Oil pressure t piece

[oliver90owner]

That 3psi is the difference between the inlet and outlet sides of the filter. On an engine with good bearings, that difference should be minimal. All oil should pass through the filtration medium. I doubt, with an unblocked filter, that the PRV will not be bypassing excess pressure and at lower volumes the filter will be passing the lesser volume with less than 3psi pressure drop.

Question is : Which is better - some lube or none at all? A lot of my engines have either no filtration (splash lubricated internals relying on sedimentation of larger particles in the oil) or bypass systems where only a proportion of the pumped lubricant is filtered and returned directly to the sump.

Much of the sub-micron particles are carried around within the lubricant (in detergent oils) anyway.

[philthehill]

The 3 lb/in sq is the pressure required to overcome the oil filter bypass valve so a portion of the oil will bypass the oil filter especially if the oil filter is contaminated with crud. Only if the pressure is equal either side of the oil filter bypass valve will the valve remain inactive/closed.
The oil pressure relief valve if the pump is in good condition should be off its seat most of the time with the engine running.
The oil pump is designed to move more oil than the system can cope with and the oil pressure relief valve allows that excess oil to discharge back into the sump. Moving all that oil requires power and therefore generates heat which consumes power. The capacity of the oil pump should reflect the requirements of the engine. So there is no requirement to fit a large capacity turbo oil pump to a 803cc 'A' Series engine.
Fitting an adjustable oil pressure relief valve to limit the oil pressure is a good move.
Detergent oils (for example diesel engine oil) can be used in a 'A' Series engine but the filter may have to be changed on a more regular basis as the sludge particles picked up and carried round the engine by the detergent oil are more efficiently collected and restrained by the filter.
Lubricant should be available to all mating bearing surfaces at all times - how that is delivered and in what quantities depends upon the application.
The same pictures as I tried to post yesterday are now able to be attached so here is a non modified filter head and and blanked oil filter bypass valve filter head.

filter head 1 (1).JPG (1.42 MiB) Viewed 303 times

filter head 2 (1).JPG (1.49 MiB) Viewed 301 times