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post #1 of 14 (permalink) Old Jul 29th, 15, 03:08 PM Thread Starter
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another holley power valve question

I have a big block with 850 double pumper. The cam is 290 510. And produces 13 inches vacuum in park idle speed is 1200 rpm. In drive the car produces about 6 inches vacuum idle is 750 rpm. The idle mixture is set at 1.5 turns out and the carb has a 6.5 power valve now.

The engine had a cluster of problems I had to fix to get it to run ok. Bad fuel filter, carb gasket had a tear causing a vacuum leak. All this is fixed. the car seems to be running ok. The carb sneezed one time. Now here are my questions?
1 How do you choose the correct power valve? I am thinking of trying a 4.5 power valve.
2 Maybe float level?
3 Timing is set to 4 degrees btdc
I am leaning towards checking the float level and timing one more time. But just wanted to get opinions on the power valve selection?

Thanks

Jerry
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post #2 of 14 (permalink) Old Jul 29th, 15, 03:31 PM
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Re: another holley power valve question

1.5 turns out on the IAB's is only a baseline to start (mine are only 1/2-3/4 turns out)
I think your timing is too low. I would think 8-12 would be better (if this is a no power adder motor)
Idle dropping that much tells me your converter is too tight.
And yes, PV should be less than your vacuum at idle. (usually 2 or more inches) otherwise its dumping fuel.

I would recommend an AFR gauge to help in your quest.

I would also like to see more about your combo for better suggestions.

68 Camaro~LSx ~all motor
1.54 60'--6.95 @ 98.45 660'--10.96 @ 121.53
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post #3 of 14 (permalink) Old Jul 29th, 15, 05:09 PM
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Re: another holley power valve question

power valve has no effect on idle vacuum, you can have 6" vacuum and a 8.5 power valve, it has nothing to do with idle. Power valve adds fuel to the main circuit. and the main circuit is not open at idle. Only way a power valve can dump fuel at idle is if the power valve or pv gasket is bad or the throttle blades are open to far. Power valve should be selected by cruise vacuum with a gauge, then you can tell when the engine needs the extra fuel. It doesnt matter what size power valve, it will only flow what the power valve restrictors are.

1967 camaro, 327, 9.7 CR, comp cam xs274solid, vortec cylinder heads, edelbrock vortec performer rpm, 750 annular carb, HVH 2" spacer, msd pro billet w/6A box, turbo 350 trans, 3500 stall, 4:10 gears, caltracs suspension, m/t drag radials.
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post #4 of 14 (permalink) Old Jul 29th, 15, 05:16 PM
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Re: another holley power valve question

Basing the PV selection on idle vacuum is BAD advise that has floated around for a long time. If you take CRUISE vacuum, divide it in half and add about 2" you will be close. You want the PV to be closed while running a steady cruise, and open when the load is enough to require more fuel. Idle has nothing to do with it, even if the PV is open at idle the idle feed restriction is so much smaller the open PV will have no significant effect. Much like drinking through a coffee stirrer, it don't matter whether it's in a cup or a gallon jug, you will only flow so much through it. This is by one of the best carb tuners out there.
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1967 camaro, 327, 9.7 CR, comp cam xs274solid, vortec cylinder heads, edelbrock vortec performer rpm, 750 annular carb, HVH 2" spacer, msd pro billet w/6A box, turbo 350 trans, 3500 stall, 4:10 gears, caltracs suspension, m/t drag radials.
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post #5 of 14 (permalink) Old Jul 29th, 15, 05:30 PM
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Re: another holley power valve question

First if you have an aftermarket camshaft with a duration between 220 and 240 set the timing at 16* BTDC. Use a vacuum gauge to adjust the idle mixture screws to the highest vacuum reading. Adjust the curb idle to 900 rpms. A Distributor Advance Kit would help. Check your total timing at 3000 rpms and idle it to 3000-4000 rpms to make sure it is all in at 3000. You will want a 16* initial timing, 22* mechanical advance for a total of 38*. If you have a vacuum advance you want around 10-12 degrees. The total of 38 + 12 = 50 degrees. If you have 12 hg of vacuum at idle a 6.5 power valve is fine which comes in an 850 Holley. Here is Holley's instructions http://documents.holley.com/techlibr...wer_valves.pdf

Last edited by Cdminter55; Jul 29th, 15 at 05:42 PM.
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post #6 of 14 (permalink) Old Jul 29th, 15, 06:14 PM
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Re: another holley power valve question

Quote:
Originally Posted by Cdminter55 View Post
First if you have an aftermarket camshaft with a duration between 220 and 240 set the timing at 16* BTDC. Use a vacuum gauge to adjust the idle mixture screws to the highest vacuum reading. Adjust the curb idle to 900 rpms. A Distributor Advance Kit would help. Check your total timing at 3000 rpms and idle it to 3000-4000 rpms to make sure it is all in at 3000. You will want a 16* initial timing, 22* mechanical advance for a total of 38*. If you have a vacuum advance you want around 10-12 degrees. The total of 38 + 12 = 50 degrees. If you have 12 hg of vacuum at idle a 6.5 power valve is fine which comes in an 850 Holley. Here is Holley's instructions http://documents.holley.com/techlibr...wer_valves.pdf
X2 on Doug's advice.

Your vacuum will improve if you set the timing like he recommends.

THEN you can select a power valve like the holley tech document says.

