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Baffled by which powervalve to order.

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Moloko

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Discussion starter · #1 ·
Im trying to figure out which powervalve I need for my car. At idle I have about 11Hg of vacuum. Cruising at 45mph about 15Hg. I've heard so many different ways to "pick" a power valve.

-"Half your cruising vac"
-"2'' under your cruising vac"
-"the point at which the car bogs"

What is the way to check for sure? Im probably going to order a few if I need too, along with that accel pump tuning kit. I really want to figure this car out so I can finally actually drive it.
 
#7 ·
Originally posted by 1bad67:
What is the idle rpm in park vs. in gear? It should not drop that low. Is it rich now? Give us a little info on the combo.
Click to expand...
Are you saying if a car is running rich the vacuum drops quite a bit from park to drive? Interesting.

I always thought that vacuum had a tendency to go UP the richer the engine ran? Seems like when you set the idle mixture that the "highest vacuum" always seems to be on the rich side of things.

If the car DOES have 5 inches HG in gear, then 2.5 pv would be in the ball park. I sure havent seen alot of those around, though. Everything is like 6.5-8.5. Might be a special order kind of deal on that one.
 
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#10 ·
Droping from 11" out of gear to 5" in gear would seem excessive. If it is actually dropping that much the butterflies may be out of adjustment, or your initial engine timing may be too low. You really should double check this before changing your PV.
 
Discussion starter · #11 ·
Been messing with this since june now, its gotten to the point where im going to just start throwing parts at it, like powervalves, possibly even a new carb.
 
#12 ·
There is something wrong with your engine, no way should it only have 5 inches of vac in gear for the engine you put together. 11 even seems low, and didnt you say before the vac reading was jumping around? It should be rock steady. From all the problems you have listed I bet you have a mechanical problem in the valvetrain and are never going to fix it throwing external parts at it. Something just isnt right with it. I did a flat top 350 with the CC 280 magnum cam which has a very lumpy idle and didnt have to touch the box stock holley I put on it. What are your cam specs?
 
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#13 ·
Originally posted by Moloko:
Been messing with this since june now, its gotten to the point where im going to just start throwing parts at it, like powervalves, possibly even a new carb.
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Buy an Edelbrock carb!!
 
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#14 ·
I agree with BG, you need to get the butterflies set right, timing set, then tune the carb. Can't help without the motor info. Sounds like to much cam and to little converter stall, is it a driver or a street strip deal?
 
#15 ·
The dyno operator that Ive used told me this...
Get your cruising vacuum figure and divide by half. Then add two.
For instance if your cruising vacuum is 16..
half would be 8..
add two ...
would give you a 10...
 
Discussion starter · #16 ·
It has stedy vac now that the timing isnt set to 40 advanced anymore. The cam is a Comp cams XM280. 280 duration, .480 lift. 110 seperation.

cliff notes on what the car is doing;
No bog from a stop.
Bog when getting off gas, then getting back on
Low speed stumble when cruising.
All problems gone by 2500 rpm, smooth sailing from there.

Engine;
350
10.4:1 forged pistons
vortec heads w/ upgraded springs
XM280 cam
Holley 670 street avenger
edelbrock rpm air gap intake
mallory HEI distributer
 
#17 ·
Yep, same cam I ran, real dog below 3000rpm. I had no problems idle to 6500 with it but it had very poor low end torque even with 3:73 gears, which this cam needs to get up into the powerband. What rear gear are you running? I couldnt imagine running with less than a 3:55.
 
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Discussion starter · #19 ·
Originally posted by drptop70ss:
Yep, same cam I ran, real dog below 3000rpm. I had no problems idle to 6500 with it but it had very poor low end torque even with 3:73 gears, which this cam needs to get up into the powerband. What rear gear are you running? I couldnt imagine running with less than a 3:55.
Click to expand...
I believe 3.42
 
#20 ·
FROM THE 2000 HOLLEY CATALOG

A COMPETITION OR RACE ENGINE WHICH HAS 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 UPAT IDLE. IF THE VEHICLE HAS AN AUTOMATIC TRANSMISSION, TAKE THE VACUUM READING WITH THE ENGINE THOROUGHLY WARMED UP AND IDLING IN GEAR. IN EITHER CASE, THE POWER VALVE SHOULD HAVE AN OPENING POINT ABOUT 2” HG BELOW THE LOWEST 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 VAVLE, THE VEHICLE SHOULD BE DRIVEN AT VARIOUS STEADY SPEEDS AND VACUUM READINGS TAKEN. THE POWER VALVE SELECTED SHOULD HAVE AN OPENING POINT ABOUT 2” BELOW THE LOWEST STEADY SPEED ENGINE VACUUM OBSERVED. HOLLEY HAS A 6.5” HG POWER VALVE P/N 125-65, WHICH USAUALLY WORKS OUT WELL FOR MOST DRIVING SITUATIONS.

HOLLEY MAKES STANDARD POWER VALVES IN THESE RATINGS…
1.0”, 2.5”, 3.5”, 4.5”, 5.5”, 6.5”, 7.5” 8.5” 9.5”, 10.5”.

THE HIGH FLOW UNITS ARE AVAILABLE IN THESE RATINGS..
2.5”, 3.5”, 4.5”, 5.5”, 6.5”, 8.5” 10.5”

THERE ARE ALSO TWO STAGE POWER VALVES AVAILABLE BUT THEY ARE NOT RECCOMMENDED FOR PERFORMANCE APPLICATIONS.
 
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Moloko
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