Secondary power valve

[Charger 69/72]

I just picked up a 950 Ultra HP #80676 and it has a power valve in the front and the rear. What's the point in having one in the rear? For one thing, if you plug it and go up eight jet sizes in the rear, then you don't have to worry about having four of each jet size on hand for tuning purposes. In other words, you would only have to buy ONE jet assortment which consists of two of each jet size. Am I wrong in thinking this way?

[crisser]

Nothing wrong with your thinking, kinda. I wouldn`t remove it just to have 1 jet assortment. Holley uses a pv in the rear on the large venturi models to keep jet sizes in reason. That`s why you don`t see them on the 800 and smaller carbs. Most jet kits stop at 99 or 100 so what if you needed larger? You can`t drill into the threads,can you? Thankfully, most carbs don`t need to go that high, so removing the rear pv is a good way to have a better tuning capability. Also, there is no chance that the valve will close at the top of the track and lean out the motor. Then, bad things will happen. On the street, the pv in the rear is not a bad thing. Race is another story. Hell, most pure race motors have the front pv blocked as well. Most have cams so large that the pv will stay open all the time, loading the engine with fuel.HTH

[crisser]

By the way, sometimes 6 sizes is enough to cover the pv removal.

[Charger 69/72]

Thanks for the info, Crisser.

[69Sixpackbee]

Do it. I have done it several ways by up-jetting.

[mhenesian]

I built the equivalent of an HP750 Ultra using Proform, Quick Fuel, and Holley parts with Proform billet metering blocks. I use and would recommend the rear metering block with a power valve, as Holley originally intended when they designed the first double pumpers back in the late '60's. Eliminating the rear power valve saved Holley money, and allowed them to sell a cheaper carb, but IMO causes performance problems with big double pumpers.

On the street or track a rear power valve prevents flooding when you're off the gas and on the brakes, since the rear power valve will close and help prevent fuel from drizzling out of the rear venturi boosters. This flooding can "kill" the engine, and if you have power brakes/power steering you'll be in real trouble.

For example, in warm weather I run '70's in front, 76's in the rear metering block with an 8.5" Hg power valve in front and a 6.5" Hg in back. Use 0.027" to 0.029" high speed air bleeds front and back for crisp throttle response. In cold weather, or at the track with open headers, just replace the 70's in front with 74's and your good to go !

Incidently, the Proform/Holley 750 cfm (actually 830 cfm) center sections come with 0.036" air bleeds, which is way to large, requiring primary jet sizes in the 74 to 76 range, otherwise you'll get a "lean surge" at part throttle. It doesn't matter what metering blocks you're running (old Holley, new Holley, Proform, Quickfuel,...) You'll be a lot happier with smaller main air bleeds.

Mark H.

[Charger 69/72]

Mark,

Thanks for the input - I really appreciate the tip on smaller main air bleeds.

[crisser]

Be very careful on changing main bleeds. Small changes can make a big difference. The main bleed size is connected to the emulsion package built into the metering block. The idle bleed can be changed more freely as it acts more directly like a jet.

[krazykuda]

Keeping the secondary power valve will give you better economy and throttle response on the street. It only kicks in when the engine is under load. Increasing jet size makes you run richer all of the time.

[turbododge]

On a double pumper, having a power valve on the secondaries can be very handy on a street driver, particularly if you have a manual transmission and like to drive curvies. With a power valve in the secondaries you have a smoother transition onto the secondaries with less chance of going too rich as they open. If you are into cornering with the throttle, you will quickly see the advantage, as you can easily be at a throttle setting that has vacuum in the secondaries enough to hold the pv closed, but still making power. If you did it all with the jets, you would be way rich under those circumstances.

If you are doing drag stuff and not partial throttle things, it is a lot less necessary.

When I was running a 650dp on the TT340, I actually used a two stage power valve in the secondaries to get smooth transitions, as under boost almost all the richening is done in the secondaries. I ran 61 primaries, 93 secondaries with the power valve restriction drilled out besides, and controlled secondary fuel flow at transition with the first stage of the pv.

[Charger 69/72]

Thanks to all so far for all of the responses - I now see the importance of keeping the power valve on the secondary side!