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Oil pressure, how does it work?
GaryM - 10/8/22 at 11:23 AM

Somethings been bugging me for a while now…..The more I try and understand oil pressure (specifically relating to my xflow) the more it baffles me.

I often read that oil pressure can be used as a measure of engine wear, specifically the wear in the bottom end of the engine (main and big end bearings). The theory seems to be that as the gap between the shell bearings and the face of the crankshaft increases the oil pressure decreases. On the face of it this seems logical, however, the oil is not only pumped to the these bearings, it is also pumped (on a Xflow at least) to the rocker assembly, the camshaft bearings and the distributor shaft…maybe more places?

It strikes me that none of these additional oil flow destinations are made using the same precision tolerances as the crankshaft /crankshaft bearings so what’s to stop a worn and/or loose fitting rocker or distributor shaft (for example) being the reason an engine has low oil pressure?

Sorry if this is a stupid question.

Gary

Mr Whippy - 10/8/22 at 11:40 AM

The vast majority of the oil is to the main bearings, plus the oil feed holes to the cam etc are quite precise holes/drillings, so should not vary much over the life of the engine. When is the last time you've ever heard a cam rattling around in its bearings?

If the pressure is dropping greatly when the engines hot and idling, then chances are it's the main bearings. That is if the oil pump isn't worn out or your filter isn't blocked or your relief valve isn't stuck open or you've got the wrong grade of oil in there.

Don't get too obsessed with the oil pressure gauge though, if your worried about the oil pressure all the time just fit a low pressure light instead.

adithorp - 10/8/22 at 12:16 PM

quote:
Originally posted by GaryM
...so what’s to stop a worn and/or loose fitting rocker or distributor shaft (for example) being the reason an engine has low oil pressure?

Sorry if this is a stupid question.

Gary


Nothing in theory.

None of those parts are subject to the same loads and hence don't wear at the same rate. Balance of probabilities, if its not supply side its main bearings.

coyoteboy - 10/8/22 at 12:45 PM

It's based on the assumption that nothing else is wrong. It's a very very rough guide. Pressure varies not only with outlet restriction but also oil age, pressure regulator wear etc. On my old engine, after 3k miles of hard driving, top quality oil would go from a hot idle of 24 psi at 0 miles age to 10psi at 3k miles. Unless you can quantify your oil degredation rate and state, it makes no sense to use it as a measure.

[Edited on 10/8/22 by coyoteboy]

hobbsy - 10/8/22 at 06:55 PM

Wow, I don't think I've seen a noticeable oil pressure change between new and old oil.

Was this mineral / semi or fully synth?

What engine

Did concern me a bit how low the pressure got at a very hot idle with my R1 engined fury, single figures psi.

But was all good

GaryM - 11/8/22 at 08:17 AM

Thanks for the replies.

Interesting reply from Mr Whippy suggesting that the oil flow destined for the rocker shaft, camshaft etc is limited by restrictive drilling’s in the block (not liable to wear) that effectively create pressure (which I assume is higher than the pressure needed for the bottom end). This would mean that wear or bad tolerances on the rocker assembly, camshaft etc could not reduce the pressure available to the bottom end.

Have I understood that correctly?

coyoteboy - 11/8/22 at 08:50 AM

quote:
Originally posted by hobbsy
Wow, I don't think I've seen a noticeable oil pressure change between new and old oil.

Was this mineral / semi or fully synth?

What engine

Did concern me a bit how low the pressure got at a very hot idle with my R1 engined fury, single figures psi.

But was all good


Fully synth 10W40, top of the line. But it was a turbocharged engine (3SGTE) and the turbo would regularly be almost translucent with heat, so it degrades heavily between changes. Probably less noticeable on cars that don't stress their oil so much (and/or maybe don't have as much wear on the engine overall). But that's kind of my point, there's too many variables unless you know your car/engine and have watched it for a few years with this closely in mind.


quote:

Interesting reply from Mr Whippy suggesting that the oil flow destined for the rocker shaft, camshaft etc is limited by restrictive drilling’s in the block (not liable to wear) that effectively create pressure (which I assume is higher than the pressure needed for the bottom end). This would mean that wear or bad tolerances on the rocker assembly, camshaft etc could not reduce the pressure available to the bottom end.

Have I understood that correctly?


Yes you understood correctly - if the drilling in the casting is the primary restriction and the outlets up top are wide open by design, you'll see no change in pressure due to wear there, totally depends on the system in question though. In other engines, cams are fed the same way the mains are, and contribute to the pressure - I can't speak for your engine though.

Bear in mind this is sometimes done as a safety - i.e. you can't lose bottom end pressure completely if you lose top end pressure due to failure - the restriction limits the potential for lower end damage. In those cases, the restriction doesn't actually have to create the final pressure at the top end, it can just prevent the pressure in the system dropping too low in failure. Depends on design. Generally less oil is needed in the top end and huge flows up there aren't helpful, and the restrictions leading to the top end are there to limit flow rate, not fix pressure - that's just a side effect.

It's a very multi-variate beast. This is why you can't rely on it.


[Edited on 11/8/22 by coyoteboy]

[Edited on 11/8/22 by coyoteboy]

[Edited on 11/8/22 by coyoteboy]