holley power valve

[70gtx]

what is the correct size power valve procedure. check vacuum at idle and deduct 2?

[amoparguy]

Use vacuum at idle divided by 2. If you have a lumpy cam it will cause manifold vacuum to fluctuate a little bit, causing the power valve to open. You want to keep from having the power valve open while idiling, I don't think 2 steps lower than manifold vacuum is enough.

[JANDJ]

SUBTRACT ONE NUMBER FROM YOUR VACCUUM AT IDLE IN GEAR. EXAMPLE IF YOU HAVE 4.5 IN GEAR AT IDLE USE A 3.5 POWER VALVE.

[jfs mopars]

Deduct 2'' from idle for performance. Half of vacuum at idle will help milage very slightly. As long it is lower than vacuum at idle you'll be fine.

[amoparguy]

I dragged out one of my Holley books (HP Books - Holley Carbs, manifolds & Fuel Injection 4th Edition) and J and J is correct. The chapter on performance tuning includes this paragraph on page 129...

"Lets use as an example a car equipped with a camshaft that provides such low manifold vacuum at idle or part throttle that the power valve opens, giving richer mixtures or perhaps flutters open and closed due to vacuum fluctuations. In this instance, a power valve that opens at a still lower vacuum should be installed. If a 65 power valve is in the carburetor and the vacuum occasionally drops to 5 in.Hg at idle or part throttle, install a 4o power valve (opens at 4 in.Hg). This ensures that the power valve will not open until its added fuel is needed."

It goes on to say...

"The power valve must have a higher opening point than the highest manifold vacuum attained during the run, especially at the end. If this is not the case the valve will close and the engine will run lean. Disaster will result, usually in the form of holed pistons."

Basically they are saying if you have a 30 power valve and vacuum goes up to 4 in.Hg...

Well, you get the idea!

Sorry about the long post but I thought this was interesting information.

[HOLESHOT]

GENERALY ALL THESE ANSWERS ARE CORRECT BUT DO TAKE INTO CONSIDERATION THE WEIGHT OF THE CAR,GEARING SIZE OF CARB ETC.MOST STREET CARBS WILL RUN A 6.5 EXCEPT THE 3310 WHICH USES THE 10.5(GENERALY TO QUICK).MOST HIGH PO CARS WILL USE 7.5-8.5.THE 2" RULE IS AS A RESULT OF A 13-15" IDLE VACCUM IN NUETRAL WHICH MAY GO TO 12" IN GEAR AND THE NEXT SMALLER CHOICE WOULD END UP TO BE 9.5(10.5 IS .5 LARGER).IF THE CAR IS HEAVY THE DELAY FOR ENRICHMENT IN A STREET CAR WOULD BE A BENEFIT AND AS SUCH 7.5 WOULD BE A BETTER CHOICE OVERALL.WHAT CARB?CAR,GEARS,DO YOU TOW WITH THIS?

[70gtx]

no towing, street n strip couple times a year (unless i just cant stay away).70 GTX,6pack (2300) center,76 jets,auto,3500 stall,391 gears.jets are 0n the boarder of being to rich(run 14 - 14.4 on dyno meter.i did a silly thing and bought a napa rebuild and the power valve didnt have any marking on it. it ran fine on the dyno but now its in the car it is flooding at idle.i dont know if it is the float or power valve yet.i'm going to get it running tommorrow nite and look it over.maybe float is to high now motor is shaking around in car and dyno held it still.
how do you tell difference if it is float or power valve.
it did cough when i put msd ignition on because it had gas in cylinders. ythanks walt