power valves tuning

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RyanUpdike70

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Would going from a 4.5 to 6.5 gain anything on my quickfuel 850Carb i going to change man the 76 jet to 74 to see if it will pick up the motor is 440 30 over stock 452 heads 509 cam m1 intake
 
Really impossible to answer your question with the info provided.

Here is Holley info on power valve selection. Quick Fuel also bas a pile of information.

Selection is really vacuum related and we don't know what you have going on.

[ame]http://www.holley.com/data/TechService/Technical/power_valves.pdf[/ame]
 
You need a vacuum gauge hooked up to full manifold vacuum (center of carb, brake booster port). Tune your engine to maximum vacuum and back it off a little. Take your vacuum reading and divide by 2, that's what you need for a power valve.
 
If your car picked up e.t. by changing the power valve something is drastically wrong. The power valve is for low speed to part throttle cruising. At WOT it's always open, or at least it should be open. Only way it wouldn't be open at WOT is if somehow it has enough manifold vacuum to hold it closed.

Have you read the plugs or have a air/fuel ratio meter installed? I have to run 76's in the primaries on my mild 408 in a 750 carb. Seems like your 440 should want at least 76's as a minimum with an 850 carb. Be careful you don't jet it too lean and hurt the engine.
 
You can also remove the power valve completely and block it off with a dummy. You will then have to increase your primary jet size, usually to the same size as the secondary jets, called "squaring it up". This will make your car more consistant, but use more fuel and be running slightly rich when street driving.
 
RyanUpdike70 said:
Would going from a 4.5 to 6.5 gain anything on my quickfuel 850Carb i going to change man the 76 jet to 74 to see if it will pick up the motor is 440 30 over stock 452 heads 509 cam m1 intake
Click to expand...

FYI A power valve is simply that, a "valve" that is either open or closed, depending on the vacuum. The number determines at what vacuum reading the valve opens to richen the mixture when needed under load/accelleration. The amount of fuel flowing through a power valve doesn't change with the number it's rated at. It's to keep your plugs from loading up at idle and light cruising speeds.
 
Hi Myron. Do you run a Thermoquad on your Dart Sport in the Stock Eliminator class? If so, how is it modified? If not, what do you use?
 
75 Duster 360 said:
You can also remove the power valve completely and block it off with a dummy. You will then have to increase your primary jet size, usually to the same size as the secondary jets, called "squaring it up". This will make your car more consistant, but use more fuel and be running slightly rich when street driving.
Click to expand...


You're pretty close. The power valve is like what Loco said. It's a spring-loaded-open valve that engine vacuum holds shut. Vacuum drops when the throttle's opened, and fuel gets pulled through it. But, it's not really metered, and it dumps unevenly into the intake. Race cars don't need part throttle transitions, so in order to get more accurate tuning, the power valves are pulled and the metered jetting is used to compensate for the fuel the power valve dumped only in a much better controlled fashion. The normal increase is 8 jet sizes when the PV is repalced with a plug. Squaring the jetting means having the same jets in primary and secondary.
 
I'll toss some pennies in the pool here too.

Like mentioned, going to a higher or lower number only changes the point in time that the valve opens. If you need more fuel from the PV circuit, then the channel restrictor needs to be altered.

The guidelines from holley and get something that will work. 1/2 of idle vacuum or 1.5" below idle in gear will usually get you an acceptable PV. Call a couple times and you'll get all sorts of advice from the tech line, one different from the other. I have cars that like a PV with a higher reading than either of those way to choose.

The only real definitive way to get the correct PV is to use an A/F meter and run the engine under a mild load with a vacuum sensor/gauge hooked up. You can see when the PV opens because the A/F drops and whether the engine goes very lean before it does.
 
Sounds like you need to go to your local book store & pick up a holley perf. tuning book & read it, you'll be surprized what you'd learn, not trying to sound sarcastic, but from what your asking it sounds like you don't really understand how a carb works.
 
75 Duster 360 said:
Hi Myron. Do you run a Thermoquad on your Dart Sport in the Stock Eliminator class? If so, how is it modified? If not, what do you use?
Click to expand...

My Dart Sport is in the IHRA-only crate motor segment of their Stock class where they allow a 750 Holley on small blocks.We run with the rest of the "regular" Stockers, just with different HP ratings and indexes. They also have a Stock GT class that allows regular Stocker engines in different year bodies, like NHRA SS/GT. I went with the crate motors because all I ever worked on was Holley carbs. I kinda wish I had the time and patience to learn about TQ's so I could have the option of dabbling in NHRA. But that creates another whole set of problems with time and money.

All we can do is change jets, squirters and pump cams. Choke plate is retained and no grinding, polishing, narrowing, etc. I run without a power valve and jet "square". I'm always tinkering because any changes, like X-pipe or cam, has changed the optimum set-up a bit. Of course, winter to summer can also change jetting a bit. I've been using the 4779's for many years but switched to the legal aluminum Super Street versions, which they also pay contingency for if I'm in a final. They seem to be a little easier to tune and don't run as rich as the 4779's.

I learned a lot from the Holley books published by HP Books, not just about HP tuning but the basics on how they work and how changes affect things. They are found with soft-covers.
 
Thanks for the info, Myron. I was unsure of the IHRA rules in your class. I know some of the other Stock drivers such as Macy, Alford, Beard, Peterson, etc. that I bracket race against in this area, but you're probally my favorite, though I have never seen you race. I read the DRM magazines each month and keep track of the Stock and Super Stock classes. Your car is definetely deadly and so are you on the tree. I hope to see you at a Pro Am event in North Carolina someday. Keep up the good work!
 
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