carb power valves

[moneymaker]

What does all of this mean? I am looking to rebuild my carb for circle track application and found out they are a bunch of different sizes available. What is the benefit of different sizes?....

The standard by which all other power valves are measured. QFT® exclusive in-house design has become the power valve of preference by professional race teams in virtually every circuit where carburetors are used. Not only does the FOUR DOOR™ power valve have a much greater flow capacity (vital for alcohol applications), but a quicker response rate and tighter limit bands allow engine builders to precisely tune the power enrichment phase of their calibration.

Each FOUR DOOR™ power valve is hand tested and set to the stamped value. The opening and closing specifications are ½ inch HG above and below the stamped value. The thicker molded diaphragm adds increased protection against backfire induced rupture. No one offers a broader selection of opening values than QFT®.

[MadBill]

The rating stamped on a power valve is 10x the vacuum at which it will open to add fuel as the throttle is opened and the vacuum drops. A fairly typical number for race engines is 45, which will open at 4.5" Hg. Many street engines use 65s.

The valve does not control flow. Flow is determined by the size of the Power Valve Channel Restrictors (usually ~ 0.050" -0.065") which can be seen in the metering block when the PV is removed. In most cases (not including alcohol apps) the 'window' (Vs. drilled feed holes) Holley style is more than adequate to ensure metering is controlled by the P.V.C.R.s, not the valve.

Typically, if a PV is used only on the primary, ~6-8 sizes larger jets are required on the secondary to make up the difference.

[SS402]

The # on the power valve is usually your highest vacuum minus 2,
so if your manifold vacuum at idle is 10inches you would select a #8
power valve..

George

[Warpspeed]

What does all of this mean? I am looking to rebuild my carb for circle track application and found out they are a bunch of different sizes available. What is the benefit of different sizes?....

The power valve is an economy device only, and is not really applicable for ANY type of racing application. As soon as you floor the throttle, manifold vacuum will drop so low, any power valve is going to snap fully open.

All a power valve does, is allow the engine to run economically lean at high constant road speeds with very small throttle openings.

And you are certainly not going to be doing anything like that with a circle track engine.........

Whatever came with the carb will work fine, and there is no advantage in changing it for something different.

Power valve tuning can be really good for a street car that does a lot of freeway miles at constant speeds. It has the potential to improve fuel economy, that is it's only purpose.

For on/off the throttle fast driving, the power valve does nothing at all useful, it is for economy only.

[revolutionary]

I have no problems using PV's in race carbs but you have to be careful especially in circle track application that require some kind of small carburetor where you are running a big cam. If you select a low PV like a 25 or 35 because you have really low vacuum at idle and are resrticed with a small carb, your PV may close back up at high rpm creating a lean condition.

[Shurshot]

I have no problems using PV's in race carbs but you have to be careful especially in circle track application that require some kind of small carburetor where you are running a big cam. If you select a low PV like a 25 or 35 because you have really low vacuum at idle and are resrticed with a small carb, your PV may close back up at high rpm creating a lean condition.

That sounds a lot like why I was told I broke 4 pistons and the person who told me of that possibility happening had no idea that he had just described the scenario of when the smoke started coming out my left exhaust

I had told a friend that a carb manufacture tech line had told me "return to square" the 950 CT carb I had gotten from them and put the 4.5 PV back in my 496's secondaries with a 680 lift XE grind solid roller. My friend responded that if I did that and went from WOT to 7/8 throttle the PV would incorrectly detect a cruise mode and cause the motor to go lean at a fairly high rpm. (6500)

That is exactly what happened and as a result I broke 4 pistons but by the grace from above there was no damage my block....... still I do not have to tell anyone here the price of all the parts plus the major hassle I had to go through.

I have dealt with some people that have followed up bad mistakes with good moves that have made things right but to this date QFT has not stepped up to the plate (other than not charging me for removing the PV and re-jetting it after I sent the carb back to them)

Doug

[jmarkaudio]

Another to avoid is running a PV in the rear on a drag engine. No extensions for the PVCR's, they can be uncovered at launch... Jet up 6 to 8.

[MadBill]

Actually, there are (or were) power valve caps for just this reason. They fit(ted) snugly over the rear valve and have/had a feed hole at the bottom, where air couldn't easily be picked up...

[jmarkaudio]

That may be, but I don't see the need on a drag engine for a rear PV. When will you ever be going from the secondaries open part way to full throttle unless you had 1 to 1 linkage with an inline stop? Just one more thing to possibly fail and cause a problem anyway.

[moneymaker]

Thanks for all of the replies.... now the question is, do I replace them as normal maintenance or how do I tell if they are still good?

