larry b
Well-Known Member
i have a 750 proform carb how do you select the right power valve size
power valves
- Thread starter larry b
- Start date
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1968FormulaS340
Well-Known Member
[ame]http://www.holley.com/data/TechService/Technical/power_valves.pdf[/ame]
stumper
Well-Known Member
Rule of thumb - take your vacuum at idle and devide it in half. Get the PV that is nearest that value. Round up if necessary.
If engine vacuum at idle is 6, half is 3 but PVs only come in 2.5 and 3.5 size so go to 3.5. etc.
If engine vacuum at idle is 6, half is 3 but PVs only come in 2.5 and 3.5 size so go to 3.5. etc.
Blue Grey Duster
Well-Known Member
This is from FBO tuning guide. He is working with a Demon carb, but I think it's all relevant.
http://www.4secondsflat.com/
Power Valve
http://www.4secondsflat.com/
Power Valve
Let's first of all look at the function of the power valve…when you initially stomp
the throttle the front end of the carb is running 8 jet sizes smaller (right?) so it
needs to catch up to the rear…we'll explain this off balance situation later.
To determine the optimum power valve hook up your trusty vacuum gauge to the
lower or below the throttle plate port, (on a Demon it's the small port that's
closest to the firewall on the passenger side) now fire it off warm it up and drop it
in gear. You'll probably see a reading of 4-20 depending on the amount of
the throttle the front end of the carb is running 8 jet sizes smaller (right?) so it
needs to catch up to the rear…we'll explain this off balance situation later.
To determine the optimum power valve hook up your trusty vacuum gauge to the
lower or below the throttle plate port, (on a Demon it's the small port that's
closest to the firewall on the passenger side) now fire it off warm it up and drop it
in gear. You'll probably see a reading of 4-20 depending on the amount of
overlap on your cam for this example we'll use 8. This is of course is after you
have everything else set correctly in the idle circuits and the ignition timing.
The formula is 1.5 to 2.0” below idle in gear vacuum, so we would use a 6.0-6.5
power valve in this example. This is the correct method and you should not vary
from it. For a gear car (Standard tranny) we use 2” to 2.5” below manifold
vacuum at idle in neutral.
If you look at a metering block where the P/V screws in, you’ll see two tiny holes,
have everything else set correctly in the idle circuits and the ignition timing.
The formula is 1.5 to 2.0” below idle in gear vacuum, so we would use a 6.0-6.5
power valve in this example. This is the correct method and you should not vary
from it. For a gear car (Standard tranny) we use 2” to 2.5” below manifold
vacuum at idle in neutral.
If you look at a metering block where the P/V screws in, you’ll see two tiny holes,
these are called the "Power Valve Channel Restrictors", big name for a couple of
little holes. This is where the fuel dumps through the power valve to richen the
front half of the carb when you smack the throttle. Remember what I said about
the 8 size smaller jets in the front…well the "Power Valve Channel Restrictors"
dump fuel through the valve at a rate equal to a #8 jet sizes which gives you the
correct balance we're looking for.
Having the incorrect jet split will cause the P and S ends to operate at
different Air/Fuel ratios especially during the transition causing stumble,
hesitation and at WOT causing the engine to possibly lean out on ½ the engine
this will result in poor and inconsistent ET’s.little holes. This is where the fuel dumps through the power valve to richen the
front half of the carb when you smack the throttle. Remember what I said about
the 8 size smaller jets in the front…well the "Power Valve Channel Restrictors"
dump fuel through the valve at a rate equal to a #8 jet sizes which gives you the
correct balance we're looking for.
Having the incorrect jet split will cause the P and S ends to operate at
different Air/Fuel ratios especially during the transition causing stumble,
hesitation and at WOT causing the engine to possibly lean out on ½ the engine
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