I have a Professional Product RPM airgap and its SQUARE BORE.
Now i am getting a Holley 650cfm spreadbore.
I cannot find a square bore 650 vacuum for the life of me..So does an adapter work as a spacer also or is it just an adapter?

-Eric

The adapter is a one inch spacer.

Holley just recently started making their squarebore 650 vacuum secondary carb. Why not use an 1850, 600 CFM carb?
Or get a 750 vac. sec. How do you know 650 is the right size?

I did the calculations and i need around 615cfm..but that doesn't mean anything..My best friend has basically the same setup and is running a 650 and runs good but might be able to go to a 750.
Setup is 9.5:1 compression 360-375(EST) chevy 357(.040).
Another thing is i am having trouble with finding an adapter from spread bore to square bore. All i can find is square bore to spread bore intake.The performer RPM airgap is a square bore intake so thats why i need it..If you have any other suggestions/carbs it would be appreciated.

-Eric

Flip the adapter over.......................

Adapters are notorious for giving carb problems....should avoid them, if possible.

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I could avoid it by going with the 650 Double Pumper route which is square bore.I only have $150 to spend on a carb(rebuilt).

-Eric

Do not follow that formula. That formula does not work for performance vehicles.

I am sure you could run a vac. 750 or a 650 DP with no problems.

I would rather have one a little small over a mis match with a spacer,at least it would run good and have good response.

If it is making 360 to 375 hp, a 650 CFM DP would be conservative.

I would use the adapter as a temporary thing until you can get a good square-bore holley. I used an adapter once and my motor ran fine. But when I finally got a carburetor that didn't need an adapter the motor ran a lot better at the top.

Conservative puts your shoulders in the seat all the way threw,not just begging for a high rpm rush with an indecisive screwdriver. Adapters are indecisive.