will recommend a vacuum secondary carb if you are using a converter that stalls at less than 3000 rpm's. Over 3000, there's enough airflow to support all four barrels and get a good mix, assuming you have sized the carburetor properly for the size of the motor. For max power street and strip, a good rule of thumb is cubic inches times 2 = CFM.

I knew someone would come on that could explain it a lot better than I could. Thanks, Richard

Alright, good answer. I have seen Cfm = (engine size/2) x (maximum rpm/1,728)] / VE % will give the correct cfm. Using this formula I would need a 546cfm carb. Using CID x 2 I would need a 710cfm carb....Big difference. A debate on correct sizing is not what I'm looking for here. Here's what I'm after... I have a 390cfm double pumper that came off a circle track(short track) car with a 355cid engine that lived in the 5500 - 8500 rpm range. I'm wondering if it will be worth the trouble to put it on my motor. It has a 355cid built for 2000 - 6000 rpm range, but I have an automatic. The carb is one of THESE.

the beauty of a vacuum sec carb is it gives the engine no more than what it ask for .. but a big enuff engine can easily swallow a double pumper .. i run one on my 429 .. the windsor loves a 750 vac carb..