another question.....correct Power Valve??

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aengineguy

aengineguy

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I have been tuning a little at time in the past few months. I had left the Power Valve alone, but realized I was pretty far off. Original PV in the Holley 750 HP was 6.5, but after checking the vacuum at idle in gear it was hovering around 5 inches (does this seem too low??). I went to a 3.5, but am thinking to go to a 2.5.

Watchathink?

408
10.65 CR
24 initial 34 total
Intake .610 with 252 @.050

Thanks......!!!!
 
That's what I figured, but did not want to go too far. Yup.......Flat Solid Intake CL is 103 and LSA is 106, pretty stinkin radical idle. I have a sound byte recently in the exhaust section.
 
I have the MP .557" solid cam in my 340 which is also 252 @ 50. I usually get 5-7" of vacuum depending on how the idle is behaving when I check it. No amount of fiddling will increase it.

What is the car doing/reasoning that would make you think the PV needs to be changed?

Do you have a wide band 02 meter to monitor your AFR? I think the vacuum @ idle check is basically a ballpark verification to make sure it does not open too early. Checking AFR under load would be definitive.
 
Thanks rmchrgr.....

If I am running at 5" and currently have a 3.5, I am a little concerned that the PV may be too close of a threshold and may be bouncing open a little. I did not go to the trouble installing bungs in the collectors.....yet, so I don't have an 02 meter. I know it's the way to go and may do it in the future.
 
aengineguy said:
Thanks rmchrgr.....

If I am running at 5" and currently have a 3.5, I am a little concerned that the PV may be too close of a threshold and may be bouncing open a little. I did not go to the trouble installing bungs in the collectors.....yet, so I don't have an 02 meter. I know it's the way to go and may do it in the future.
Click to expand...

I think then the only way to know if it's correct is to do some back-to-back testing. Try the 3.5, make a blast down a back road, shut if off and read the plugs. Then switch it out for the 2.5. Repeat procedure and compare.
 
What are you running for initial timing? Can you use more?
 
67,

Currently @ 24 initial with 10 mechanical total 34. Don't know how far I want to push it. not bucking back on start or pinging.
 
3.5 should work OK.

Unless you have an A/F meter to see when it open and how it changes ratios, you may not notice a difference between a 2.5 and 3.5.
 
You shouldn't notice a difference at idle. The main restriction for idle and transition is in the idle well. The fuel comes from the main jet, but as long as the main jet is about 4 times the size of the idle feed restriction, opening the PV isn't changing anything. The rule of thumb about idle was a best guess for less radically cammed cars.

Agree that you can try to sop test to feel if it 'kicks in' as you squeeze on part throttle going getting close to full. If it kicks, go higher, that's an indicator it was going lean than it wanted for the increasing load. (edit. Starting with a high power valve, the kick would be from hitting rich to early making it momentarily lazy until more throttle opening.)

Important thing of course is to make sure it stays open at WOT through top rpm.

Wide band is most useful with a MAP and/or throttle position sensor, along with rpm of course - just in case you decide to go that route at some point.
 
Unless the vac is jumping all over the place, i'd stick with the 3.5 for now. Your cam has some radical numbers and i'd like to see you stick with the PV for the street, although your close to running a blocked PV with a jump in jet to compensate.

Just a thought that hasn't been mentioned, can you open your valve lash a hair, might give you a "little" extra vacuum at idle.
 
Thanks for the input all...

Still on occasion playing with the lash. Agree, possibly give it a little more and see what that gives me.
 
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