Holley power valve selection

Hi,Cuz I bump my head alot AND I do not run a power valve,I forget how to tell my pal how to select the correct one for his Chevelle. Can any one help me out here so I can help him? Tanks!

 

vac*** reading in gear, cut that number in half will give the p.v. you need. well at least that how i do it with good luck everytime

 

That's how I've been doing it with good results.

 

Best way is to connect a vacuum gauge and actually drive it. In gear and stopped will only get you close.

 

x2 on this method,,, I tape a gauge to the windshield, drive part throttle accel and then full throttle accell. Take half that reading and you will right on.

 

Just to be clear, don't you mean "half-way between" those two readings?

Other than that, unless the engine is modified, stick with what the factory recommends. On my O/T 327 Corvette, the vacuum gauge method told me it needed an 8.5. Put it in. didn't work as well as what Chevrolet recommended from the factory (6.5). Don't ask me why. I'm sure they had a lot more time and money into figuring it out than I did, so I'll take them at their word. On a modified engine, use the va***m gauge method to start, and move up and down a couple of numbers to find the one that feels best on the "seat of your pants"

 

Power valves stay "closed"...ie; no enrichment, under vacuum. Sooo....if you're idling around 5.5 -7.0" in gear...the logical choice would be; #45 ie; 4.5". As the cam "lopes"...you want your P/V number ( opening ) to be lower.

Simple...really. Generally, even with a very hot cam, ( unless an automatic car w/no hi-stall converter ) a # 55 will do the trick....but check .Vacuum gauges are $10.....& you need one anyway.

P/V block-offs will hurt mileage & low-speed "load-up" situations. I wouldn't.

 

Cool Thanks to all who take the time to help out! I will relay the info! R~R

 

I would strongly advise against running power valves with sideways mounted carbs on a tunnel ram. Could be wrong, but if memory serves, thats what the O/P runs.

 

I generally do part or half throttle in high gear, than a full throttle in high gear, I take a look at both readings lets say it drops to 8 inches at part and6 inches at full. depending on how the car acts i'll put a 6-7 valve.

 

Say the idle vacuum is 5 inches; what is supposed to happen if an 8 is put in?

 

I thought choice of power valve was determined by what al***ude the engine was running in.
Hence part of the issue when moving up or down in al***ude with poor engine performance.