quote:Originally posted by Hemihunter:
A decent Vacuum is about 15 -18
Yes but the largest factor is going to be in the timing of the camshaft, the overlap being the most effect on it.
A 292 cam probably has an overlap in the 70 to 75 degree range. That right there will drop you down to the 10-12 range.
A 60-65 overlap, 12-14.
You sometimes can gain about 2 inches by running around 15 to 16 degrees of initial advance.
Also the more static compression the more vacuum you will gain at idle. If you are running this cam with the iron open chamber heads that is around a 2 inch loss.
If you want 17 inches and a hotter cam, you need to get very tricky.
Simplest way to do that with a Cleveland now is to get away from the off the shelf cams from the major manufacturers and use George's special "Cobra Jet" cam.
Clevelands are not your average engines because of the canted valves and port configurations.
You need to trick the engine and the only way you can do that is with a very special cam grind.
Incidentally, I would expect anything with under 12 at idle, the brake booster needing an additional vacuum reserve reservoir. In fact with 10 inches, you may have problems with the brakes causing the engine to stall in bumper to bumper traffic?
What will happen is that the engine won't be able to fill the vacuum booster for more than one or two pumps of the brakes without it.
You probably wouldn't notice that unless you were in bumper to bumper traffic? Every time you apply the brakes, the engine vacuum will drop 3-4 inches and with anything under about 8 inches at idle, it may be stalling on you?
Having said that though, the Pantera is not a car that really needs power brakes to begin with.
I personally think they were put in because of the concept of marketing the car as a "luxury sports car" through Lincoln-Mercury?
You can chase this low vacuum phantom all you want, but it's the overlap of the cam that you have that is causing it.
There is no other way to bring up idle vacuum then to reduce the camshaft overlap.