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OK, I'm no carb. guru but I am trying to get the best run out of a Holley carb. I have looked and listened to a lot of peeps on the boards to try and figure out a proper setup. I am running a 4781-6 Holley 850 on a 468 with a pretty healthy cam and it has power valves front and rear and my vacuum runs around 8-9 inch. at 1100 RPM. Should I be running power valves or not and what size would be recomended for this vacuum? I do know to jet up if not using PVs! WT:)devil

Coby
 

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OK, I'm no carb. guru but I am trying to get the best run out of a Holley carb. I have looked and listened to a lot of peeps on the boards to try and figure out a proper setup. I am running a 4781-6 Holley 850 on a 468 with a pretty healthy cam and it has power valves front and rear and my vacuum runs around 8-9 inch. at 1100 RPM. Should I be running power valves or not and what size would be recomended for this vacuum? I do know to jet up if not using PVs! WT:)devil

Coby

I suggest running power valves unless it is just a drag race piece. Part throttle drivability and fuel milliege will suffer with out them.
With your vaccuam (depresion:D) reading I would run 6.5's front and rear.

IMHO;)
 

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Discussion Starter #3
That is what is in the carb now. I was looking to test this weekend and get the jetting set with plug readings! Hopefully LMAO! Thanks Warp WT:)devil

Coby
 

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E-7 Sheepdog (ret)
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I agree with Warp.

Figure on jetting up 6 to 8 sizes by the time you get it right again after blocking the power valves off, and then spending weeks jacking arround with accelerator pump sizes, squirter sizes, air bleed sizes, just to get it to drive as well as it did before you decided to make it better.
 

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I agree with Warp.

Figure on jetting up 6 to 8 sizes by the time you get it right again after blocking the power valves off, and then spending weeks jacking arround with accelerator pump sizes, squirter sizes, air bleed sizes, just to get it to drive as well as it did before you decided to make it better.
I personally have dumped the power valves many times and have never had an issue. At the same time I dump the power valves I go one step up on the squirter and two steps on the jet size. If you are only pulling that vacuum at 1100 rpm when you crack the throttle the vacuum drops and power valves go for a nap. Back fire the motor good one time and they go tits up. IMO M
 

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E-7 Sheepdog (ret)
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I thought when the vacuum drops the power valve opens to add fuel. And don't all holleys built after like 1990 or so have power valve protection for back fire?
You are correct, on both counts.

Also a kit exists to retrofit it to older carbs.


Widetrack, what is your MAX vaccumme at cruise? Power valves are ususally advised to be at 1/2 this value.
Say, your best cruising vac. is 14", then the basic "starting point" is recomended to be a PV of 7.0.

6.5 is close enough.

if your max cruise vac. is only 8 or 9, then you should try a 4.5 or so.
 

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I thought when the vacuum drops the power valve opens to add fuel. And don't all holleys built after like 1990 or so have power valve protection for back fire?
hEY RAT, it would be my thought and thats it. The power valve kicks in as vacuum comes back up and with that said, With a big carb and especially a large cam with a butt load of overlap the vacuum stays in the very low range. Just to think the equasion out. If you where a carb builder would you want to dump alot of feul to a motor with zero vacuum and for that matter what would that do to the efficiancey of how the motor is and how it will respond to a throttle change in that situation. For example, alot of feul that has not had a chance to atomize and mix with air to become a more volatile product for the combustion chamber. Sorry I'm ranting on the silly edge. M
 

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Cantard
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You are correct, on both counts.

Also a kit exists to retrofit it to older carbs.


Widetrack, what is your MAX vaccumme at cruise? Power valves are ususally advised to be at 1/2 this value.
Say, your best cruising vac. is 14", then the basic "starting point" is recomended to be a PV of 7.0.

6.5 is close enough.

if your max cruise vac. is only 8 or 9, then you should try a 4.5 or so.
Nothin, didnt read the whole thread.
 

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Resident Ford Nut
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For what it's worth, depending on your throttle linkage I'd leave the secondary power valve out which is easier if you have progressive linkage.ou

If you have a hard leaving boat the secondary power valve will unload. They don't make power valve extensions but they do make jet extensions.

Sleeper CP :D
 

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E-7 Sheepdog (ret)
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hEY RAT, it would be my thought and thats it. The power valve kicks in as vacuum comes back up and with that said, With a big carb and especially a large cam with a butt load of overlap the vacuum stays in the very low range. Just to think the equasion out. If you where a carb builder would you want to dump alot of feul to a motor with zero vacuum and for that matter what would that do to the efficiancey of how the motor is and how it will respond to a throttle change in that situation. For example, alot of feul that has not had a chance to atomize and mix with air to become a more volatile product for the combustion chamber. Sorry I'm ranting on the silly edge. M
From Holley's tech site:
Holley
carburetors utilize a vacuum operated power enrichment system
and a selection of power valves is available to “time” this system’s
operation to your specific needs. Each Holley power
valve is stamped with a number to indicate the vacuum opening
point.
For example, the number “65” indicates that the power
valve will open when the engine vacuum drops to 6.5" or
below.


In a "zero vaccume" condition, there would be NO fuel flow anyhow, because there would be NO fuel pulled from the main venturi circuit, so your "dumping excess fuel" simply does not, cannot happen.
When you are running flat out, with only 1-2" vac, you WANT lots of fuel flow, and you can do it one of 2 ways. Bigger main jets, or all 3 "jets" (the mains and the PV in a Holley) flowing.
Same for running 3" vac. dragging that 35' 5th wheel or Fountain up a steep hill, you are tryibng to make max power, and want full fuel flow, same 2 choices for how to do it.

