9" of vacuum???

rainman69 · 01-15-2002, 03:25 PM

I don't have a vacuum leak as far as I know, as I checked the vacuum at the manifold with all the accessories disconnected/plugged at the manifold. I know I'm at 6000', but shouldn't I still see around 12" idle...unless there's quite a cam in there.

Any ideas on what kind of cam? Power band is 2200+ and really is a rocket over 3000.

drive it · 01-15-2002, 04:15 PM

I just recently checked my vac.- I live at 7000ft.-at home 11in. and down the hill (about 1000ft) was 17in. Thats with cam specs of 222 and 228 at .050 on a 114 with .494 lift. So yup, altitude makes a big diff! And it sounds like your cam must be pretty "healthy". Oh yea-thats at an idle speed of 750rpm.

[Modified by drive it, 1:15 PM 1/15/2002]

Gordonm · 01-15-2002, 04:55 PM

That is about what I pull 9" in my car at about sea level. My cam has a 248 duration at .050. I idle at 800 to 850 rpm

gkull · 01-15-2002, 05:22 PM

At your alt. You need to run more advance. At least 4 more than sea level. Then adjust your idle mixture screws with a vacuum gauge. When your dialing it in you have to keep lowering your idle speed.

RMS73 · 01-15-2002, 05:33 PM

Check to see if you have vaccum in the crankcase: this would come from a leak on the underside of the intake manifold. Remove the dipstick and put the vac gage on the tube. My Vette was pulling 9.5" Hg on the intake and 8.5" Hg at the dipstick tube. Replace the intake manifold gaskets and intake vac went to 13.5"Hg and 0 at the cranckcase. Also the carburetor mix screws then had control over idle speed/quality.
Roger

rainman69 · 01-15-2002, 07:51 PM

Will do crankcase check.

I am at 16 degrees initial, 37 degrees total.

74vetteman · 01-15-2002, 11:42 PM

Check your vacuum gauge manual. My manual for my gauge mentioned for every 1000 ft of elevation you lose 1 inch of mercury. If your gauge is the same you would add 6 more inches at 6000 ft. I was having low readings as well and searched for vacuum leaks until I was blue in the face. :mad

rainman69 · 01-16-2002, 01:30 PM

Hmmm...ok. Next question along the same lines:

So, I did a compression check. All eight were at 90 +/- 2 psi. I'd say that was pretty consistent across the board, but a little low. I'm guessing that there's a good bit of overlap in my cam...is that a logical guess?

mountainmotor · 01-16-2002, 04:02 PM

Post your cam card specs and I will be glad to tell you how much valve overlap you have :)

rainman69 · 01-17-2002, 01:20 AM

ummm....yeah... :jester if I had the cam card, I could tell myself the overlap. :lol: I was just hoping to get a ballpark figure, but from what I'm seeing here and what I'm simulating in DD2000, my current best guess is:

CompCams XE284H, 240/246 @ .050", .507/.510, Hydraulic (Valve overlap 23 degrees)

This results in a 383 Fly HP (SAE Net) at 6000, which makes sense. Also, helping to make sense is that while an examination down the pushrod hole shows a hydraulic lifter top, I have aftermarket dual valve springs. The heads had to be modified (by previous owner) to make them fit...

So...Maybe I'm on the right track. I just wish I knew if I still had the factory forged crank and forged domed pistons (11:1 comp ratio) or if it was bored out any... :smash:

mountainmotor · 01-17-2002, 11:13 AM

The 23 degrees you posted is not actual overlap when running.I estimate 66-70 degrees of running overlap with that cam. :rolleyes: