[reelknots]

I have a Holley 4779-6, 750 cfm that I cannot find the number that identifies the power valve. I looked all over with a magnifying glass and the only numbers I found were a 4, 5 and 6 stamped in different quadrants on the brass disc just under the hex that you use to remove the valve. Could this be an aftermarket valve and not a Holley part?

Thanks
Steve

[dubs68camaro]

That is actually the way Holley does it. It's a 6.5 hg power valve!

[Pro-Street69Camaro468]

I was taking about this last night and a buddy of mine told me vacuum at idle divided by two is the size.Not sure if this is correct but sounded like a good lie to me.Heres a question on an accellorator pump 30 in the front should there be any preload or should it be a couple thousanths clearence.Thanks if no answer I will start a thread.

[reelknots]

Thanks for the response. I'm wondering what effect a 6.5 power valve has on an engine that pulls 17" at idle? As for a pre-load on the acc. pump, Holley says there should be 15 thous. clearance at wide open throttle so the pump diaphram doesn't bottom out.

[Skeeter55]

Originally Posted by Pro-Street69Camaro468

I was taking about this last night and a buddy of mine told me vacuum at idle divided by two is the size.Not sure if this is correct but sounded like a good lie to me.Heres a question on an accellorator pump 30 in the front should there be any preload or should it be a couple thousands Clarence.Thanks if no answer I will start a thread.

You should be able to hold the throttle linkage wide open and still have about 020.0 free play so you don't over extend the diaphragm. Check this link out www.4secondsflat.com

[Tom P]

Originally Posted by reelknots

Thanks for the response. I'm wondering what effect a 6.5 power valve has on an engine that pulls 17" at idle?

I believe the 6.5 power valve was stock - @ 17" vacuum you should be OK

YouTube - Carburetor Power Valves Video - Holley Carb Tuning DVD

YouTube - Carburetor Power Valves Video - Holley Carb Tuning DVD

[reelknots]

Very informative video. Thank you very much.

[markw]

I can't believe Holley would spread a myth like "half the vacuum at idle" for a power valve. Bull. If you have an air/fuel meter and a vacuum gage you can find the exact valve needed. Put in a really low power valve like a 3.5". Go up a hill in high gear (to put a load on the motor) and hold the vacuum at 8" and watch the air/fuel meter. Then hold it at 7", then 6", etc. When the motor leans out (you'll feel it surging at that point even if you don't have an air/fuel meter) look at the vacuum gage and pick a power valve 1-2" higher. I can make my idle vacuum change 5" just by turning the idle speed screw. What does that have to do with the power valve? You need to find the point where the motor needs to transition from the 'cruise' mode mixture to the 'power' mode mixture. A heavy vehicle needs to transition sooner than a light vehicle. 2.56 gears need to transition sooner than 5.13 gears. Some high overlap cams idle at 5" but cruise at 15" and need 8.5" power valves. Put a 2.5" valve in there and watch the motor rattle the bearings out going up a hill at 6" vacuum. The motor tells you what it needs. Argue with it and see who wins.

[Skeeter55]

Originally Posted by markw

I can't believe Holley would spread a myth like "half the vacuum at idle" for a power valve. Bull. If you have an air/fuel meter and a vacuum gage you can find the exact valve needed. Put in a really low power valve like a 3.5". Go up a hill in high gear (to put a load on the motor) and hold the vacuum at 8" and watch the air/fuel meter. Then hold it at 7", then 6", etc. When the motor leans out (you'll feel it surging at that point even if you don't have an air/fuel meter) look at the vacuum gage and pick a power valve 1-2" higher. I can make my idle vacuum change 5" just by turning the idle speed screw. What does that have to do with the power valve? You need to find the point where the motor needs to transition from the 'cruise' mode mixture to the 'power' mode mixture. A heavy vehicle needs to transition sooner than a light vehicle. 2.56 gears need to transition sooner than 5.13 gears. Some high overlap cams idle at 5" but cruise at 15" and need 8.5" power valves. Put a 2.5" valve in there and watch the motor rattle the bearings out going up a hill at 6" vacuum. The motor tells you what it needs. Argue with it and see who wins.

Mark with my BBC 498 cam duration 254 @ 50 with a 660 lift and 110 LSA, running a Pro-System carb 4150 1000hp and 972 true cfm with a 6.5 power valve and about 10"-12" vacuum at 900rpm's. Should this be a good 6.5 or should i try another power valve.

[markw]

Skip, do the vacuum 'crowds" and feel where the motor leans out. You don't even need the air/fuel meter, you'll feel it. Opening too soon will just waste gas. Opening too late may hurt the motor or at least give you a dead spot that you have to 'step over'. The only reason I can think of for Holley to say "half the vacuum at idle" is because that might be a safe number for the average motor. I've heard those "Thumper" cams give really low idle vacuum but recover to a more normal vacuum at cruise. That would give you a dangerously low power valve number. But it's not really that hard to find the right number with some experimenting.

[Skeeter55]

Originally Posted by markw

Skip, do the vacuum 'crowds" and feel where the motor leans out. You don't even need the air/fuel meter, you'll feel it. Opening too soon will just waste gas. Opening too late may hurt the motor or at least give you a dead spot that you have to 'step over'. The only reason I can think of for Holley to say "half the vacuum at idle" is because that might be a safe number for the average motor. I've heard those "Thumper" cams give really low idle vacuum but recover to a more normal vacuum at cruise. That would give you a dangerously low power valve number. But it's not really that hard to find the right number with some experimenting.

The only stumble that i have is when im releasing the clutch from a dead at about 1000rpm's to 1400rpm's if i dont give it some help from the throttle. But anything past this rpm it smooth with big balls of fire, now i may be leaning towards accelerator pump or air bleeds.

Heres my carb specs.