[MadBill]

That may be, but I don't see the need on a drag engine for a rear PV. When will you ever be going from the secondaries open part way to full throttle unless you had 1 to 1 linkage with an inline stop? Just one more thing to possibly fail and cause a problem anyway.

The only benefit I can see is that if you're really trying to get the jetting right on, it's hard to know exactly the number of jet sizes to step up to compensate for the secondary P.V.C.R.s. With two PVs, you can just match the restrictor and jet sizes for P & S. (assuming full removal of choke housing etc. to equalize airflow)

[lluciano77]

That may be, but I don't see the need on a drag engine for a rear PV. When will you ever be going from the secondaries open part way to full throttle unless you had 1 to 1 linkage with an inline stop? Just one more thing to possibly fail and cause a problem anyway.

The only benefit I can see is that if you're really trying to get the jetting right on, it's hard to know exactly the number of jet sizes to step up to compensate for the secondary P.V.C.R.s. With two PVs, you can just match the restrictor and jet sizes for P & S. (assuming full removal of choke housing etc. to equalize airflow)

There are a lot of Holleys that came with different size venturi in the primaries and secondaries. They are more like squarebore/spreadbore carbs and will require stagger jetting front to back. I have a 4609 with this feature on top of having truck style boosters in the front and downleg in the rear.

You can jet the primaries for cruise and find your lean limit just like you would setting the vacuum at idle. As soon as a surge is felt go back up a size or two. I start the secondaries at the stock jetting and then adjust them more for top end enrichment. There is a goofy crossover for finding secondary enrichment. Some people disconnect the link and tune the primaries by themselves. Then they add in the secondaries. This would work in a perfect world, but there is no way to isolate the two and simulate the same airflow that would be present had the two been linked. The primaries will pull more air as the secondaries are closed. When the secondaries get added back in, the primaries are metered for working alone and the secondaries will contribute based on that. I am not knocking this method, just pointing out its downside. I don't really have a better method that truly isolates how P versus S contribute to the overall mixture.

A wideband 02 sensor has been my best aid in tuning my engine. Innovate Motorsports has a board with more accurate info than anywhere else IMO. If you really want to get dialed in and fully adjustable, that site and an 02 meter will get you there.

[norton]

I have been told in drag racing if using a tranny brake and a two step take the power valves out completely and jet accordingly???

Haven't really tried it both ways. I seem to have enough hassles getting my combo worked out without throwing another change in the mix

[Doug Schriefer]

The power valve is an economy device only, and is not really applicable for ANY type of racing application. As soon as you floor the throttle, manifold vacuum will drop so low, any power valve is going to snap fully open.

A Power Valve is an Economy and Drivabilty device and is 100% used in certain types of racing.

And you are certainly not going to be doing anything like that with a circle track engine..........

Actually the Power Valve sizing is EXTREMELY important in an Oval Track engine for Drivability.

Whatever came with the carb will work fine, and there is no advantage in changing it for something different.

That is like saying whatever jets come in a carburetor are good enough. Fine tuning is just that fine tuning.

Power valve tuning can be really good for a street car that does a lot of freeway miles at constant speeds. It has the potential to improve fuel economy, that is it's only purpose.

Again 100% false most Circle Track and Road Race engines will benifit from Power Valve tuning.

For on/off the throttle fast driving, the power valve does nothing at all useful, it is for economy only.

Again false...

[Doug Schriefer]

That sounds a lot like why I was told I broke 4 pistons and the person who told me of that possibility happening had no idea that he had just described the scenario of when the smoke started coming out my left exhaust

I had told a friend that a carb manufacture tech line had told me "return to square" the 950 CT carb I had gotten from them and put the 4.5 PV back in my 496's secondaries with a 680 lift XE grind solid roller. My friend responded that if I did that and went from WOT to 7/8 throttle the PV would incorrectly detect a cruise mode and cause the motor to go lean at a fairly high rpm. (6500)

That is exactly what happened and as a result I broke 4 pistons but by the grace from above there was no damage my block....... still I do not have to tell anyone here the price of all the parts plus the major hassle I had to go through.

I have dealt with some people that have followed up bad mistakes with good moves that have made things right but to this date QFT has not stepped up to the plate (other than not charging me for removing the PV and re-jetting it after I sent the carb back to them)

Doug

The vacuum building and closing the Power Valves normally only happens with small restricted carburetors (390's, 500's, etc).

I would find it Extremely hard to believe that with a 950 carburetor you could build up enough vacuum at 1/2 to 3/4 throttle to close the Power Valve.