Originally Posted By: kilkm68
A friend of mine has an unusual oil pressure problem I never heard of. It's a new rebuild of an AMC 390 in a '69 S/C Rambler. When he accelerates his oil pressure goes DOWN. We checked with a different gauge and same result. Ran it a quart over full, still same result. A friend suggested maybe the screen came off the oil pump sump. Any thoughts on this?9
Excessive bearing clearances will do that, was this engine set up for racing with loose tolerances? Could also just be a measurement error and someone got the clearances too big.
When the engine is under load, there's a much larger net downward (away from the heads) force on the crankshaft than at idle or light load, which opens up the clearances on the oil feed side of all the main bearings and lets the pressure drop a bit. I've seen that on Mopar, GM, and Ford engines on several occasions. Normally increasing RPM offsets that effect and you never notice it, but if you're cruising with a more or less constant RPM and then apply throttle, before the RPM come up, the oil pressure may drop a few pounds. The real fix is to fit the main bearings better. May require undersize bearings... SHOULD require undersize bearings if the crank has been turned.