http://forums.corvetteforum.com/showthread.php?t=1436650

I read this thread over at Corvette. Not a member there so I brought it here. With my DZ pulling 8-9" hg at idle should I really have a 4.5 PV installed instead of the 6.5? If so why did Holley isntall 6.5 PV's for GM?

If the PV opens so much, say at every shift and moving from a stop does the engine get more fuel than needed? Is fuel mileage lowered by too high of a PV number?:confused:

The 6.5 power valve should be fine. It seems that everyone has their own theory on power valves. Your 6.5 PV will enrich the fuel/air mixture sooner than a 4.5 would. That being said, your mileage depends on how you drive. Hook up a vacuum guage and drive around. Are you below 6.5 a lot of the time? If so, try a 4.5 and see if there is a difference. With the 4.5 installed, did the engine develope a lean "stumble". If so, change back to a 6.5 and do not worry about it.
My 427 pulls 16 inches of vacuum at idle. I have a 10.5 PV installed. When I had a 6.5 PV in there, it had a stumble (lean condition), so I went back to the 10.5.
Once you hook up a vacuum gauge to your engine and see what is going on, you can select a PV that suits your engine's fuel requirements and your personal driving style.

I suppose since GM had the 302 running lean for smog so they helped it to run a little better by opening the PV a little more.

I plan on getting a long vacuum hose to watch the vacuum at idle and all ranges this weekend. I have no lean stumble, in fact this engine runs so well it's spooky. Idle has been my only problem and I am pretty sure I have that fixed.

What GM did 38 years ago MAY not have been the best thing to do...

The power valve feeds the main wells, and the main wells DO NOT move any fuel at idle, ever, this is an old wives tale.

The power valve does not determine how much fuel is added, only "when"

"really" on a street car, you want to jet the primary's for part throttle operation, using a vacaum guage and a/f ratio meter, then adjust the power valve channel restrictor for proper wot (primary's only) a/f ratio, then set the correct power valve number based on what vacaum level "needs" the enrichment, ie: goea lean on the a/f mixture.

This is a tremendous hassle, and rarely if ever happens. It is for sure one of the reasons that efi gives better part throttle performance, driveability, and economy.

I actually did tune a holley like that once, tho before a/f meters existed. This was a very hi comp motor on very bad late 70's gas. I could tell it was going lean when it started to surge and ping, and was able to tune that out with the power valve, just by watching the vac guage while I drove the car under controlled conitions.

If the engine goes lean at 6" of vac, put the 6.5 in it. if it goes lean at 10, put in a 12....

Your vacaum at cruise will be much higher than 8-9, and the mains will not flow any fuel at idle.

Thats why Q-jets with adjustable power pistons make great street carbs. It sure is easier to tune.

I went through this with my DZ 302. I installed a slightly larger cam and the motor makes 7-8" vacuum at idle. With the idle mixture screws properly set and stock 6.5 PVs, it was eye-watering rich at idle and when pulling away from stop signs. The Holley tech line said to use 3.5 PVs (1/2 idle vacuum). I tried these and it did clean up the idle. The later-opening PVs caused a lean stumble on tip-in and I had to change the pump shooter from .025 to .028. This combo works well for me, but I might try 4.5 or 5.5 PVs and stock shooter just to compare.

I really don't have any issues. I just was asking for learning sake. I did not realize the PV opens and closes as much as it apparently does. So maybe a good approach for tuning is to go as low a number as you can without causing a tip-in stumble.

Installing a Holley 2 Stage power valve will help with fuel economy and get rid of the engine stumble. :>)