Reply to "Mysterious Cleveland alu rocker valve train"

I think that the rocker arm nuts are unrelated to the idle vacuum issue.

IF the idle rpm is the same for all vacuum readings whether it is high or low than it is the idle characteristics of the camshaft profile.

IF the vacuum is rising at the engine is idling at a higher rpm, then again it is the camshaft profile.

My engine will fast idle cold at 1700 rpm. Vacuum at that point is lousy, 12 to 15 rpm.

As the engine starts to warm, the choke begins to open and the engine rpm starts to drop in about 200 rpm increments.

As it does this the vacuum rises to a high point of about 17 at about 1200 rpm.

The engine finally will idle down to about 700 rpm and at that point the vacuum has droped back down to about 12.

First of all with this camshaft 700 rpm is really low to get it to.

It could easily be idle up, 800, 900 rpm. At those rpms the vacuum will rise as well. Probably to no higher that about 15 though.

Not knowing what your camshaft profile is, I suspect that these variations are caused by it at idle.

In my experience what you are describing is not out of character with a real high performance cam being used on the street.

I think that's one of the compromises that one needs to live with for them.

Incidentally on the subject of the Porsche. I just test drove a standard Caymen.

You have to hear the idle on this thing. It has a real rumpity rump idle for a 6 cylinder.

It had the PK transmission with the paddle shifters on the column.

In order to generate 275hp at 7,600 rpm, there were some compromises made. The idle is something to thing about though. I suppose Porsche wants to impress the gearheads?