picking a power valve

Holley says 1/2 of engine vacuum...others say 1 inch less ..others say 2 inch less...how do you pick a power valve..im thing use what ever works the best..lol..

whats the combo? weight, stick auto? more info please

BOSS 429 wrote:whats the combo? weight, stick auto? more info please

auto c4..rpm intake..9 inches of vacuum..3000 pounds(guessing) 500 lift roller cam..heads flowed 306@600... 3.73 gears 26in tire..650 proform carb

power brakes? 1,check for vac leak, does it stay running smoother if choke is on, or if your hand is over carb, or test it by having someone in the car,and lightly push acc pump.

if its a lumpy cam,and fuel is the prob id start with a picture window valve,or some call it a big block valve, 6.5,make sure ports are clean in carb ,air bleeds etc.

No "Golden Rule" applies to correct P/V selection, IMO. Install a vacuum gauge and monitor it during operating conditions, not at a no-load, idle condition. Also several variables, power valve enrichment circuit diameter, type of P/V (window size and stage or stages) and of course opening rate of P/V. Although the opening point is stamped on a P/V, it could vary under operating conditions. The numbers on the P/V are much like the numbers stamped on a main jet, a simple reference point.

Happy motoring,
Dave.

LivermoreDave wrote:No "Golden Rule" applies to correct P/V selection, IMO. Install a vacuum gauge and monitor it during operating conditions, not at a no-load, idle condition. Also several variables, power valve enrichment circuit diameter, type of P/V (window size and stage or stages) and of course opening rate of P/V. Although the opening point is stamped on a P/V, it could vary under operating conditions. The numbers on the P/V are much like the numbers stamped on a main jet, a simple reference point.

Happy motoring,
Dave.

ok, so at what point do i check the vacuum gauge number? full throttle..cruise? part throttle?

take a reading at idle in gear with tire chalked around 1000rpm save #. cruise car @ 1500rpm, save # 2000rpm save # 2500rpm save# then average the 4# and 1/2 that is your pv

quick 52 wrote:take a reading at idle in gear with tire chalked around 1000rpm save #. cruise car @ 1500rpm, save # 2000rpm save # 2500rpm save# then average the 4# and 1/2 that is your pv

ok..ill try it

As "quick 52" wrote of his theory to check vacuum as we are concerned, that may not be "johndickjr's" habit of driving. If your driving habits follow the criteria that "quick 52" posted, then that would be a good starting point. My suggestion, install a vacuum guage at the base of the carburetor (direct source, not ported), a vacuum guage installation that will allow you to observe the vacuum reading while performing everyday driving. Take note to the lowest vacuum reading and at what point the engine speed is and the status of your application (i.e;stopped at idle in gear, pulling away from a stop, cruise (steady speed) and last but not least engine speed during cold start up condition (choke applied).

The PVEC simply adds fuel to the engine during acceleration aiding the main jetting. In most non-modified carburetors, a 8 to 10 jet SIZE increase of fuel is allowed to enter the engine when the power valve is open. My point? If your main jetting is close, then your power valve opens to quickly, can your engine with stand the additional amount of fuel the PVEC offers? A over fueled engine can be a dog, much less accelerate wear on given internal parts or possible failure.

If I were working on the application you mention, I would find the lowest vacuum reading during normal driving, is the lowest reading consistent and depending on my driving habits, install a power valve with an opening rate 3 to 4 inches of vacuum higher than my lowest reading. Remember, the power valve will (more than likely) open during medium to hard acceleration anyway, regardless of the power valve's rating!

Dave.

LivermoreDave wrote:As "quick 52" wrote of his theory to check vacuum as we are concerned, that may not be "johndickjr's" habit of driving. If your driving habits follow the criteria that "quick 52" posted, then that would be a good starting point. My suggestion, install a vacuum guage at the base of the carburetor (direct source, not ported), a vacuum guage installation that will allow you to observe the vacuum reading while performing everyday driving. Take note to the lowest vacuum reading and at what point the engine speed is and the status of your application (i.e;stopped at idle in gear, pulling away from a stop, cruise (steady speed) and last but not least engine speed during cold start up condition (choke applied).

The PVEC simply adds fuel to the engine during acceleration aiding the main jetting. In most non-modified carburetors, a 8 to 10 jet SIZE increase of fuel is allowed to enter the engine when the power valve is open. My point? If your main jetting is close, then your power valve opens to quickly, can your engine with stand the additional amount of fuel the PVEC offers? A over fueled engine can be a dog, much less accelerate wear on given internal parts or possible failure.

If I were working on the application you mention, I would find the lowest vacuum reading during normal driving, is the lowest reading consistent and depending on my driving habits, install a power valve with an opening rate 3 to 4 inches of vacuum higher than my lowest reading. Remember, the power valve will (more than likely) open during medium to hard acceleration anyway, regardless of the power valve's rating!

Dave.

i bought a new nice vacuum gauge set, i understand it will open anyway, what a person is wanting is it to open at the correct time..i forgot to say that this is strictly a bracket race car, i can drive it to test changes ive done

johndickjr wrote:i bought a new nice vacuum gauge set, i understand it will open anyway, what a person is wanting is it to open at the correct time..i forgot to say that this is strictly a bracket race car, i can drive it to test changes ive done

Enough said john! Install the guage and do your thing at the strip, watch the guage and adjust accordingly!

Dave.