Holley Carb Power Valve
Thread Starter
Racer
Joined: Jul 2003
Posts: 301
Likes: 2
From: Memphis TN
Holley Carb Power Valve
Hey guys, I need a new power valve for my 81's Holley carb. I tested the vacuum on it and it is around 9" hG. What would be a correct valve size? Is there a special equation to figure it out or just minus 2" of hG... I've heard a couple of ways to figure out the size, which is correct?
Thanks a bunch!
Thanks a bunch!
Drifting
Joined: Jan 2002
Posts: 1,500
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From: N.Y
Re: Holley Carb Power Valve (jetlag)
If its a stock motor there is a whole procedure on the right way. but if it has a cam and makes a low amount of vacuum then get a reading in gear and divide it in half.
Thread Starter
Racer
Joined: Jul 2003
Posts: 301
Likes: 2
From: Memphis TN
Re: Holley Carb Power Valve (78 Vette)
Ok Thanks guys!
That was a quick response. I just posted expecting to get an answer in the morning, but dang!
I just bought the 1981 Electron Blue vette, and I absolutely love it. I can get that 5$ valve tomorrow and be ready to roll!
That was a quick response. I just posted expecting to get an answer in the morning, but dang!
I just bought the 1981 Electron Blue vette, and I absolutely love it. I can get that 5$ valve tomorrow and be ready to roll!
Le Mans Master
Joined: Mar 1999
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From: delmont pa
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Re: Holley Carb Power Valve (jetlag)
check the vacuum at WOT because if you go too low it can close at WOT and lean out the engine. if the carb is set correctly the PV has no effect on the idle :chevy
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Le Mans Master
Joined: Dec 2003
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From: Fremont CA
Re: Holley Carb Power Valve (jetlag)
9" of vacuum seems REALLY LOW to me. I have a very big cam in my car and can still pull 12-14" at idle. You car is an automatic I presume, if you don't have the totally wrong carb/camshaft/intake manifold/engine combination then I would first take a can of carb cleaner and check around the carb and head/intake mating surfaces for intake leaks or a vacuum hose or system problem. The car will not be street driviable with 9" of vacuum, you will need a steep set of rear gears and a BIG accelerator pump shot to prevent an off ildle lean condition when applying throttle. I could be wrong and you could be discussing a race car, if that is the case, I would run a 4.5 or 3.5" power valve but have never had to run one that low except when playing around with Z/28 Xrams and tunnel rams with two 4bbls on them. My opinion, good luck!
Team Owner
Joined: Apr 1999
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From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Re: Holley Carb Power Valve (Matt Gruber)
What Clem and Matt are talking about. drive around with a vacuum gauge duct taped to your window. Do some WOT runs locked in second gear from 2500 - red line
Your lowest vacuum will be right at 2500 and vacuum should increase as the RPM goes up. You want a power valve that is still open and dumping fuel past peak TQ rpm.
The poster should do a couple of things to increase the vacuum.
1. increase the idle rpm. My motor likes a 900 rpm idle hot.
2. increase the idle spark advance. hot cams like 14-18 initial
3. Make sure that your idle fuel air mixture is correct.
[Modified by gkull, 3:08 PM 3/30/2004]
Your lowest vacuum will be right at 2500 and vacuum should increase as the RPM goes up. You want a power valve that is still open and dumping fuel past peak TQ rpm.
The poster should do a couple of things to increase the vacuum.
1. increase the idle rpm. My motor likes a 900 rpm idle hot.
2. increase the idle spark advance. hot cams like 14-18 initial
3. Make sure that your idle fuel air mixture is correct.
[Modified by gkull, 3:08 PM 3/30/2004]
Le Mans Master
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From: delmont pa
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Re: Holley Carb Power Valve (jetlag)
if you use too low a PV number you can also have a hesitation because the power valve is opening too late. with a hot cam and a auto trans you may need a higher stall speed convertor. :chevy
[Modified by clem zahrobsky, 12:50 PM 3/30/2004]
[Modified by clem zahrobsky, 12:50 PM 3/30/2004]
Le Mans Master
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From: delmont pa
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Re: Holley Carb Power Valve (jetlag)
if the engine RPM you need to keep the engine running at idle and in gear and you have to hold the brakes to keep the car from moving you need a higher stall speed convertor. :chevy
Drifting
Joined: Jan 2002
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From: N.Y
Re: Holley Carb Power Valve (clem zahrobsky)
P.V dose nothing at idle? :crazy: :confused: . Go to a local speed shop and buy the holly book It will tell you everything you need to know. 99% of the time if the motor makes vac at wide open throttle its because it is a small carb on a larger motor. :thumbs:. What I posted earlier is right out of the book.
Thread Starter
Racer
Joined: Jul 2003
Posts: 301
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From: Memphis TN
Re: Holley Carb Power Valve (78 Vette)
Do you think the Holley 600CFM carb on my motor is too small for it?
New 350 , Bored .30 Over, 10.50:1 Compression, Polished Holley 4bbl. Carburetor, 64cc Vortec Heads w/Polished Performer Intake, SRP Flat-top Pistons, Xtreme Energy Comp Cam 12-246-3(.487/.490 Lift),
New 350 , Bored .30 Over, 10.50:1 Compression, Polished Holley 4bbl. Carburetor, 64cc Vortec Heads w/Polished Performer Intake, SRP Flat-top Pistons, Xtreme Energy Comp Cam 12-246-3(.487/.490 Lift),
Team Owner
Joined: Apr 1999
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From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Re: Holley Carb Power Valve (jetlag)
I had that cam in a 355 ci motor and never really liked it. CS XE274H-10 It had poor vacuum and it made very peaky HP. when combined with my 750 cfm carb on a single plane intake. It was a motor that I was almost happy when it blew up because then i didn't have to fix it.
To answer your question: 600 cfm is great. That wimpy intake manifold that starves the motor at just over 5000 rpm is a serious mismatch with your high winding cam.
To answer your question: 600 cfm is great. That wimpy intake manifold that starves the motor at just over 5000 rpm is a serious mismatch with your high winding cam.
Team Owner
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Re: Holley Carb Power Valve (jetlag)
Carbs are complicated :(
Drifting
Joined: Jan 2002
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From: N.Y
Re: Holley Carb Power Valve (gkull)
:iagree: your carb might be slightly small but I don't think its much of a problem. Just do like I mentioned earlier and your pv issue should be ok. IF there is still problems there my be something else wrong but don't stress your self out a Holly is a pretty simple carb to work on. You really should buy the book. My vette is a 12.50 car and I drive it every day and have been for the last 2 1/2 years with a 750 and haven't had one problem since the day it was installed. :thumbs: Good luck
[Modified by 78 Vette, 6:27 PM 3/30/2004]
[Modified by 78 Vette, 6:27 PM 3/30/2004]