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  #1 (permalink)  
Old 11-30-2009, 10:23 AM
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Default How to modify your Holley Power Valve Circuit

Here are some pictures on how to modify your Power Valve Channel Restriction (PVCR) circuit on your Holley carb. It is not very difficult and can make an incredible difference on how your car drives. To tune this circuit you will need a wide band O2 sensor.

Step #1: Drilling out your PVCR


Step #2: tap out the PVCR, We like to use 6-32, some prefer 8-32.


Step #3: I like to use a cordless drill to hold the set screw while I drill out the center.


Step #4: Here you can see the jet we are about to install into the PVCR


Step #5: The completed PVCR mod
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Old 11-30-2009, 10:53 AM
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Great tech tip Tom. Jeremy did this on my carb.

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Old 11-30-2009, 11:13 AM
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Any tips or rule of thumb on determining a starting size for the new jet? I ass/u/me you want to start some significant size smaller than stock and increase one drill size at a time.

Should the inserts be removed from the metering block for redrilling?
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Old 11-30-2009, 11:33 AM
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Great Tip!!!

If you use the 8-32 screws, you can have a wider range of sizes.

I would recommend a small amount of loctite blue if the set screws are easy to turn.

Thanks for the tip!

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Old 11-30-2009, 12:05 PM
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Question

is this done on the primary metering block only ? or do you do both ?.
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Old 11-30-2009, 12:39 PM
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Quote:
Originally Posted by Gunner View Post
Any tips or rule of thumb on determining a starting size for the new jet? I ass/u/me you want to start some significant size smaller than stock and increase one drill size at a time.

Should the inserts be removed from the metering block for redrilling?
Every carb is different. You will need a wide band O2 sensor to get to the right size hole. It is safer to start with bigger. A quick way to see if this is what you need to do is insert a piece of wire in the PVCR to make the effective hole smaller.

In the stock Holley metering block I am not aware of any inserts in the PVCR. Of course in the idle circuit their typically is a brass insert.
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Last edited by Tom Kirkham; 11-30-2009 at 12:47 PM..
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Old 11-30-2009, 12:43 PM
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Originally Posted by trularin View Post
Great Tip!!!

If you use the 8-32 screws, you can have a wider range of sizes.

I would recommend a small amount of loctite blue if the set screws are easy to turn.

Thanks for the tip!

Typically we do not tap the entire length of the hole. This way we can bottom out the jet and Locktite is not needed. However if you tap all the way through you will need Locktite to keep the jet from falling through...
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Old 11-30-2009, 12:45 PM
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Quote:
Originally Posted by wrench87 View Post
is this done on the primary metering block only ? or do you do both ?.
Both, if you have a PV in the secondary.
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Old 11-30-2009, 01:56 PM
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Originally Posted by Tom Kirkham View Post
In the stock Holley metering block I am not aware of any inserts in the PVCR.
Sorry, I wasn't clear. If you need to drill out these plugs you've added a step or two, is it safe to do so in the block or should they be removed?
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Old 11-30-2009, 02:30 PM
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Thanks Tom,
Great Post, the photos make it so much easier to follow & understand.

Craig
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Old 11-30-2009, 02:47 PM
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Quote:
Originally Posted by Gunner View Post
Sorry, I wasn't clear. If you need to drill out these plugs you've added a step or two, is it safe to do so in the block or should they be removed?
Sorry I am still confused as to what you are asking. All Holley carbs that have PV all ready have metering holes in them. Now if the metering block does not have a PV then their are no holes. Do you have a Holley or do you have a copy? A lot of the Holley copies have billet metering blocks with this mod all ready done. If this does not answer your question, could you post a picture?

Thanks,
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Old 11-30-2009, 04:23 PM
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Default Wide band o2 sensor

Does anyone make a reasonably priced wide band O2 sensor (and meter) that fits within the exhaust pipe? I really do not desire to weld O2 sensor bungs in my exhaust pipes.

