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Author
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Topic: POWER VALVE ?
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johnmustang
Gearhead Posts: 1586
From: Vancouver Island , British Columbia , Canada
Registered: Nov 2001
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posted 05-28-2002 03:46 PM
   
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 ? JOHN  ------------------
65 2+2 FASTBACK
68 COUPE
87 TAURUS WAGON
98 F150 XLT TRITON V8 4.6, 4 WHEEL DRIVE
Member:Vancouver Island Mustang Association
M&M #1710
MyPhotoPage
MY TRUCK
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capri man
Gearhead Posts: 1495
From: doerun, ga.
Registered: Nov 2000
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posted 05-28-2002 03:50 PM
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.
http://prestage.com/site/site_display.asp?SiteID=141
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kid vishus
Gearhead Posts: 2361
From: not where I want to be
Registered: Oct 2000
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posted 05-28-2002 04:13 PM
I always thought that was the proper way to figure it.
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kcode
Gearhead Posts: 2099
From: alvaton,ky,usa Suburb of Bowling Green, M&M #79, MCA #29208
Registered: Jun 99
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posted 05-28-2002 11:44 PM
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.Mike
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Dusty Kiser
Journeyman Posts: 71
From: Bethel,Oh USA
Registered: Mar 2002
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posted 05-31-2002 10:44 PM
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
Gearhead Posts: 304
From: St. Louis, MO
Registered: Aug 99
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posted 06-01-2002 10:05 PM
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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