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Old 04-24-2005, 06:34 PM
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What Size Powervalve?

I Have A 350 Small Block Chevy With A 750 Double Pumper Holley. In Park It Has 15 Vacumm And In Drive At Idle It Has 12 Vavumm. I'm Not Sure What Size Power Valve I Should Have In The Carb! Thanks

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Old 04-24-2005, 07:15 PM
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power valve

The stock 6.5 is fine.
A 750 cfm is too large for a 350 cu in motor. You should have a 650. It is better to have too small of a carb as opposed to too big.
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Old 04-24-2005, 07:19 PM
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What's in it now? It has been said to use a PV of two steps lower than your lowest vacuum reading. They're cheap enough so maybe u can buy a few different valves. I'd be heading for an 8Hg power valve in your case. It's better to be too low than to high.
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Old 04-24-2005, 07:30 PM
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I disagree

Holley recommends a power valve 1/2 the vacuum in gear. If you are at 12 Hg you would use a 6 Hg power valve. Since the carb comes with a 6.5 from the factory. It would be a waste of money to replace it.

The 2 Hg below vacuum is at driving speed not idle.
http://holley.com/HiOctn/TechServ/TechInfo/CI-121.html
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Last edited by bracketeer; 04-24-2005 at 07:46 PM.
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Old 04-24-2005, 07:52 PM
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Bracketeer, after reading your reply i questioned my memory as I was sure what I said was valid. So upon checking the Holley site I'm relieved that my memory's not gone.....yet! But I agree, I'd stick with the 6.5Hg valve too.

The folowing's quoted from Holley tech page

CARBURETOR
POWER ENRICHMENT SYSTEM

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 have a vacuum opening point about 2" Hg below the intake manifold vacuum reading taken.
A stock engine, or one that is only mildly built for street use, will have high
manifold vacuum at idle speeds. To determine the correct power valve the
vehicle should be driven at various steady speeds and vacuum readings
taken. The power valve selected should have an opening point about 2" Hg
below the lowest steady speed engine vacuum observed.

Last edited by club327; 04-24-2005 at 08:12 PM.
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