Thanks
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Old 12-01-2009, 01:07 PM
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pvcr are sometimes too small also. in this scenario you can enlarge the pvcr one drill bit size at a time. these channels are very sensitive. then if you go too big you can drill and tap for the bleed as tom has shown. fyi
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Old 12-01-2009, 01:16 PM
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Sorry I did not include this picture earlier. It might have cleared up some confusion. This is a picture of an unmodifed metering block. The two little holes inside the black ring are the PVCR.

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Old 12-02-2009, 05:18 AM
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Just curious
Are you doing this modification to decrease or increase fuel volume while still retaining the original PV vac setting ie. 6.5 etc.
Or,is this for track use such as where performance suffers coming out of corners and a boost from more fuel is required.
Or,are you just seeking better overall street driving.
I realize many Cobra engine combos out there have cams that provide poor at best vaccum and wonder what PV's are being used,how low of a setting?
The reason for the questions is the 2 -715 Holleys on my Boss 429 run pretty well and the engine produces 10 in of vac at idle with 6.5 PV's in both carbs.
However,when I accelerate at full throttle I look like I'm leaving a vapor trail out of the exhaust.
No hesitation,just acceleration,but adding insult to injury as I bested a ZO6 and washed him with the vapor trail at the same time.He was peeved,I might say I was embarassed ,or not.
I also realize that the sewer size ports combined with dumping all that fuel at WOT may not be a cureable situation but am exploring all options.
Anyone else with 2X4's experiance this?
Carbs are from Carl's,and overall I'm very happy with the performance.
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Old 12-02-2009, 10:27 AM
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Quote:
Originally Posted by 69boss429 View Post
Are you doing this modification to decrease or increase fuel volume while still retaining the original PV vac setting ie. 6.5 etc.
Or,is this for track use such as where performance suffers coming out of corners and a boost from more fuel is required.
Or,are you just seeking better overall street driving...
We do this modification to get the air fuel ratio where the engine needs it. Most of the time (but not always) Holley has the fuel curve set very safe (rich.) We are just trying to get the air fuel ratio back so the engine runs right. This mod does not change when the power valve opens; only how much fuel is being delivered to the engine when the power valve opens.

Quote:
Originally Posted by 69boss429 View Post
...I realize many Cobra engine combos out there have cams that provide poor at best vaccum and wonder what PV's are being used,how low of a setting?...
I have seen the PV selected in two different ways. The first way is to have the PV come in before the main jets such as at cruise to give a slight enrichment when climbing hill or passing a car on the interstate. The second way is to have the PV come in after the main jets are activated. This is typically done in a more aggressive situation. What we try and do is to get the air fuel ratio fairly lean during cruise and richen up to about 12.5:1 under full throttle. Better combustion chamber designs will require a leaner mixture for max power.

Quote:
Originally Posted by 69boss429 View Post
...The reason for the questions is the 2 -715 Holleys on my Boss 429 run pretty well and the engine produces 10 in of vac at idle with 6.5 PV's in both carbs. However, when I accelerate at full throttle I look like I'm leaving a vapor trail out of the exhaust. No hesitation, just acceleration, but adding insult to injury as I bested a ZO6 and washed him with the vapor trail at the same time.He was peeved, I might say I was embarassed ,or not.
You are running way rich and giving up horsepower, fuel economy, and throttle response.

