The Stovebolt Site Forums The Shop Area Engine Shop Unsettled Oil Pressure

I have a new GM 350 crate with only 235 miles on it, and a 700R4 tranny. When I start it and run it at about 1500 rpm the oil gauge (new Classic) will bounce as much as 25 lbs. I hold it at 2000 rpm and the pressure sits at about 90 lbs. When I drop it to 1500 rpm the pressure drops to 75 lbs. At that point it will vacillate between 65 and 90. Sometimes the movement is erratic and sometimes it moves around slowly. At a slow idle it can drop to 20

Jim, before making "snap" decisions" do you have an oil pressure guage you can hook up to verify what you see? Maybe one of those old SW guages...or get one off the performace isle at the local flaps.
I suspect a guage problem. Perhaps its hanging up or dragging on the face?

My first thought is that the gauge is inaccurate.I would get a mechanical test gauge and hook it with a short flex line at the port near the distributor.
Be aware that a gauge without a orifice in the line will not damp out the rapid fluctuations and rapid changes that don't allow smooth gauge reaction to normal pump pulses.
Probably your gauge is electrically controlled and the sending unit is usually the weak link--they are often unreliable and being electrical in nature are sensitive to voltage fluctuations of any cause.So we want to test with a mechanical gauge.
The normal SBC needs at least 10 PSI hot at idle but a new engine would probably be near 30 psi.I don't think over 60 PSI, hot, at speed is normal or desirable.Relief valve setting(max pressure) will be reached usually before 1500 RPM.
Excessive oil pressure wears the cam and pump drive abnormally and wastes horsepower.
I think the gauge defective. My 2 cents worth!

I was wondering what the relief pressure should be on that motor. 90 lbs seems a little high.

Even with a fairly low pressure relief spring, the cold oil pressure will spike up above 60 PSI on most engines. After a few minutes of warmup, if it's still over 50 at cruise RPM a lower-pressure relief spring is a good idea. Hot idle oil pressure can be anywhere from 15-30 PSI, sometimes lower. On engines with an idiot light instead of a gauge, the light turns on below 7 PSI. I've seen race engines actually twist the tang off the pump drive shaft if the pressure is regulated at too high a number. It's also rough on the distributor drive gear and the camshaft gear.
Jerry

Thanks guys. It seems the consensus is the gauge. I'll hook up a mechanical gauge and we'll see what's going on.
When you think you've finished your project and you begin to think how bored you are going to be not having anything else to keep you busy. However it seems you will always have "stuff" to do on that old project that will keep you busy.