Thread: Power valve do I need one?

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I'm running a 68 impala 4 door with a 381 sbc pump gas engine has a QF 950 runs a single power valve in the front. Almost everyone tells me to plug it, but I am skeptical because this engine sees different plenum vacuum readings when staged and at WOT going down the track. when foot braking the plenum is at 8 in and at WOT crossing the line at 6800rpm the plenum is pulling 2 to 3 in, take in to account this engine gets shifted at 7200rpm.

So the Main system in a carb basically operates of vacuum a pressure drop in the main venturis(boosters/doglegs) pulls a mixture of air and fuel from the main system in the carb. As engine rpm increase you displace more over a giving time. More air is moved through the intake in a giving time, the flow is faster, the pressure drop gets higher over the venturis and pulls more air and fuel.

THE POWER SYSTEM
I am on the understanding that the power system exists in a carb because at WOT throttle when the plenum vacuum drops this can cause the pressure drop to lower over the venturis and make the the main system inefficient and not deliver enough fuel where as air has more of a tendency to just fall in the carb and cause a leaning effect and power loss. So it basically corrects Air:Fuel ratio at high engine loads

Would it be beneficial to have in my car? I could justify not having one if this was a car that only saw 6000rpm and up all the way down the track, but the engine in my car operates between 3000rpm to 7200rpm the duration of the run

What are the specs of your engine

Compression ratio
Heads (type, intake/exhaust sizes)
Cam specs
Intake
Ignition

Has the car been on a CHASSIS (not engine) dyno?

Once we have these items, we can then decide whether you need a new power valve, or perhaps a small carb.

Bill S.

Originally Posted by Jed Rumbolz

I'm running a 68 impala 4 door with a 381 sbc pump gas engine has a QF 950 runs a single power valve in the front. Almost everyone tells me to plug it, but I am skeptical because this engine sees different plenum vacuum readings when staged and at WOT going down the track. when foot braking the plenum is at 8 in and at WOT crossing the line at 6800rpm the plenum is pulling 2 to 3 in, take in to account this engine gets shifted at 7200rpm.

So the Main system in a carb basically operates of vacuum a pressure drop in the main venturis(boosters/doglegs) pulls a mixture of air and fuel from the main system in the carb. As engine rpm increase you displace more over a giving time. More air is moved through the intake in a giving time, the flow is faster, the pressure drop gets higher over the venturis and pulls more air and fuel.

THE POWER SYSTEM
I am on the understanding that the power system exists in a carb because at WOT throttle when the plenum vacuum drops this can cause the pressure drop to lower over the venturis and make the the main system inefficient and not deliver enough fuel where as air has more of a tendency to just fall in the carb and cause a leaning effect and power loss. So it basically corrects Air:Fuel ratio at high engine loads

Would it be beneficial to have in my car? I could justify not having one if this was a car that only saw 6000rpm and up all the way down the track, but the engine in my car operates between 3000rpm to 7200rpm the duration of the run

Is this a track only car, or do you depend on it for street operation and want it to have good street manners, too?

The use of a power valve lets a carb (Holley in this case) be set up to run leaner during normal everyday to the market/school drives for more effiency but will richen up the mixture equal to 5 -6 jet sizes at a low vacume which occurs at higher throttle positions-

These days-------fuel sucks, no such thing as vacume(well if there was a perfect vacume there wouldn't be anything so evedently there isn't any such thing as vacume)

How much etonal does your fuel have in it? If you are racing why not use 100+ race fuel? For racing you could lock the advance in dist and only retard it for starting--

Have you verified tdc and all the markings on your crank pulley???

..................

Originally Posted by mrmustang

What are the specs of your engine

Compression ratio
Heads (type, intake/exhaust sizes)
Cam specs
Intake
Ignition

Has the car been on a CHASSIS (not engine) dyno?

Once we have these items, we can then decide whether you need a new power valve, or perhaps a small carb.

Bill S.

Billy

We run a 11.5:1 static compression ratio with 91 octane pump gas
heads are iron 190cc darts ported to about 200c, they run a 2.05'' intake and a 1.68'' exhaust
cam specs- solid roller, both intake and exhaust lifts are in the .660'' lift range and the duration for both are around 260 degrees @.50'', 109 LSA, and the Intake centerline I think is 108 degrees
Intake manifold is a 5.8'' tall holley strip dominator single plane
Carb is a 950 QF with a 1'' spacer underneath
headers have 1-5/8'' primaries with a 3'' secondaries
ignition- MSD 6Al-1 box, MSD distributor, coil is a master blaster

Bill the car has not seen a dyno of any kind. I agree, I think it needs a larger carb, this is a father son drag team and pops doesn't want to try a 1050 at the time

Originally Posted by rspears

Is this a track only car, or do you depend on it for street operation and want it to have good street manners, too?

It did see quite a bit of street use but now only on once in a while

Originally Posted by jerry clayton

The use of a power valve lets a carb (Holley in this case) be set up to run leaner during normal everyday to the market/school drives for more effiency but will richen up the mixture equal to 5 -6 jet sizes at a low vacume which occurs at higher throttle positions-

These days-------fuel sucks, no such thing as vacume(well if there was a perfect vacume there wouldn't be anything so evedently there isn't any such thing as vacume)

How much etonal does your fuel have in it? If you are racing why not use 100+ race fuel? For racing you could lock the advance in dist and only retard it for starting--

Have you verified tdc and all the markings on your crank pulley???

Jerry TDC has been verified,
I have heard locking the advance WOT makes for a poor running engine at idle and makes unnecessary heat, I cant run through the pits like a nascar after all you gotta drive like granny going to church
$3.00 a gallon is nice price for fuel going down the track, I wouldn't consider using race fuel unless we jacked the compression ratio.

but----------you have jacked the compression ratio and with iron heads-------

With your engines specs, you actually need a smaller carb (850cfm properly tuned), not a larger one. If strictly a drag car, then a QF-R 850cfm carb , if a street car, then a 850cfm street carb . You;ll find that when it comes to carburetion , bigger is not always better, a few hours on a chassis dyno with someone that knows how to tune the carb, will be the proof needed.


Bill S.

Like Bill says, and the dyno will also solve your ignition/timing curve questions.

And you'll need the power valve to drive like granny going to church in the pits, without fouling your plugs.