vacumm in gear is 9 to 10 inches of vacuum.what size power valve to use.carb is an 850 aed db pumper on a 418 stroker small block solid roller cam,current pv is 3.5 or 4.5 have to open carb up to verify which one.

Street car? If so, the cruise A/F and vacuum are the biggest pieces of the puzzle. DPs are typically very rich on the cruise and transitions and as such like a later opening PV than a more street oriented carb.

Do you know the A/F ratio and vac reading at cruise. Tranny and rear gear info would help, too.

As long as your not idling on the transition circiut any power vallve should work well, I would probally try a 8.5 or maybe a 7.5 to be safe I do work on the all the circuits to get them to work the way I want them to Lean is mean Fat is fat

I have always had good luck with a 3.5 in the front and the rear blocked.

I don't focus on idle vacuum but more cruise and go a bit below that so if you cruise at 10 inches of vacuum for example then a 7.5 or 8.5 should be close BUT you need to experiment to adjust how fast you need the enrichment to suit your needs.

For my heap, I monitor finish line vacuum. For a street/strip, as stated, lots more to consider.

I used to run a 3.5" too and thought I was doing good, had always been told to run a PV that was 1" less than my idle vacuum.

After several phone conversations with Dom he finally got it through my thick head that idle vacuum had nothing to do with it. I now have a 6.5 in it now and my LM2 recordings look better.

for some odd reason an 8.5 has always been magic p/v in a 7-800 cfm carb for me.

I might add that all PV's aren't the same. Many aren't even close to what is stamped on them - especially those that come with a new carb kit. Most accurate ones I have found come from Quick Fuel. They only sell the 4-door Hi-flow valves.

Re: what size power valve

Just to clarify: power valves don't have sizes, those numbers are vacuum levels. The "size" is not in the valve, but in the carburetor body, it's the PVCR (power valve channel restriction) and without going to aftermarket the adjuster is a drill.

PV selection depends on what type of driving you are doing. For drag racing it doesn't matter too much since you're at WOT all the way down the track. You can keep the engine from loading up by running a PV on the primary and leaning out the main jets.

For a street car you set the PV so that the enrichment circuit kicks in when it is needed. You have to drive the car to figure out where that is. Drive the car with a vacuum gauge hooked up and see what it tells you.

Nobody can tell you what the PV needs to be in your car since we don't know how much it weighs and what gears are in it and how modified the engine is or anything else.

So true and I only use quickfuel 4 Window pvs now which flow more fuel and allow more precise tuning of the pvcrs.

vehicle is 88 dakota 3340lbs w/driver.9.7 compression ratio, 340 stroked to 418,cranecustom solid roller cam,weiand single plane intake,aed 850 db pumper,timing is 20*at idle and 36*all in by 1800rpm.3.91 rear gears/295 55 15 m/t drag radials.727 reverse pattern, full manual valve body.10 inch converter flashes 3600 on motor and 4200 on 100 shot.best 1/8 mile 7.24@95.5 in street trim on motor.at 40 mph at a cruise im pulling roughly 14 to 15 inches vacuum.at idle in gear 9 to 10 inches vacuum.ive never played with the pv in this carb just trying to figure out if a higher number pv would make any diference.thanks mike

If you drive it on the street a fair amount then you want to set the power valve so that it comes on at about 1/2 the cruise vacuum level. So for you something in the 7,8 or 10 range should work for you since you cruise at 15 inches.

But if you drag race the truck then you want to make sure that the PV is open when you need it to be. A 8 should work just fine but you really need to double check the vacuum during a run to see where it is. Going down the track you should be at WOT and the PV should be open the entire time, but at the starting line it may or may not be open depending if you foot brake or not.

Vacuum at idle shouldn't really matter for PV selection. If having the PV open at idle causes a problem then you have other issues with your carb.

I agree with Andy's answer. The only thing I would add is that IF you had a WBO2, the best way to set the PV opening is just before the lean spike happens. And the cleaner (leaner) your car (truck) cruises, the sooner that happens so the sooner the PV needs to open to compensate.

If however, your carb is rich in the transitions and cruise, that earlier opening point is giving it more fuel than it really needs or wants. But most guys do not put that fine a point on it. If you have to guess, then you are better off with an earlier than needed opening rather than a later than needed opening.

Our engines are much more tolerant of overly rich conditions than they are of even slightly lean conditions. So, if in doubt and without any A/F info, there is less chance of a problem with the earlier opening.

If you are experiencing no driving problems on the street with the PV you have, making a change will probably not help anything. But at the track, changing to an earlier or later opening may impact the 60'. Only testing will tell.

I am with Dom on this .its load dependant , if you are running a high stall it will affect choice as well .i have about 4" @idle but i use a 6.5 QF pv to get drivability and tip in correct . I have a 5000 stall vert

Tex