Quote:
Originally Posted by 69boss429 View Post
I also realize that the sewer size ports combined with dumping all that fuel at WOT may not be a cureable situation but am exploring all options.
Anyone else with 2X4's experiance this?
Carbs are from Carl's,and overall I'm very happy with the performance.
I feel like a broken record but you need to get a wide band O2 sensor, and see what your engine is doing. We had a dual 4 427 (482 CID)come in and it was terrible. It would hardly run until 1/2 throttle at which point it would take off like a rocket. The car was undrivable. When it left it was almost as good as factory EFI (once it was warmed up). You will be much happier once the engine is tuned up.
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Old 12-02-2009, 02:07 PM
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The availability of the billet metering blocks,make this and other metering block mods easy,but you need a good wide band A/F meter,like an INNOVATE LM-1,to verify results on the track or street.
What you try to do,with the Power Valve Restriction, is use the smallest main
jet size,for best cruise performance,and then adjust the PVR, for best performance at heavier throttle loads,before secondaries start to open.
I've been tuning with the LM-1 for 4 1/2 years,and it can be addictive,chasing the ultimate combo!
Wait till you start tuning the emulsion bleeds/jets! You do that to straighten out the fuel curve.

Last edited by Mus408; 12-02-2009 at 02:12 PM..
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Old 12-02-2009, 03:31 PM
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Quote:
Originally Posted by Tom Kirkham View Post
We do this modification to get the air fuel ratio where the engine needs it. Most of the time (but not always) Holley has the fuel curve set very safe (rich.) We are just trying to get the air fuel ratio back so the engine runs right. This mod does not change when the power valve opens; only how much fuel is being delivered to the engine when the power valve opens.




I have seen the PV selected in two different ways. The first way is to have the PV come in before the main jets such as at cruise to give a slight enrichment when climbing hill or passing a car on the interstate. The second way is to have the PV come in after the main jets are activated. This is typically done in a more aggressive situation. What we try and do is to get the air fuel ratio fairly lean during cruise and richen up to about 12.5:1 under full throttle. Better combustion chamber designs will require a leaner mixture for max power.



You are running way rich and giving up horsepower, fuel economy, and throttle response.


I feel like a broken record but you need to get a wide band O2 sensor, and see what your engine is doing. We had a dual 4 427 (482 CID)come in and it was terrible. It would hardly run until 1/2 throttle at which point it would take off like a rocket. The car was undrivable. When it left it was almost as good as factory EFI (once it was warmed up). You will be much happier once the engine is tuned up.
"Safe is good,"
"At cruise I'm in reality running a 2 barrel and the power band is very steady as I accelerate with no stumbling or hesitation. "
"Yes,I'm sure it's running rich when I floor it and everything opens up,but the secondaries are vaccum and I'm thinking maybe the power valves are dumping to much fuel all at once at initial WOT,after the initial blast out the pipes everything is clear as a bell.The plugs and tailpipes both have the same coffee with cream coloring and driveability is actually fantastic. "
"I'm sure the 482 FE is cammed differently.I can drive off idle with no issues,and if and when I've achieved 1/2 throttle I'd be exceeding the speed limit in any gear including first.And if I was any happier when driving it I'd likely get arrested
Thanks for your thoughts,I may try closing down those holes a bit"

Last edited by 69boss429; 12-02-2009 at 03:35 PM..
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Old 12-16-2009, 06:15 PM
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Quote:
Originally Posted by Tom Kirkham View Post
Every carb is different. You will need a wide band O2 sensor to get to the right size hole. It is safer to start with bigger. A quick way to see if this is what you need to do is insert a piece of wire in the PVCR to make the effective hole smaller.

In the stock Holley metering block I am not aware of any inserts in the PVCR. Of course in the idle circuit their typically is a brass insert.
Tom,

can you envision just using the air bleed jets instead of a brass set screw ? Is there enough material in the metering block to set the bleed jets in deep enough to clear the power valve ?

Z. Ray
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Old 12-16-2009, 07:42 PM
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Quote:
Originally Posted by zrayr View Post
Tom,

can you envision just using the air bleed jets instead of a brass set screw ? Is there enough material in the metering block to set the bleed jets in deep enough to clear the power valve ?

Z. Ray
I believe you will need to counter sink for the head to clear the PV. Besides they are expensive...
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Last edited by Tom Kirkham; 12-16-2009 at 07:49 PM.. Reason: spelling
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