A 11 lb autiomotive valve can only open and allow unfiltered, uncooled oil to bypass the cooler and filter resulting in engine damage if its original with standard volume oil pump, unless bypass is hung open, i doubt its your problem though. Im of the understanding mercruiser used the stiffer bypass valves but not 100% sure. The lines, cooler and bypass were sized appropriately from factory for stock clearances and standard oil pump and usually work for hundreds of hours. Now, if someone put a high volume oil pump in, then it could be bypassing the cooler and filter, smitty

Gotcha, I'll check with another oil pressure gauge first as I'm lost as to what else could cause the low pressure, aside from what you mentioned.

Back to the top. Great thread! This saved me from going from a bad situation (not of my doing) to a much worse one. I remember reading this back when it was first posted and decided go over it again recently. My engine started life as an HP500 and I see it still has the bypass valve on the outer side of the oil filter pad. Being I'm now going with a Melling 10778 I will remove and block with a Melling 13/16 plug and go with a larger oil cooler as well as a high capacity filter pad, lines and filter mount from Hardin Marine thanks to the information found in this thread. One more question though, being I have a choice between an HP4 or HP6 style oil filter, which would be the better option for a blown 509 making 855hp. Sincere thank you to Articfriends and all who contributed here.

210-220°f is optimal...so it has enough thermal capacity to cool the engines parts...mainly the springs.

Standard HP4 is what I would use. We ran the HP6 on the race boat, but for a recreation boat I would just run the HP4. They are readily available anywhere. You never know when you might need to change the oil. Crack in the oil cooler, stuck injector, or bowl seat not shutting off.

Thank you Rookie. Much appreciated.

Don't forget you can run 2x oil filters in parallel. I'm not a fan of the 62 micron ultra flow racing filters. Maybe as a first state, but we don't tear down our engines every few runs.

Lots of good filtration options.

Having a personal experiemce of spinning a main bearing from oil bypass...I don't like it at all. Bypass to oilpan ok of course...but not bypass filtration.

Check out David Vizards newer vids on setting up the oil pump and cleanking up the oil flow path. Worth the watch..and would help you setup your new oil pump.

What would be the best option for a high flow/good filtration Hp4 style filter? I would imagine, like everything else, there is a reasonable compromise?

Just to clarify as I saw some conflicting information in this long thread.

The 30psi bypass valve is part# 25161284.
The 11psi bypass valve is part# 25013759