[dv657172]

Basically stock 1971 GTO 400

Just replaced the head gaskets and my compression levels are all 150 +/- 5 PSI however my vacuum level is only 12 in HG. Is this too low? Car has power brakes and they seem ok at least in the driveway....

I took vacuum reading from 3 way fitting at back of carb which is below throttle plates (manifold vacuum) with car in park.

Car has an old General Kinetics cam in it (based on an old catalog someone on this board had its 272/282, 214/224 at 0.050, Valve lift of 0.448/471 taking into account Comp Cams Roller Rockers, LSA 112). I think that is similar to Summit 2801.

Stock distributor, coil with initial timing at 14 degrees. Still need to check what the mechanical adds to it.

[steve25]

150 psi sounds good for a low comp 1971 motor.

If you check for timing chain slop and you don't find any excess then that level of vacuum is what the overlap of that Cam is leaving you with.

Maybe someone with a similar combo will chime in.

One other thing to note is with the stock valve spring installed height of those number 96 heads, that .470" valve lift dead on at thel limit .

You may want to investigate if you have a issue there unless you know that valve springs other then stock are being used.

[Half-Inch Stud]

May monitor vacuum level while raising the idle. Vac may raise with increased idle, in neutral. Highway cruise vacuum ought be higher.

[dv657172]

Steve25m - in regards to valve springs I have an old receipt from a previous owner which indicates "High Performance Valve Springs" were installed during an engine rebuild but no details unfortunately. Anyway to determine anything about them?

Half-Inch Stud - What would you expect to see for a vacuum level on a set up like this?

[Formulajones]

12 inches isn't terrible for a low compression 400 using a mild cam. Depends on your elevation when the reading is taken. Anything above sea level is going to be lower anyway.

There are things you can do. Timing is a big one, from there carb adjustments, primarily idle circuits, have a pretty big influence. I'm sure you could tweak things and raise vacuum an inch or two, but 12 inches doesn't make me concerned.

[steve25]

DV, if you look on line there are many videos to show you what to look for in terms of the valve spring install.

The pure stock installed height on your heads from what I have seen is 1.586”
If a valve job has been done to the heads and the stock valves where recut then your installed height might be a tad over 1,600” .

[dv657172]

Thanks Steve, will do some investigating.

DV