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I've got a 454 with a 950 HP Holley, the duration on the camshaft is 302. The car is in storage and has been fogged down so I can't start it to see the vacuum, but I'm going to be placing an order through Jegs and thought I would order jets/powervalve. I plan on getting 79 or order 80 jets (factory size). What size powervalve should I get? Anyone out there have a similar size cam in a big block and know their powervalve size. Factory size is 6.5, but I think the cam I have cuts down on the vacuum considerably. Any suggestions would be appreciated on powervalve size.

Thanks,
Zane
 

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66 El Camino 57 Chevy pickup 2004 Tahoe
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the idle vacuum reading has nothing to do with the PV, never has. this is an old magazine legend that has no connection to how Holley idle circuits work in the real world.

Why do you think your carb needs changes? If it's a HP950 it'd probably very close with the factory jets.

BTW, all Holley PV are the same size, the flow rate is set by a restriction in the metering block. This does not cover PV's made by cheap carb kit marketers, they're junk when new. The different PV numbers indicate the vacuum at which they open.
 

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I have a newly built 468 with the same 950HP carb. I get 7 inches of vacuum at idle. When I called Holley and checked everything on their website, they recommended I change to from the 6.5 to a 3.5. Why would Holley say that if it really isn't necessary? Why do I not care if the PV opens so quickly on this carb and engine setup? Please help me understand.
 

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the idle vacuum reading has nothing to do with the PV, never has. this is an old magazine legend that has no connection to how Holley idle circuits work in the real world.

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This confuses me??? If the idle vacuum is below the vacuum of the power valve the engine will not idle. It will be extremely rich and flood the plugs.
On really healthy cams I have been forced to run a 4 or less power valve since idle vacuum in gear was below say 6.

Vacuum holds the power valve closed and if the vacuum drops below the value on the side the valve is open and it floods the motor.

The idle circuit has alot to do with high speed running, as does the transfer slots. If both are rich the running circuit is also rich unless you really jet down on the main jets to pull fuel out of the mixture.
Get the idle circuit right to begin with.
 

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Holley recommends that because it causes you to out and buy Holley parts.

"Need a PV, might as well pick up some gaskets and a few jets just in case....."

Doesn't take long for $30 - $50 to leave your pocket and arrive in theirs. This is a marketing strategy.

Problem is, magazines picked this up years ago, started stating it as Gospel without ever questioning whether or not it made any sense. They're writers, not carb experts.

PV's add fuel to the main wells under conditions of low vacuum. the idle fuel is pulled from the main wells also. So, the PV opening and closing at idle screws up the idle, right? Wrong. The main wells already hold something like 50 times more fuel than the idle circuits can ever pull, so making it like 60x by opening the PV is irrelevant. But this BS deal is so deeply embedded in the hot rod culture it may never go away completely.
 

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Iowa, how did it get to the 92 from the 79's? Even if somebody removed the PV completely and installed a plug it would only require 8 - 10 jetsizes to make up the difference. Was this carb on some other engine before?

Recommendation: install the jets and PV's it's supposed to have in it, see how it runs. If a condition becomes apparent that requires a change you can make it then. At least that way you'll be starting from a known baseline.
 

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Discussion Starter · #9 ·
Tom,
The engine is brand new and was in the car when I bought it. The guy I bought it from had no idea what to do and didn't have enough sense to ask someone when he had a problem. The jets are 92 and 93 in the carb. Not sure on the power valve. Plan on putting the jets back to 79 and then putting a power valve of 5 in the carb. Unless you have any other ideas? Should I just plug the powervalve and put bigger jets in? The car runs very rich right now and the plugs get real black and the car will only run for about 2 minutes and then dies and doesn't want to start again and the smoke in the open garage will about kill you.
Thanks,
Zane
 

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>> "Should I just plug the powervalve and put bigger jets in?"<<

Not unless you only drive it 1/4 mile at a time and trailer it to and from the track. Appears that's already been done or something like it.

Install the correct jets and PV for the carb. You can look them up on the Holley site. Then you'll have a known place to start if it turns out to need any changes. That's a good carb you have there.

So, did this guy build the engine too? Maybe that was his warm-up before exercising his magnificent skills on the carb.
 

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good deal there.
 

