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Author Topic:   POWER VALVE ?

Posts: 1586
From: Vancouver Island , British Columbia , Canada
Registered: Nov 2001

posted 05-28-2002 03:46 PM     Click Here to See the Profile for johnmustang   Click Here to Email johnmustang     Edit/Delete Message   Reply w/Quote
What is the proper formula to figure out which power valve to put in your carb. I thought it was, Take your manifold pressure at idle , divide it in half ,and , add .5 , so , 14 divide in half = 7 (+.5) = power valve of 7.5 , I am now told this is wrong , but , was not told how to do it . Can anybody please help me out with this ?


Member:Vancouver Island Mustang Association
M&M #1710

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capri man

Posts: 1495
From: doerun, ga.
Registered: Nov 2000

posted 05-28-2002 03:50 PM     Click Here to See the Profile for capri man   Click Here to Email capri man     Edit/Delete Message   Reply w/Quote
sounds prety close to me john. most holleys come with a 6.5.

mike r
racing is real
everything else is just a game.
81 capri-7.56 @88mph 1/8
1.56 60 ft.

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kid vishus

Posts: 2361
From: not where I want to be
Registered: Oct 2000

posted 05-28-2002 04:13 PM     Click Here to See the Profile for kid vishus   Click Here to Email kid vishus     Edit/Delete Message   Reply w/Quote
I always thought that was the proper way to figure it.

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Posts: 2099
From: alvaton,ky,usa Suburb of Bowling Green, M&M #79, MCA #29208
Registered: Jun 99

posted 05-28-2002 11:44 PM     Click Here to See the Profile for kcode   Click Here to Email kcode     Edit/Delete Message   Reply w/Quote
According to Holley, the power valve should have a rating that is 2" Hg below the lowest steady vacuum reading. Again this depends on motor build. Assuming you have a reading of 9" at idle, you could get by with a 65 or 6.5". Most of the time it will be trial and error.


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Dusty Kiser

Posts: 71
From: Bethel,Oh USA
Registered: Mar 2002

posted 05-31-2002 10:44 PM     Click Here to See the Profile for Dusty Kiser   Click Here to Email Dusty Kiser     Edit/Delete Message   Reply w/Quote
If you've ever driven a car with a vacuum gauge, you know it takes very little application of throttle for the vacuum to drop like a rock. So' you could look at it this way, no matter how low you go it's going to open eventually. If you go too high you're going to know it as soon as you start the engine!

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Daniel Jones

Posts: 304
From: St. Louis, MO
Registered: Aug 99

posted 06-01-2002 10:05 PM     Click Here to See the Profile for Daniel Jones   Click Here to Email Daniel Jones     Edit/Delete Message   Reply w/Quote
When I built the temporary engine for the Pantera, I ran up and
down State Route 125 (couldn't have been too far from you Dusty,
I was turing around right near Bethel high school) with the vacuum
gauge hooked up to figure out what power valve to use. I had a
Holley 735 vacuum secondary 4150 off my dad's '69 428CJ on the
engine. Unless I was WFO, the vacuum really didn't seem to vary
that much. Something about that engine combo (351C, mild cam
218/222 @ 0.050", 110 LSA, 0.530" lift, single plane intake,
ported Aussie 2V heads, 1 3/4" diameter headers, mechanical advance
only) in the low drag, short geared (4.22:1 final drive though 5th
is 70% overdrive) Pantera but it just seems unburdened. It's quite
happy to pull 5th gear from just off idle. I ended up using the 2"
below part throttle cruise/steady acceleration rule that Holley
suggests to pick the power valve but I'm pretty sure I can lower
on the PV rating. I probably ought to swap out the stiff 428CJ
secondary spring one of these days too. I can feel the power dip
before the secondaries open.

A friend put a mild 302 in a 1st generation Mazda RX7 and had
trouble with the carb during cruise. With the 3.90:1 gears,
tiny 13" diameter tires, and low weight it wanted to cruise on
the idle circuit alone. I wonder what it's vacuum readings would
look like?

Dan Jones

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