74 vert

Is 17 normal at the manifold at idle?

zwede

I would expect more from a stock or near stock L48. Depends on the cam specs. 17 is normal for a mild performance grind.

Double Aught

When I had my L-82 dyno-tuned last year, they measured 14" of vacuum at idle. Is that normal for an L-82 cam? My mechanic insists that the cam in my engine isn't stock. The dyno sheet shows 220hp/250tq at the rear wheels. The factory rated the L-82 at 220. Maybe I do have an aftermarket cam.

mountainmotor

That would be pretty good for 29 year old L-48 that has never been into if that's the case .

A good single pattern cam with unsymetrical cam lobes with a duration of 216 @ .050 on a 110 LCA can make 17 inches of vacuum in a 355 inch motor just for example.


[Modified by mountainmotor, 5:44 AM 7/19/2003]

zwede

Keep in mind that the state of tune makes a big difference on idle vacuum. Both idle mixture and timing advance can knock off several inches/hg if it is off just a bit. Also, if the idle rpm is lower than spec your vacuum will be lower.

The most important thing is that the vacuum is steady. You can then try changing the idle mxture and advancing the timing and see what it does. Most any engine benefits from advancing the timing 2-4 degrees more than spec. Only time that's bad is if you fill it up with the absolutely worst gas you can find (which is how GM came up with the spec. Worst case scenario).

zwede

Sounds like an aftermarket cam... I would expect a little more vacuum with a stock L82. Of course idle mixture/speed could be off.