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I'm baseing my info on holley's website: http://www.holley.com/data/TechService/Technical/power_valves.pdf


My jets are 92-93. Factory jets are 79, so I'm definitely changing them. Right now the car is flooding badly on gas and will only run for a short while until it chokes itself to death. I'm thinking of going to a 4.5 power valve or something in that range.
Dropping the jetting to 80 would be good. If you are flooding the motor the PV might be open. It is definitely a good idea to go with the 4.5
If you still can't get your idle circuit right you can install .015 wires in the idle circuit. The mixture screws on the sides of the carb should be out 1 1/2 turns from closed. Slight adjustments from this is ok but not alot.
Holley even recommends the wires in the idle circuit to lean out the mixture and make the idle adjustment screws more sensitive.
I run a pair of holley 850 double pumpers and my motor idles cleanly. I do drill a 8 throttle plates to about .100 and run wires in all the idle circuits.
I pull down 15 mpg with over 800 horse, well over.
 

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Finally someone who understands idle vacuum has nothing to do with pv tuning.
the idle vacuum reading has nothing to do with the PV, never has. this is an old magazine legend that has no connection to how Holley idle circuits work in the real world.

Why do you think your carb needs changes? If it's a HP950 it'd probably very close with the factory jets.

BTW, all Holley PV are the same size, the flow rate is set by a restriction in the metering block. This does not cover PV's made by cheap carb kit marketers, they're junk when new. The different PV numbers indicate the vacuum at which they open.
 

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it's really hard to get guys to understand this. That old BS is burned into the hot rodders mind like with a branding iron or something.
 

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How does the power valve work and how do you tune it???

The power valve does NOT meter fuel. It is strickly an open closed valve. When open the hole in the metering plate meters the actual fuel. The power valve merely opens or closes.
It is held open by a spring and vacuum holds it closed. The number on the side of the power valve is the value in inches of vacuum where the valve opens.
I jet my carbs lean using a fuel ratio gage to about 15-1. The range of modern cars with fuel injection. I like my plugs white while cruising under light load.
You monitor the average light load cruising vacuum and say it is 8 for lack of a number. You would select a power valve just 1 1/2 or 2 points below this or a 6 or 6.5 in this case.
While cruising under light load you fuel ratio sits at the 15 or so you jetted for but passing, even under moderate throttle or going up a hill the vacuum drops slightly below the power valve value and the power valve opens richening the mixture and in my case using the stock drilled metering block it drops to 13-1. Immediately after passing and resuming light load cruising the mixture goes back to the 15-1.
You want to run lean for good gas mileage and a clean running motor. This prevents washing of the cylinder walls and longevity of your motor.
At idle the vacuum can NEVER drop below the value on the power valve or the motor is flooded with fuel causing fouled plugs and stalling.

Crusing jetting is a combination of idle circuit, yes it feeds the engine all the time, the transfer slots and the main jets. A rich idle mixture causes a rich cruisng mixture. Get the idle right to begin with. If the mixture screws are almost closed or open the idle circuit needs attention before you can get the car to run right.

Power valves are needed in a street car, do not even attempt to run without one. Not if you want the motor to run right and live a long healthy life.
I run about 940 horse, twin 850 double pumpers and pull down a solid 15 mpg without ever fouling a plug.
As for running lean I often do plug checks and like them to run white on long cruises. I pulled the motor down after 7 years and it looked great inside, pistons clean, no excess carbon and not a trace of burning pistons.
 

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Norval, this part: "At idle the vacuum can NEVER drop below the value on the power valve or the motor is flooded with fuel causing fouled plugs and stalling."

this is not true and it never has been. have you ever really thought about this?

if you really believe this is so, please post an explanation of exactly how the excess fuel enters the intake manifold. Are you thinking there's a separate idle discharge orifice that opens up only when the PV is is open? maybe an extra idle transfer slot that receives fuel when the PV is open? Please tell exactly how the PV being open floods the engine. keep in mind the PV feeds the main wells which are already full. exactly how does this extra fuel make the leap into the manifold?
 

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Norval, this part: "At idle the vacuum can NEVER drop below the value on the power valve or the motor is flooded with fuel causing fouled plugs and stalling."

this is not true and it never has been. have you ever really thought about this?

if you really believe this is so, please post an explanation of exactly how the excess fuel enters the intake manifold. Are you thinking there's a separate idle discharge orifice that opens up only when the PV is is open? maybe an extra idle transfer slot that receives fuel when the PV is open? Please tell exactly how the PV being open floods the engine. keep in mind the PV feeds the main wells which are already full. exactly how does this extra fuel make the leap into the manifold?
Tom I see what you are saying. I have a sectioned holley carb on my desk that I refer to to look at how things work and I do not see how the fuel from the power valve gets to the idle circuit or into the intake. I see how it bypasses the jets to richen that part up but not the idle circuit.
I see a few blown power valves and in that case the fuel gets into the main body of the carb through the broken diaphragm but I can not figure out how it affects idle if it opens.
Looks like you are right Tom and my assumption that opening during idle will flood the engine??
You are right:thumbsup: :thumbsup: The power valve doesn't affect the idle circuit.:beers:
 
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