From discussions on this board (and the local speed shop agreed), the concensis seems to be to pick the power valve based on 1/2 idle vacuum. For instance, I have about 9.5-10 inches at idle in drive, so a 4.5-5 PV would be indicated by that formula. The carb came with 6.5, so I'm running a 4.5 right now. I did not see any improvement over the 6.5, and some off-idle degradation.

The Holley web site ( http://www.holley.com/HiOctn/TechSer...fo/CI-121.html ) says:
"A competition or race engine which has installed a long duration, high overlap cam will have low manifold vacuum at idle speeds. If the vehicle has a manual transmission, take the vacuum reading with the engine thoroughly warmed up and at idle. If the vehicle is equipped with an automatic transmission, take the vacuum reading with the engine thoroughly warmed up and idling in gear. In either case, the power valve selected should have a vacuum opening point about 2" Hg below the intake manifold vacuum reading taken.

"A stock engine, or one that is only mildly built for street use, will have high manifold vacuum (17" to 21" Hg) at idle speeds. To determine the correct power valve, the vehicle should be driven at various steady speeds and vacuum readings taken. The power valve selected should have an opening point about 2" Hg below the lowest steady speed engine vacuum observed. Holley has a 6.5" Hg power valve, P/N 125-65, which usually works out well for most driving situations."

I think I went the wrong way.

 

a 6.5" is good enough for most motors. Unless you wanna be real picky or have a big enough cam to open a 6.5" at low loads I say leave it be.

If you really wanna get the PV dialed in, you really need an A/F gauge, and sort of balance it with the jets. Plug the PV, and tune the jets so that it reads dead on at lower RPM's...then falls off lean as RPM's rise. Then tune the PV so that it opens just as the jets fall off lean. Get the latest opening valve that leaves no lean gap inbetween.

 

Five7 , may I suggest you try a 8.5 or a 7.5 power valve? Give it a try and post results. On my carb setup I have 8 inches at idle in drive and I've got a 6.5 no kidding! I have not had any problems with this setup.

 

The holley method usually works well. Where are you lossing power at? Is it idle, mid or WOT? Which secondary spring are you using? If you were to use the 6.5 and change to a firmer secondary spring 1 step up it might help.
What exactly is the car doing?

Oh yea, I posted on the (ATTENTION:SSC by Jester) about the spacer of course it got erased when the board crahed. If you decide to try a spacer look at the square to spread bore adaptor. They have them at autozone for $18 with everything gaskets bolts. They are great spacers 4 small holes on top the back two get larger as they go down. These are perfect for that action+'s spread bore design.
SSC

 

The problem is an off-idle flat spot, also initial acceleration from cruise. Sounds like the PV to me, with what I've learned here. (SSC, I thought you recommended a PV using the 1/2 idle vacuum a few months back when I asked...)

The car actually acts like it would like the secondaries to open sooner. I have the spring kit from Holley, went both heavier and lighter from what came in the carb, have lightest in now. It has never bogged when nailed.

As for the spacer, the crash got both your suggestion and my response on that other thread. Jester recommended an open spacer, what you're talking about is not.

So, what effect would a 4-hole spacer have vs. open? I assume no effect on the PV (just to keep it on-topic).

 

4 hole spacers are designed to increace low end just a bit by increasing the velocity of the air through the carb. Kinda sorta like making the runners longer.

Open spacers effectivly increase plenum volume, adding a few RPM up top and helping bandaid otherwise choked motors.

and no..niether effects the PV

 

I might go ahead and try that spacer (no big loss if it doesn't help), and get a 7.5 PV as well.

If the carb needs both low end help and high end flow, could I take the die grinder to the lower half of the 4-hole spacer and get the best of both worlds?

Thanks for the help, all. Might I suggest we steer people to the Holley page whenever PV choice questions come up in the future?

 

no..you get a decent manifold and carb

The Holley page is a good source of info..certainly couldn't hurt.

 

Thats why I reccomend this spacer. The 2 back bores wont choke the engine at higher rpms because they gradually increase in size (o-O) and all 4 bore are angled for max velocity.
Ive tryed open spacers but I like this one better, Its got a more tourqy feal and its a very good design. I'll see if i can scan the sucker and show ya what im talking about.
SSC

 

stuff that wouldent work.

 

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Edited to make the photos work here.
SSC