jetlag

Hey guys, I need a new power valve for my 81's Holley carb. I tested the vacuum on it and it is around 9" hG. What would be a correct valve size? Is there a special equation to figure it out or just minus 2" of hG... I've heard a couple of ways to figure out the size, which is correct?


Thanks a bunch!

78 Vette

If its a stock motor there is a whole procedure on the right way. but if it has a cam and makes a low amount of vacuum then get a reading in gear and divide it in half.

jetlag

It does have a cam and a lot of other goodies.

I put it in Drive and tested the vacuum, and it was around 10-8

So..would a 4.5 Powervalve do good?

78 Vette

Yep that would be a good choice. mine has a 3.5 if I remember correctly I make about 6-7 in of vac. :thumbs:

jetlag

Ok Thanks guys!

That was a quick response. I just posted expecting to get an answer in the morning, but dang!

I just bought the 1981 Electron Blue vette, and I absolutely love it. I can get that 5$ valve tomorrow and be ready to roll!

78 Vette

Just seen it it looks great

clem zahrobsky

check the vacuum at WOT because if you go too low it can close at WOT and lean out the engine. if the carb is set correctly the PV has no effect on the idle :chevy

Matt Gruber

i would expect the 6.5 to work fine.
gkull pulls a lot of vacuum at WOT as clem suggests, but that is unusual.
some tips in sig.

jetlag

Oh, so I should check my vacuum at WOT instead?
So what is the real way to find out what size power valve I need? Divide in half? Subtract 2" ?

Solid LT1

9" of vacuum seems REALLY LOW to me. I have a very big cam in my car and can still pull 12-14" at idle. You car is an automatic I presume, if you don't have the totally wrong carb/camshaft/intake manifold/engine combination then I would first take a can of carb cleaner and check around the carb and head/intake mating surfaces for intake leaks or a vacuum hose or system problem. The car will not be street driviable with 9" of vacuum, you will need a steep set of rear gears and a BIG accelerator pump shot to prevent an off ildle lean condition when applying throttle. I could be wrong and you could be discussing a race car, if that is the case, I would run a 4.5 or 3.5" power valve but have never had to run one that low except when playing around with Z/28 Xrams and tunnel rams with two 4bbls on them. My opinion, good luck!

gkull

What Clem and Matt are talking about. drive around with a vacuum gauge duct taped to your window. Do some WOT runs locked in second gear from 2500 - red line

Your lowest vacuum will be right at 2500 and vacuum should increase as the RPM goes up. You want a power valve that is still open and dumping fuel past peak TQ rpm.

The poster should do a couple of things to increase the vacuum.

1. increase the idle rpm. My motor likes a 900 rpm idle hot.

2. increase the idle spark advance. hot cams like 14-18 initial

3. Make sure that your idle fuel air mixture is correct.



[Modified by gkull, 3:08 PM 3/30/2004]

clem zahrobsky

if you use too low a PV number you can also have a hesitation because the power valve is opening too late. with a hot cam and a auto trans you may need a higher stall speed convertor. :chevy


[Modified by clem zahrobsky, 12:50 PM 3/30/2004]

jetlag

Carbs are complicated :(

Clem I do have a 2200 Stall Convertor

clem zahrobsky

if the engine RPM you need to keep the engine running at idle and in gear and you have to hold the brakes to keep the car from moving you need a higher stall speed convertor. :chevy

78 Vette

P.V dose nothing at idle? :crazy: :confused: . Go to a local speed shop and buy the holly book It will tell you everything you need to know. 99% of the time if the motor makes vac at wide open throttle its because it is a small carb on a larger motor. :thumbs:. What I posted earlier is right out of the book.

jetlag

Do you think the Holley 600CFM carb on my motor is too small for it?

New 350 , Bored .30 Over, 10.50:1 Compression, Polished Holley 4bbl. Carburetor, 64cc Vortec Heads w/Polished Performer Intake, SRP Flat-top Pistons, Xtreme Energy Comp Cam 12-246-3(.487/.490 Lift),

gkull

I had that cam in a 355 ci motor and never really liked it. CS XE274H-10 It had poor vacuum and it made very peaky HP. when combined with my 750 cfm carb on a single plane intake. It was a motor that I was almost happy when it blew up because then i didn't have to fix it.

To answer your question: 600 cfm is great. That wimpy intake manifold that starves the motor at just over 5000 rpm is a serious mismatch with your high winding cam.

jetlag

What is a good intake manifold you recommend?

BSeery

Yeah -- try fuel injection some time.

78 Vette

:iagree: your carb might be slightly small but I don't think its much of a problem. Just do like I mentioned earlier and your pv issue should be ok. IF there is still problems there my be something else wrong but don't stress your self out a Holly is a pretty simple carb to work on. You really should buy the book. My vette is a 12.50 car and I drive it every day and have been for the last 2 1/2 years with a 750 and haven't had one problem since the day it was installed. :thumbs: Good luck


[Modified by 78 Vette, 6:27 PM 3/30/2004]