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Username Post: How do you determine the correct Power Valve to run in your carb?        (Topic#222990)
Jim.Biron 
"7th Year" Platinum Supporting Member
Posts: 2525
Jim.Biron
Age: 64
Loc: Saugerties, NY
Reg: 04-10-08
08-04-09 04:39 AM - Post#1747608    

I've seen a lot of talk about jets and the like but haven't seen too much discussion on Power Valves. How do you determine what the right Power Valve should be in your carb?
Jim
www.HudsonValleyCruise.com


 
61ragtop 
Member
Posts: 622

Age: 38
Loc: Langley B.C. Canada
Reg: 07-30-03
08-04-09 05:00 AM - Post#1747626    
    In response to Jim.Biron

Determine your manifold vacuum at idle then go down about 2 inches of vac for the valve. So if manifold vac is 10 you will want a 8 power valve or lower.

 
rumrumm 
"13th Year" Silver Supporting Member
Posts: 1902
rumrumm
Loc: Macomb, IL
Reg: 10-18-01
08-04-09 06:42 AM - Post#1747692    
    In response to 61ragtop

The Holley tech guy told me several years ago to find your vacuum in inches with the car under load, and divide by 2. But I have also been told that it is only a starting point and you have to experiment from that point on.
Lynn

"There's no 12-step program for stupid."


383 sbc, Eagle, AFR, SRP, CompCams, Edelbrock, Sanderson. Dyno #'s: 450 hp @ 5700, 468 ft. lbs. torque @ 4300.

http://photo.net/photos/Lynn%20Johanso n


 
Jim.Biron 
"7th Year" Platinum Supporting Member
Posts: 2525
Jim.Biron
Age: 64
Loc: Saugerties, NY
Reg: 04-10-08
08-04-09 08:24 AM - Post#1747746    
    In response to 61ragtop

  • 61ragtop Said:
Determine your manifold vacuum at idle then go down about 2 inches of vac for the valve. So if manifold vac is 10 you will want a 8 power valve or lower.



Same for a carb with 2 power valves?
Jim
www.HudsonValleyCruise.com


 
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 16233
grumpyvette
Age: 66
Loc: FLORIDA USA
Reg: 03-16-01
08-04-09 09:28 AM - Post#1747787    
    In response to Jim.Biron

http://forum.grumpysperformance.com/viewtopic.php?...

if its a manual trans take the reading vacuum reading in neutral, if its an auto take the reading at idle in gear with your foot on the brake,then select a power valve that 1/2 that vacuum number as a starting point for tuning
EXAMPLE if the vacuum reads, at 15-16" then youll want about a 7.5 power valve
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


Edited by grumpyvette on 08-04-09 12:11 PM. Reason for edit: No reason given.

 
Big Jacks Auto 
Contributor
Posts: 314
Big Jacks Auto
Loc: Syracuse, N.Y.
Reg: 08-14-07
08-04-09 10:28 AM - Post#1747813    
    In response to Jim.Biron

Basically there are a couple of different theories on how to select the proper power valve in a holley carburetor

If you are running a automatic for example

Warm the car up hook up a vacuum gauge and put the car in gear if it is a stick car just keep it in neutral
If you are running a big cam you may be somewhere around 5 inches of vacuum at idle you would want to select around 1 number lower say a 40 power valve
The 40 power valve will open at 4”’s of vacuum and stay open until it goes above 4”’s of vacuum

When the power valve opens it dumps or adds the equivalent of ten jet numbers
So if you have 80 jets in the front and 90’s in the back you can cruise around town on 80 jets and get pretty decent mileage (well better mileage anyhow) then when you accelerate and need the additional fuel the power valve opens up and now you basically are running 90 jets front and rear

what you want to happen is to have the power valve come in just as you are accelerating
this of course differs if you are just leaving the light or leaving at 5,000 rpm with a 8" converter
You may have to experiment each car and engine combo is different and I have run into quite a few different scenarios in the last 25 years or so of running different variations of these cars on stock naturally aspirated as well as nitrous and blown applications