All the power valve DOES is add fuel to the main metering circuit, like having a "third jet".
When vac. signal drops, so does fuel flow thru the main metering circuit jets, purely due to physics of differential pressure. The low-vac. condition opens the power valve to allow more fuel flow to fill in this "hole" in the gasoline flow. As vac. comes back up, the pull on the main circuit increases, pulling harder from the jets, and the PV closes off.

The PV only flows fuel when a heavy load is holding engine RPM down much lower than the throttle opening would (such as pulling a trailer uphill, a WOT start against an auto trans tq. converter (or a prop), and at wide open throttle. Otherwise it sits there, held shut by engine vaccume.

The fuel it passes goes to the main boosters, not anywhere else (unless a diaphram is ruptured, in which case it leaks constantly out the vac. port in the bottom.

I was incorrect here, I said cruising RPM, should have said IDLE vac. level. Again, straight from Holley Tech.:
QUESTION How do I tell what size power valve I need?
ANSWER To properly size a power valve, take a vaccum reading at idle and if it is above 12" for a standard transmission a 6.5" will be safe to use. For automatic transmissions take a vaccume reading in gear at idle and if the vaccum is below 12" divide that in half for proper size. Example 9" of vaccume in gear at idle will require a 4.5" power valve.
 

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From Holley's tech site:
Holley
carburetors utilize a vacuum operated power enrichment system
and a selection of power valves is available to “time” this system’s
operation to your specific needs. Each Holley power
valve is stamped with a number to indicate the vacuum opening
point. For example, the number “65” indicates that the power
valve will open when the engine vacuum drops to 6.5" or
below.

In a "zero vaccume" condition, there would be NO fuel flow anyhow, because there would be NO fuel pulled from the main venturi circuit, so your "dumping excess fuel" simply does not, cannot happen.
When you are running flat out, with only 1-2" vac, you WANT lots of fuel flow, and you can do it one of 2 ways. Bigger main jets, or all 3 "jets" (the mains and the PV in a Holley) flowing.
Same for running 3" vac. dragging that 35' 5th wheel or Fountain up a steep hill, you are tryibng to make max power, and want full fuel flow, same 2 choices for how to do it.

All the power valve DOES is add fuel to the main metering circuit, like having a "third jet".
When vac. signal drops, so does fuel flow thru the main metering circuit jets, purely due to physics of differential pressure. The low-vac. condition opens the power valve to allow more fuel flow to fill in this "hole" in the gasoline flow. As vac. comes back up, the pull on the main circuit increases, pulling harder from the jets, and the PV closes off.

The PV only flows fuel when a heavy load is holding engine RPM down much lower than the throttle opening would (such as pulling a trailer uphill, a WOT start against an auto trans tq. converter (or a prop), and at wide open throttle. Otherwise it sits there, held shut by engine vaccume.

The fuel it passes goes to the main boosters, not anywhere else (unless a diaphram is ruptured, in which case it leaks constantly out the vac. port in the bottom.

I was incorrect here, I said cruising RPM, should have said IDLE vac. level. Again, straight from Holley Tech.:
QUESTION How do I tell what size power valve I need?
ANSWER To properly size a power valve, take a vaccum reading at idle and if it is above 12" for a standard transmission a 6.5" will be safe to use. For automatic transmissions take a vaccume reading in gear at idle and if the vaccum is below 12" divide that in half for proper size. Example 9" of vaccume in gear at idle will require a 4.5" power valve.
Okay I explained that bass-ackwards so here we go again anal cranal intercourse. Oppps M
 

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E-7 Sheepdog (ret)
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Happens to all of us ol guy. :)
 

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I have mine set up with out power valves, runs great. A power valve was designed for fuel efficiency in mind. Why bother with it, just square the jets and tune accordingly.
 

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Resident Ford Nut
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I have mine set up with out power valves, runs great. A power valve was designed for fuel efficiency in mind. Why bother with it, just square the jets and tune accordingly.
With gas a $ 4.50 a gallon I think I what efficiency. I ran one in the primary side of a 855 hp engine for yrs. at crusing speed of 3,800-4,000 rpm's (50 mph at 4k) it used very little fuel. With the power valve out and up jetted 6 jets it sucked fuel like I couldn't beleive.

At today price for gas there is no reason to waist gas.

Sleeper CP
 

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Highaboosta
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I have mine set up with out power valves, runs great. A power valve was designed for fuel efficiency in mind. Why bother with it, just square the jets and tune accordingly.
A power valve is to give acceleration enrichment to approx 12.5:1 A/F ratio.
With gas the proper mixture for a light load cruise speed would be stoich at 14.7:1 A/F ratio.

If your saying it's better to remove the PV and rejet it to run at 12.5:1 all the time then you don't understand stoich so here you go,

http://en.wikipedia.org/wiki/Air-fuel_ratio
 

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northern member
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I have mine set up with out power valves, runs great. A power valve was designed for fuel efficiency in mind. Why bother with it, just square the jets and tune accordingly.
are you racing or do you like to waste gold.... i mean gas ? i like to tune-up a new deal square jetted without the pv but once the jetting and timing are right and i get a vacuum reading at cruise , in goes the next # smaller pv and 6 size smaller jets . if it's a race engine that doesn't get cruised then i agree you don't want the variable of the pv if your wide open all the time .
 
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