Originally Posted by jeepers creepers
I've read lot of things on the forum about the causes of oil pressure drop, oil level, windage tray, oil pick up height and its related brazing to the pumo body can afflict pressure.
Mine is very good, about 70 when cold and over 35 at idle at operating temperature, the problem pop out when engine turns close to 5000 rpm, the gauge pointer has a superfast vibration from around 5 to 35 PSI.
Looking for the cause on the web i've seen that some oil pump has problem over 5000rpm or so, any experience with them?
i'll tear down oil pan to check pick up height first (honestly i want to see bearings too) then braze the pick up or make a bracket for it.
It's not a for sure thing to say when the engine is running with no load on it. Engines make some pretty strange vibrations on the crank and when these are not damped by a damper on the front of the crank and either a flywheel/clutch assembly or torque converter on the other, these shakes can be transmitted into the cam via the timing set. These could excite the distributor gear to shake about in its clearance which would transfer the shake to the oil pump. But frankly I'd be surprised that the average oil pressure gauge would have time to respond.
I'd expect that this is either the bypass valve chattering or the pump gears doing the same, or the inlet pipe is shaking and drawing air on and off. This also could be too much or too little clearance of the pick-up to the bottom of the pan.
Chevy pump gear chatter is a common problem often fixed by milling a groove in the cover plate from the discharge side to the opposite side of the gear. The cause of this is the 7 tooth gear of the pump is just too close to a harmonic of the engine's firing order; plus the bypass returns to the inlet side of the pump so the pumps operating frequency which is imprinted on the oil outflow is fed back in to the inlet when the pressure bypass is operating. This can really shake the daylights out of the gears.
The Chevy inlet being pressed into the pump cover leaves a lot to be desired, I don't like friction fits, I've never seen them last. If this is leaking air the pump input will become unstable a condition that will worsen with RPM. The usual solutions are to braze the inlet to the cover, another thing I love is getting cast iron that hot, and using a brace to carry the structural loads of the pick up off the connection point.
Other things about the pickup is that it needs to have the anti-vortex cover with screen at the intake side. The needs to be from 3/16ths to 3/8ths inch off the bottom of the pan or 4.8 to 9.5 mm.