For the most part the 65 valve works well for most applications and the 45 or 55 works well with the larger cam setups or at least they are a good starting point

If you can hookup a vacuum gauge and cruise around town and get on the highway etc you can get a better idea of what power valve you need
say you have a 65 powervalve and you can keep an eye on what the car is doing at that point
I had a motor once that had a cam that produced an unreal amount of vacuum and I actually needed a power valve that opened around 15”’s of vacuum


The holley carbs are amazing pieces but it is amazing how many times I get a carb in that has been tuned so far out in left field it amazes me.
Hope this helps
S.H.R.A
Syracuse Hot Rod Association
55 Chevy 327 Muncie 4 speed
Prostreet Studebaker x Gasser SBC 6-71 Blown
Video of My SBC powered Studebaker


 
bobb 
Super Senior Member
Posts: 4944
bobb
Loc: paradise
Reg: 09-05-03
08-04-09 07:21 PM - Post#1748148    
    In response to Jim.Biron

i do mine by feel and use a vacum gauge too.
70 L camino 350 all forged,174 baby blower, g-force 5 spd, road rage suspension. Pray first before all else fails.


 
Weccemah 
Contributor
Posts: 169
Weccemah
Reg: 03-16-08
08-04-09 08:06 PM - Post#1748178    
    In response to Jim.Biron

I bought a book on tuning holley carbs a while bsck and it said to use a vacuum gauge and go drive and when you accelerate, say from a stop light or sign under your normal acceleration, take the lowest reading it dips to minus 2 hg of vacuum rounded down to nearest power valve.

For example, if you accelerate modestly from a stop and the gauge drops to 11.5 hg, then you'd want a 9.5 power valve. Or the gauge dips to 8 hg, then you'd want a 5.5 cause thats 6 rounded down to the nearest size.

I run a 6.5 in my 670 avenger on a 327 and it runs excellent.
1964 Chevy Impala SS


 
Jim.Biron 
"7th Year" Platinum Supporting Member
Posts: 2525
Jim.Biron
Age: 64
Loc: Saugerties, NY
Reg: 04-10-08
08-05-09 03:55 AM - Post#1748291    
    In response to grumpyvette

  • grumpyvette Said:
http://forum.grumpysperformance.com/viewtopic.php?...

if its a manual trans take the reading vacuum reading in neutral, if its an auto take the reading at idle in gear with your foot on the brake,then select a power valve that 1/2 that vacuum number as a starting point for tuning
EXAMPLE if the vacuum reads, at 15-16" then youll want about a 7.5 power valve



Got your PM Grumpy, Thanks. Good reading there.

Your instructions above are great!!! Simple and yet effective.Very cool.
Jim
www.HudsonValleyCruise.com


 
Jim.Biron 
"7th Year" Platinum Supporting Member
Posts: 2525
Jim.Biron
Age: 64
Loc: Saugerties, NY
Reg: 04-10-08
08-05-09 03:58 AM - Post#1748293    
    In response to Weccemah

  • Weccemah Said:
I bought a book on tuning holley carbs a while bsck and it said to use a vacuum gauge and go drive and when you accelerate, say from a stop light or sign under your normal acceleration, take the lowest reading it dips to minus 2 hg of vacuum rounded down to nearest power valve.

For example, if you accelerate modestly from a stop and the gauge drops to 11.5 hg, then you'd want a 9.5 power valve. Or the gauge dips to 8 hg, then you'd want a 5.5 cause thats 6 rounded down to the nearest size.

I run a 6.5 in my 670 avenger on a 327 and it runs excellent.




More good info for sure... I really should buy a book. Let's see where did I put that wallet????

Its interesting that on this new 496 I seem to be having som much "fun" trying to figure things out. Its also the first time I've ever used a Mighty Demon. In my 40+ years being a car guy I never have had to go through these things before. I guess luck was just with me. Tuning used to be a snap!!
Jim
www.HudsonValleyCruise.com


 
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