I run a 29 ft Fountain Fever with a 565 Dart big M block with Merlin heads. My issue is I have a thermostatically controlled oil filter (Mercury) and as the oil heats to 200 F I lose pressure! The engine had to be rebuilt in 2016 due to metal in the filter. I was running 25-50 Wt Brad Penn oil at that time. After the rebuild my pressure still dropped as the oil got to 200 degrees. On the dyno, without the thermostatically controlled filter, everything was fine (59-63 PSI) The engine builder had the engine pulled and replaced the Melling M77HV oil pump, put oil resticters on the the top end, inspected all oil hoses for routing, and checked main bearing clearances (all 0.003). Seeing no improvement in pressure I replaced the internal components in the thermostatic oil filter, oil pressure sending unit replaced, and oil pressure gauge verified accurate.
Still the issue prevails! The engine on start up cold has 60 PSI of oil pressure and as the oil hears up the pressure drops. When the oil is at 200 F my pressure at idle (1000 RPM) was at 0-8 PSI and at 3500 RPM and beyond I was at 35 PSI.
The next change was to go to straight 50 Wt oil. This helped immensely, at hot idle I now have 15-20 PSI but on the upper end 3500 and up I still have 35-40 PSI. The engine is out of the boat again (different builder as the other retired) and is sitting complete on the floor waiting its turn. This time it will be run with the thermostatic oil filter in place. The engine has all good parts and produces 707 HP and 742 ft/lbs of torque on the dyno. But this issue has me watching the oil pressure gauge like a hawk and not the speedometer or tach! I am thinking the internal leakage almost has to be in the lifter oil galley. Lifters were changed when the engine was rebuilt from Comp Cams to Crower extreme duty, of by the way it is a mechanical roller.
I am looking for any ideas that I have not thought of!