Determining correct power valve

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pjc360

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I have read a lot of different theory's on how to select the correct power valve for a holley style carburetor. I have read that if you have an auto you put it in gear and take the vaccum reading when it's in gear and divide it by 2 and that's the power valve you should run. I have read you take the vaccum reading you get while cruising down the highway at 65 and divide that by 2 and then add 2 and that's the power valve you should select.
I have also read you take your idle vaccum and minus 2 from that and that's the correct power valve.
I have a quick fuel super street series 680cfm vaccum secondary carburetor and I am running it on a 300 hp crate 360 magnum in a 1991 half ton dodge power ram 150 4x4 truck. The way I determined what power valve to run was I put my truck in drive (it's and auto) and I took the vaccum reading which is 15.5 to 16.0 hg and I divided that by 2, so I decided to put an 8.5hg power valve in it. But now I am wondering if I did that wrong. How do you guys select the power valve you want to run?
I am now wondering if I should hook up a vacuum gauge and go cruise around on the highway and take my cruising at 65 vaccum and divide that by 2 and then add 2 to that and select a power valve based on that.
I was just curious to see how you guys go about selecting power valves and am curious to know if you think I did it the right way, or if you think I should select a power valve based on my cruising vaccum numbers.
 
The only way i do it is drop it in gear take the reading and drop down 2 sizes.this is critical for a holley.
 
8.5 is not bad for your use, it means you will not get full enrichment of fuel until the vacuum drops below 8.5 in's of mercury, if you want better fuel mileageand betetr drivablity like when towing a trailer go up to a 10.5 two stage power valve
 
8.5 sounds reasonable for your combo, although it could be a bit thirsty. Hook up a vacuum gauge and drive normally. If you spend a lot of time with less than 10" of vacuum, you might want to try a 6.5. It's a compromise between throttle response and MPG. How lean/rich your main jets are will also make a difference.

Think of all the "Rules" as a starting point, and tune from there. If more than one of the "rules" leads you to the same starting point, all the more reason to start there.

With the Holley on my truck it would run the best with 8.5 PV and 62 main jets, but got 8-10 MPG, city-highway. With 6.5 and 62 it got 10-10. With 8.5 and 60 it got 10-12. With 6.5 and 60 it got 11-12, but it was a little doggy in traffic (lean).
 
I run 64 main jets and 74 secondary jets. I will be putting a vaccum gauage on it today and go drive around and see where my vaccum is when I'm just going for a normal drive.
 
well scratch that idea, I was tying to find a spot to feed some vaccum hose into my cab so I could watch the vaccum gauge while driving and I accidently dropped the vaccum gauge and it shattered. Havent even had it for 6 months ether. Guess this will have to wait until I can get another one.
 
Well I went and got a brand new vaccum gauge for 14 bucks and some change.
I hooked it up and routed it thru the fire wall and into the cab. I did my best to read the vaccum gauge while driving but it was a little tricky with 6 inches of snow or more on the roads.
But from what I can see, when I'm on the highway at a steady cruise of 50 to 65 mph the vaccum is right around 11hg to 13hg. I am currently running an 8.5hg quick fuel power valve which is a high flowing power valve. I noticed that when I start into the gas from basically any speed the vaccum drops down to around 7hg to 5hg. So I am thinking the 8.5hg power valve I already have inn might just be perfect.
 
well today the snow finally melted off of the roads, so I was able to take the truck out the highway and check out my vaccum readings while cruising the highway.
When I first pull out on a highway and start into the throttle to get up to 65 my vaccum is around 8hg-7hg and then when I hit 65 and I keep it steady at 65 my vaccum is at 12.5-13.0. I have an 8.5hg power valve and I think that Is right on the money from what I can see, I might benefit from a 9.5hg power valve. But when I give it half throttle from a steady cruise of 65 m vaccum drops to around 8hg-7hg.
I might try a 9.5 and see how it feels, but I'm guessing I wont be able to tell the difference because they are so close.
 
Sounds pretty reasonable. If you were to try anything, swap in a 6.5. You SHOULD notice a difference (slight part throttle sag). If not, go two numbers leaner on your primary jets, and put the 8.5 back in. You should just barely "feel" the PV open, but it should still be smooth.
 
I think the hi flow PV is for alcohol carbs, still will work fine for reg holley carbs but the flow will be the same as standard PV in a regular holley the alcohol carbs have bigger PV ports
In the metering block.
 
Well the power valve I run is made by quick fuel and they call it a 4 door power valve because there is 4 different openings for the fuel to flow thru, so I am assuming that would be considered a hi-flow power valve. I don't think I want to go leaner on my jets. My plugs look pretty darn good with the 64 main jets and 74 secondary jets.
I might try a 9.5hg power valve just too see if I can even notice a difference. but the 8.5hg power valve I have in it now seems to be right where I would want it.
According to most people I have talked to you take your vaccum reading at a steady cruise of 65 on the highway and divide it by 2 and then add 2 and that will be pretty darn close. And when I divide 12.5 by 2 I get 6.25 and when I add 2 too that I get 8.25 so an 8.5hg power valve should be pretty close to perfect.
 
I was thinking since I am in a heavier 4x4 truck I might benefit to have the power valve open sooner, but I don't know if my thinking has any logic. But a heavier vehicle takes more power to get up to speed so maybe a 9.5hg power valve would be even better? I guess I wont know until I try it.
I know I need to lean down my power valve channel restrictors, I had to lean down everything else in the carburetor. and the power valve channel restrictors are the only thing I have not leaned down. My carburetor has .049 power valve channel restrictors and I was thinking about installing some .045 power valve channel restrictors. I had to go down 4 sizes on the main and secondary jets and go down 4 sizes on my idle fedd restrictors to get it to run right at my higher elevation.
 
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