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post #7 of 14 (permalink) Old Jul 29th, 15, 06:42 PM
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Re: another holley power valve question

so if you have a 6.5 power valve and your cruise vacuum is 16, what would happen?

1967 camaro, 327, 9.7 CR, comp cam xs274solid, vortec cylinder heads, edelbrock vortec performer rpm, 750 annular carb, HVH 2" spacer, msd pro billet w/6A box, turbo 350 trans, 3500 stall, 4:10 gears, caltracs suspension, m/t drag radials.
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post #8 of 14 (permalink) Old Jul 29th, 15, 06:45 PM Thread Starter
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Re: another holley power valve question

Thanks for all the words of wisdom. I have heard the story about power valve selection made by idle vacuum. I will recheck the timing again. And also check the float level. And about the number of turns out for mixtures. I had it at 1 1/4 and then went to 1 1/2 and engine responded. Throttle response was better and just sounded better.

Thanks

Jerry
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post #9 of 14 (permalink) Old Jul 29th, 15, 06:46 PM Thread Starter
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Re: another holley power valve question

Also I will stay with 6.5 power valve if timing and float helps.

Thanks

Jerryt
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post #10 of 14 (permalink) Old Jul 29th, 15, 07:22 PM
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Re: another holley power valve question

Quote:
Originally Posted by rkmjbad67 View Post
Basing the PV selection on idle vacuum is BAD advise that has floated around for a long time. If you take CRUISE vacuum, divide it in half and add about 2" you will be close. You want the PV to be closed while running a steady cruise, and open when the load is enough to require more fuel. Idle has nothing to do with it, even if the PV is open at idle the idle feed restriction is so much smaller the open PV will have no significant effect. Much like drinking through a coffee stirrer, it don't matter whether it's in a cup or a gallon jug, you will only flow so much through it. This is by one of the best carb tuners out there.
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I guess Holley doesn't know this

Yes, I should have said divided by 2 and not 2 sizes lower. I was mistaken there, but this has worked for me and my past low vacuum SBC's

Quote:
The power enrichment system supplies additional fuel to the main system during heavy load or full power situations. 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. An accurate vacuum gauge, such as Holley P/N 26-501, should be used when determining the correct power valve to use. A competition or race engine which has a long duration high overlap camshaft will have low manifold vacuum at idle speeds. If the vehicle has a manual transmission, take the vacuum reading with the engine thoroughly warmed up and at idle. If the vehicle is equipped with an automatic transmission, take the vacuum reading with the engine thoroughly warmed up and idling in gear. In either case, the power valve selected should be 1/2 the intake manifold vacuum reading taken. EXAMPLE: 13” Hg vacuum reading divided by 2 = 6.5 power valve. If your reading divided by 2 lands on an even number you should select the next lowest power valve. EXAMPLE: 8” Hg vacuum reading divided by 2 = 4 power valve. Since there is no #4 power valve you should use a 3.5

68 Camaro~LSx ~all motor
1.54 60'--6.95 @ 98.45 660'--10.96 @ 121.53
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post #11 of 14 (permalink) Old Jul 29th, 15, 07:39 PM
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Re: another holley power valve question

If you have 16 cruise vacuum and a 6.5 power valve, you will prob get a bog, raise that to a 10.5 you get the enrichment sooner when you crack the throttle. try a 10.5 power valve and 4.5 power valve, the A/f ration barely changes if at all.

1967 camaro, 327, 9.7 CR, comp cam xs274solid, vortec cylinder heads, edelbrock vortec performer rpm, 750 annular carb, HVH 2" spacer, msd pro billet w/6A box, turbo 350 trans, 3500 stall, 4:10 gears, caltracs suspension, m/t drag radials.
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post #12 of 14 (permalink) Old Jul 29th, 15, 07:48 PM
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Re: another holley power valve question

https://www.google.com/url?sa=t&rct=...UXBZuasYexQA_Q

1967 camaro, 327, 9.7 CR, comp cam xs274solid, vortec cylinder heads, edelbrock vortec performer rpm, 750 annular carb, HVH 2" spacer, msd pro billet w/6A box, turbo 350 trans, 3500 stall, 4:10 gears, caltracs suspension, m/t drag radials.
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post #13 of 14 (permalink) Old Jul 29th, 15, 08:05 PM
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Re: another holley power valve question

I'm going to say this again my cammed out 355 makes 6.5" at idle, holley's advice puts me at a 3.5" powevalve, at WOT I make more vacuum than that. BAD ADVICE! My cruise vacuum is 22" or so, full throttle blast puts me at around 14" a 16 PV works great when I was running a holley.

It gets even more into the "that'll never work" territory when i decided to put a qjet on my car
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post #14 of 14 (permalink) Old Jul 30th, 15, 04:26 PM
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Re: another holley power valve question

Quote:
Originally Posted by ap68 View Post
I'm going to say this again my cammed out 355 makes 6.5" at idle, holley's advice puts me at a 3.5" powevalve, at WOT I make more vacuum than that. BAD ADVICE! My cruise vacuum is 22" or so, full throttle blast puts me at around 14" a 16 PV works great when I was running a holley.

It gets even more into the "that'll never work" territory when i decided to put a qjet on my car
So what kind of intake you have ? or does it have a mechanical (belt driven)vacuum pump ?
I do know my vacuum always dropped when I went WOT.

Just asking.


68 Camaro~LSx ~all motor
1.54 60'--6.95 @ 98.45 660'--10.96 @ 121.53
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