Power Valve

[01somta]

I bought a motor about a month ago and the guy suggested getting the carb rebuilt. So I got the carb back about a week ago and the shop rebuilt it back to stock specs. I was wondering if with my motor specs I could get an idea of the correct or close to correct power valve size? The motor is not in the car right now and on an engine stand, but I would like to get the carb close to ready as possible.

502 Gen VI 4 bolt main, 9:1 CR
Melling hv oil pump
Crane solid roller p/n 168601 cam .615 .636 246 254 dur @ .050,
crane solid roller lifters
GMPP 12363400 Aluminum Rectangle port heads
GMPP 12361323 1.7 alum Rockers
Edelbrock victor jr 454-R intake
Crank Ext. ballanced, nitriated, crossed drilled gm p/n 10198922
Rods Forged 4340 Steel
Pistons GM P/N 12533507 Forged Aluminum
MSD Pro-Billet Dist.

[Jmartorana]

To do it right you will need to run the engine but as a starting point the standard 6.5 would probably do just fine. After it is started you need to measure the vacuum at idle and divide by 2. That will determine the correct power valve size. You could always go on to Holley's web site and get the directions from there.

Good luck

[Smittys67]

I would start with a 4.5. I always check vacum then go down 2 numbers.

[01somta]

I would start with a 4.5. I always check vacum then go down 2 numbers.

What do you mean go down 2 numbers, I thought you divide by 2.

[Smittys67]

If the vacum is 6 then use a 4 here is barry grant sheet. The power valve, located between the two main
jets, offers a boost in available fuel to the engine
in high load conditions without creating an
over-rich idle or part throttle. Power valves are
marked with a vacuum rating. This rating is the
point at which the valve will open. The lower
the number, the more the vacuum has to drop
for the valve to open. When the engine is under
a big load, it produces decreased manifold
vacuum along with an increased demand for
fuel. When the manifold depression is less than
the rating on the power valve, it opens and
allows extra fuel to enter the main circuit of the
metering block. It is a good idea to have a
power valve rating that is lower in number than
the vacuum produced by the engine at idle.

Hope this helps.