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power valve circuit myths

General engine tech -- Drag Racing to Circle Track

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mopardave
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power valve circuit myths

Post by mopardave »

If my vacuum in gear is 6.0 and i install 8.5pv's, will the pv circuit pass fuel when i'm idling in gear? I currently have 6.5pv's and it does not pass fuel at idle. I have been told a blown pv with not pass fuel at idle.
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Re: power valve circuit myths

Post by rebelrouser »

The number on the power valve is supposed to be the opening point of the valve, if you have an 8.5 power valve and you hold 6 inches of vacuum at idle the power valve should be open.
Your post reads that your power valve is not flowing? How did you determine it is not flowing?

A blown power valve will pass fuel all the time as well as pulling fuel through the rupture in the diaphragm.

I will say this, a new power valve, should open when the vacuum reaches the calibrated number, but the diaphragm over time can get stiff and change the opening point, or sometimes, they will not open at all. Seen this with street cars that run pump gas, the alcohol in the fuel can cause the diaphragm to get stiff.
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Re: power valve circuit myths

Post by jake197000 »

it will run pig rich with a blown pv
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Re: power valve circuit myths

Post by RevTheory »

If the power valve opens at 8.5 and he's idling at 6, the valve will sit there open but not flowing until there's enough depression to draw from the boosters, correct?
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Re: power valve circuit myths

Post by ClassAct »

Do not set power valve opening from idle vacuum. Set it off of cruise vacuum and be two or three numbers lower than cruise vacuum.

Do the main jets flow fuel at idle? Neither will the power valve. Setting the power valve opening (timing) the Holley way is just wrong.
mopardave
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Re: power valve circuit myths

Post by mopardave »

I was stating that my 6.5 pv’s do not pass fuel at my idle vacuum. I understand now that pv circuit is with the boosters and not the t slot. Cruise vac is 14” and I want to try 8.5’s to cover a slight hiccup at full throttle jab in high gear. I figure it needs more fuel quicker. T ram set up with twin 650’s on a 511. Thanks
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Re: power valve circuit myths

Post by Walter R. Malik »

mopardave wrote: Wed Oct 14, 2020 8:31 am If my vacuum in gear is 6.0 and i install 8.5pv's, will the pv circuit pass fuel when i'm idling in gear? I currently have 6.5pv's and it does not pass fuel at idle. I have been told a blown pv with not pass fuel at idle.
Only IF your throttle plates are open far enough so fuel is misting out of the main booster with that PVCR richen the idle mixture.
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rebelrouser
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Re: power valve circuit myths

Post by rebelrouser »

My experience with tunnel rams are, that your power valve change may help. But I would also try some different cams on the accelerator pumps. If you have a stumble in high gear, how far is the pedal down before you decide to stab it? The stock cams ramp may be almost used up by the time you need to stab it. Holly makes a large number of cam profiles.

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Re: power valve circuit myths

Post by dynoflo »

I would try picking up squirter size a couple of numbers. Like said before a blown power valve will go fat all the time.
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Re: power valve circuit myths

Post by GRTfast »

RevTheory wrote: Wed Oct 14, 2020 10:42 am If the power valve opens at 8.5 and he's idling at 6, the valve will sit there open but not flowing until there's enough depression to draw from the boosters, correct?
Yeah I think this is true. I had my main jets fall out once and the car ran fine until the main circuit was activated. AFRs were dead on up until then.
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Re: power valve circuit myths

Post by Hedley Bean »

mopardave wrote: Wed Oct 14, 2020 8:31 am If my vacuum in gear is 6.0 and i install 8.5pv's, will the pv circuit pass fuel when i'm idling in gear? I currently have 6.5pv's and it does not pass fuel at idle. I have been told a blown pv with not pass fuel at idle.
A blown PV diaphragm will let the manifold vacuum draw a constant flow of fuel directly into the intake via the PV vacuum chamber...bad news.
As far as a properly operating PV, it feeds through the boosters not the idle circuit. I've tested this before with several ratings of power valves and observed no change to idle AFR. Currently using a 9.5 with 8 in/hg at idle. With 14" at cruise you might even like the 10.5 depending on how fast the engine responds.
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Re: power valve circuit myths

Post by My427stang »

RevTheory wrote: Wed Oct 14, 2020 10:42 am If the power valve opens at 8.5 and he's idling at 6, the valve will sit there open but not flowing until there's enough depression to draw from the boosters, correct?
Bingo, this should be pasted all over the internet. The Holley 1/2 recommendation confuses everyone
mopardave wrote: Wed Oct 14, 2020 12:35 pm I was stating that my 6.5 pv’s do not pass fuel at my idle vacuum. I understand now that pv circuit is with the boosters and not the t slot. Cruise vac is 14” and I want to try 8.5’s to cover a slight hiccup at full throttle jab in high gear. I figure it needs more fuel quicker. T ram set up with twin 650’s on a 511. Thanks
You got it right, but, you may need to be a little more drastic using pump shot depending on how and when the hiccup happens. If it's with a stomp, potentially a change to a blue pump cam can come on quicker and last longer due to rate and volume of that cam. If it's with a roll-on, the PV should help. Small changes can also be made with float level, especially if it's too low now
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Re: power valve circuit myths

Post by oscaracme »

jake197000 wrote: Wed Oct 14, 2020 10:01 am it will run pig rich with a blown pv
at idle it won't run pig, cow or chicken rich with a blown power valve.
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Re: power valve circuit myths

Post by mopardave »

To be clear, my pv is not blown. I was making kind of an example. Yes, raising float levels help this issue in a big way. Also, going from 31 squirter to 35's helped. I tried 37's and they were no better than the 35's. Cruising 30mph in 3rd gear and then jabbing throttle wide open still causes a quick bawooo like the old quadrajets and theroquads. 1st gear, no issues. This is with an Indy tunnel ram with max wedge ports on a 12.5:1 511. The Indy has alot of volume.
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Re: power valve circuit myths

Post by Walter R. Malik »

oscaracme wrote: Thu Oct 15, 2020 7:29 am
jake197000 wrote: Wed Oct 14, 2020 10:01 am it will run pig rich with a blown pv
at idle it won't run pig, cow or chicken rich with a blown power valve.
It certainly will run super rich with a blown power valve because fuel goes right past the diaphragm in it, directly through the vacuum supply hole for it.
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