460 oil pressure issue starting at 4600 RPMs

I just installed my new 460 in my boat. The oil pressure is great at idle, and through 4600 RPM's it stays between 70 and 75 psi. I'm using a new high vol. oil pump and a new shaft.
The pressure drops to around 20 PSI like clockwork at 46-4700 RPM's.

I have a 10 quart Milodon pan on it. We set the gap on the pickup at a tick over 3/8 inch from the bottom accounting for the gasket.

This thing is FAST and sounds great. What the heck could be causing the drop? I'm using a K&N oil filter.

New bearings through out the engine, cam, mains and rods. All internals are new.

All clearances checked before the build.

Do you think trying synthetic oil say 5w 30 would help the pump? It would be easier than pumping 10W 40 regular oil and easier to get the oil through the motor.

Bad oil pressure gauge or pressure relief spring in oil pump... Maybe it can't handle the volume AND the psi at 4,700 rpms...

My 347 did that with an HV pump. I don't really know why but my theory is that the pump was over running the bleed off valve and cavitating. The HV pump was making over 100 psi right off idle. I wrecked 3 gauges before I took the HV pump out. I now run a custom built standard pump, meaning no HV or HP and have steady pressure up to any RPM. Guys (and gals?) do run HVs and HPs successfully though. Maybe they got more bearing clearance?? Bruce

I would guess he is right about the cavitating, or the pressure release valve is sticking open when it pops open at that RPM when you get that pressure built up

It just doesn't make sense, as soon as I drop the RPM's a little bit the pressure pops right back to the 70 75 range.

I'm going to put on a mechanical gauge and see what happens. The motor doesn't start lifter rattle or anything but I don't run it up there when the pressure drops. I cut the RPM's right away.

if thats the case it would seem like the pump starts to cavitate. I would guess you need to change the relief valve in the pump, its allowing pressure to get too high, and making it cavitate, my guess is that its not opening at all

What do you guys think of the synthetic oil idea? Thinner oil, easier to pump = less pressure but more volume potential???

I have plenty of pressure, if the flow is the issue thinner oil should help don't you think?

I'm using the melling high vol. oil pump. It's a new pump.

Changing viscosity will not likely change the results.
Like I said Check the Gauge first.

It will be next week till I can try it out but there will be a mechanical gauge there with the electrical one.

Thank you all for your input.

Originally Posted by electrowoman

I just installed my new 460 in my boat. The oil pressure is great at idle, and through 4600 RPM's it stays between 70 and 75 psi. I'm using a new high vol. oil pump and a new shaft.
The pressure drops to around 20 PSI like clockwork at 46-4700 RPM's.

I have a 10 quart Milodon pan on it. We set the gap on the pickup...

The problem not your oil pump; it is not because it's an HV pump (HV is absolutely the way to go in your marine engine); it is not your pump's pressure relief spring; it is not pump cavitation; etcetera.

In short, the problem rests in your Milodon 10-quart jet boat pan.

Paul