Engine Oil Pressure

[halloran]

Well we got the new engine rebuild up and running. Seems to idle fine and after some tweaking of the timing we got it pretty much dead on at 6 degrees advanced. I do have one major concern however. On initial startup the engine seemed to have consistent oil pressure at around 40 pounds at idle but after running it for about 15 minutes up and down the rpm band the pressure dropped to 20 pounds at idle (800 rpm). The oil pressure does rise to around 45 lbs at 2000 rpm but it seems to me that 20 lbs is low at idle or maybe I'm just being a worry wart. The shop book says 35-55 lbs at 2000 rpm is normal but mentions nothing about idle.

On another note the shop book says the cold cranking cylinder pressure should be 180 lbs. Our maximum on ALL of the cylinders was 120 lbs on each one evenly when cold. I do realize that the new rings probably aren't seated yet but I guess I just want to hear someone tell me what their experiences have been with oil pressure and cylinder compression.

[keithol]

Your oil pressure is perfectly normal . An old hot rod rule of thumb for oil pressure says you need 10 PSI for every 1,000 RPM. The only way to have high pressure at idle would be to install a high volume pump . But I wouldn't worry in the least about that kind of pressure. As far as compression pressure , I checked one of my old manuals also and 180 PSI. is what I found. Cranking compression tests are affected by many things, accuracy of the gauge, cranking speed, position of the throttle plates . The actual compression can be affected by ,piston rings, valves, valve timing ,piston compression ratio ,and thickness of the head gasket . Check with another gauge after the rings have had a chance to seat . If compression is actually that poor the engine would have very poor performance . Mechanics now put more stock in cylinder leakage testers ,but that only tells you if you have leakage . I am guessing that the new head gaskets are thicker than the originals for one thing, and keep in mind the ones you took out may not be the originals either. Was your timing chain replaced ? Camshaft timing can alter compression big time. Just some ideas ,I wish you the best .