Slick 60's

OK so I have been told that you should always drive with the vacume above your power valve press for best gas milage.

I have a 6.5 poer valve in my 600cfm Holley. Does this mean that I should not let the engine get to at or below 6.5 psi of vacume for best mileage?

I just ran 17.5 gal through it, when driving it REALY hard and got 11mpg.

I am hoping to get more like 16mpg this next tank, taking it easy.

any thoughts?

Dan, i was told that a backfire would blow the power valve in a holley.not sure about the vaccuum thing, might be related.all i can tell you is my 65s carb runs way too rich, you can see the black when you stomp on it.i ordered a kit , picking it up today.
it used to get a fair milage considering the FE,4x4 and low gears.was around 15 on long trips but now i think its below 10.i also took off the cast iron manifold and put a edelbrock streetmaster on.hoping between the rebuild and the improved flow of the streetmaster will be a big help.at least back up to 13 would be nice not sure the 70 posi rear and towtruck unit are not going to let it get up to 15 again.

The new Holleys have power valve blow out protection. The older ones, you can buy the blow out kit and adapt them. About the only time you blow a power valve is when the engine backfires. You should be O.K. with your 65 and dropping below that wont blow a valve. Anytime you put it to the floor, your manifold vacuum will go to zero momentarily. No harm done but it will use more gas.

The power valve is there to provide extra fuel when you step on it.
It opens a circuit to provide more fuel.
With this said...a blown power valve will allow this circuit to be open at all times and will cause a super rich condition at idle.

A 6.5 valve is the usual one used for a stock type engine. This can be checked by hooking a vacuum gauge to manifold vacuum, putting the truck in gear and reading the gauge. The inches of vacuum shown should be slightly above the number on your power valve.
That way when you step on it (your vacuum drops) and the circuit opens providing extra fuel.

Example: a modified engine may show 5 inches of vacuum in gear at idle. You should be running a 4.5 valve. If you still had the 6.5 in, you would have a rich condition sitting at a stoplight.

This goes the other way too. If you have alot of vacuum you should install a valve with a higher number.

There are lots of little tuning tricks on a Holley to improve and match it to your vehicle. This is good !

In these days of plug-and-play most people don't want to bother with all these little tricks, hence the popularity of the Edelbrock carb.

Tim Fanatic

looks like I need to do some runs with a vacume gage hooked up.

Thanks for the info guys!

I realy like the Edelbrock for its simplicity and it's ability to "plug and Play".

The more I learn about Holleys the less I am afraid of them

I will let you guys know what I find.

Thanks Again!

Tim described my problem perfectly.still have not had the time to deal wiht that.
you mentioned the 6.5 for a stock engine, i have a 390 bored to +.080 and and edelbrock streetmaster 390 manifold.guess i will have to get a vac gage hookedup after rebuilding the carb.

took my carb to a friend to get rebuilt, he told me that the vaccuum is 2x the valve size or 13 in of vac uses a 6.5 valve. there is currently a 6.5 in the carb, the kit has a 8.5 and the separate valve i ordered is a 7.5. he is going to put the 7.5 in.also of note the newer holleys have the blue reusable gaskets so its possible to change the valve without a lot of dissassembly and cleaning time.

Iceman is correct. The power valve should be approximately 1/2 of the vaccuum at idle. To big a power valve will result in a rich condition.
Also note that there are at least two different sizes of accelerator pumps, a 30cc and a larger 50cc unit. I went to the smaller pump on my center carb and the larger 50cc's on the end carbs. It has smoothed out the mixture problems I was having quite a bit. Still need a little more tuning but Nadine is getting there.
Dan, I can't believe you stole my avatar
U@ss

1) Sounds like I need to put in a smaller one cuz I don't have 13in so that could explane the bad milage when I have to stop a lot.

2) Skip, it is yours I just was using it cuz I like it. If you want I could make one of your truck, All I need is a few pictures.

thanks for the info guys, one of these days I will be good at tunning holleys!

finally got the beast back on. unfortunately it had been raining almost 2 weeks, very damp out and so to pi$$ me off it backfired right off the bat. vaccuum was 17-18 so i then put in the 8.5 valve.running good now with no big black clouds.

Skip and the others may be right about the half-of-vacuum thing when in neautral. I never messed with it that way. I always did it in gear because I never really could get going too fast in neautral. LOL
I actually would check vacuum at launch RPM at the track since I want that circuit to open as soon as possible from MASH time.

Tim Fanatic

Check a write up I have on basic holley tuning and the way youare supposed to select the proper power valve for your application is to put the vehicle in gear at idle and check the amount of manifold vacuum you have example 6 inches then the correct power valve should be a five at the most 4.5 would be even better. Basically the valve should have a setting of at least 1 inch less than your vacummin gear at idle.

guess i will have to get a longer hose for my vac gauge so i can see it inside the cab.was actually thinking of installing it in the dash anyway. then i could watch 2 gauges drastically drop at high rpm, gas and vaccuum.