Holley Power Valve Question?

[Michael M.]

http://www.holley.com/data/TechServi...wer_valves.pdf

[Monte M.]

Re: Holley Power Valve Question?

Mike,
You may get a few different responses from different people.

Other members with similar applications might be the best way to go. But, if you want to start on your own path......

First, is the car a 4-speed, or an automatic? If it is an automatic, make sure the car is in drive while doing you vacuum test.

Second, does the car have power brakes? If so, make sure your test is done with the car in neutral and the parking brake on.

Third, Sometime it is best to get a baseline by plugging all your vacuum ports and starting with the actual vacuum the engine is drawing without anything taking away from it. The is give you the actual baseline to start with.

Fourth, How do you drive? When you get down to deciding to go up or down because they do not make the exact size valve you need, how you drive will help you decide. If the car was higher horsepower and you like to enjoy the pedal on the right ever chance you get, then using the 1 1/2" - 2"Hg lower valve might be the way to go. On the other hand, if you just enjoy cruising and not pushing the car too hard, leaning on going in the up direction might be the way to go.


Hope it helps a bit.

[Jim T.]

Re: Holley Power Valve Question?

I recommend the 6.5 power valve for your Holley.
I have Holley carbs on my 68 L79 and my 70 350 since early 70's.

[John H.]

Re: Holley Power Valve Question?

Michael -

No need to agonize over the power valve - Chevrolet engineers specified the #65 (6.5" Hg.) power valve for that application and carburetor, and that's what works best. Use it.

[Anthony P.]

Re: Holley Power Valve Question?

6.5 don't mess around!! this is a stock confiqured motor right? best to leave it that way. IMO

[Timothy B.]

Re: Holley Power Valve Question?

Michael -

No need to agonize over the power valve - Chevrolet engineers specified the #65 (6.5" Hg.) power valve for that application and carburetor, and that's what works best. Use it.

I agree with John and the other posters about the #65 power valve.

Keep in mind the 1.5 or 2" less than idle vacuum is not correct. The correct way to choose the power valve if you even want to monkey with Chevrolet engineering is either 1/2 the manifold vacuum or at cruise speed in forth gear check vacuum going up a slight grade then 1.5-2" less than that vacuum reading. Anything less will result in a rich mixture because the valve opens during cruise enriching the A/F ratio when you want fuel economy and plugs get dirty resulting in misfiring.

[Duke W.]

Re: Holley Power Valve Question?

It sounds like someone is appling the VAC Two-Inch Rule to the power valve, which in most cases won't give the right answer for the power valve. If you want to experiment, try a lower rated power valve, like 5.5 or 4.5". (I'm not sure what is available.) This will delay enrichment, and as long as the engine responds cleanly to increases in throttle opening, then it's okay and might improve fuel economy slightly.

Keep in mind that the OE carbs were set up on the rich side, so they would perform satisfactorily in the highest density air like cold temperatures at sea level, so many owners who only drive in warm weather, especially if they are at high altitude can achieve better fuel economy by leaning out the carb without hurting performance.

Guys get into trouble when they put a big, high overlap cam into the engine that causes low vacuum at idle and low revs - sometimes as low at 5-7", which can cause the OE 6.5" valve to be open much of the time creating and overly rich mixture.

Duke