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Dropping oil pressure at high rpm, follow on from webinar 227

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I have a question that ive tried to find an answer for, unfortunately most of what ive found is people asking at oil pressure at low rpm.

Im hoping Andre might be able to offer a bit of insight.

I will try and provide as much info as possible

I have a 4 cylinder engine, running the factory wetsump with baffles and factory oil pump which is attached to the end of the crank shaft(like a honda b series)

I have ported the pump and also the oil filter housing to remove any sharp 90 degree angles.

We have shimmed this pump, run bearing clearances on mains and rods at 0.05mm on 48.0mm journals(mains/rod)

15w50 oil

The engine has an oil accumulator T pieced as the oil comes out of the pump, heads to a laminova, then just before entering the engine, has a T piece to the journal bearing turbo(no restrictor) and then a T piece for the oil pressure sensor.

My thoughts were i dont care what the oil pressure is as it leaves the engine, I only care what it is after any pressure drops and what is entering the block and into the bearings.

Max factory oil pressure coming out of the pump on a stock none turbo engine with hot oil is 58psi according to manual, that is an engine with tighter oil clearance(0.025mm), thinner oil(5w30) and 7000rpm rev limit.

The issue im having is we now rev this engine to 9100rpm, we can see the oil pressure drop on the dyno as well as the road so its not a g force issue, the oil level is always to the top of the dipstick mark which is pretty much the full sump plus we have the oil accumulator.

I find when the oil is at 80c, the oil pressure peaks at 60psi at 6100rpm, then slowly drops to 55psi at 8750rpm.

For reference when the oil is at 50c, its 85psi/79psi at the same rpms which shows viscocity is making a big difference.

The main question I have is, is the pump cavitating and as such putting air into the oil which would cause bearing issues, if so in the future if i stripped the engine down would I be able to see the effects in the bearing surface?

Or based on the big drop from oil temp I should either tighten up my bearing clearances or try and find a thicker oil than 15w50, im on a stock crankshaft which has counterweights to small for the piston/rod weight so there will be some flex in the crankshaft at high rpm, hence the extra clearance.

Sorry for the long winded question but I thought if I put as much info as possible it might save a 20 post Q&A before I get an opinion.

Thanks in advance, happy to provide any further information

Given what you've explained it may be that your main clearances are a touch on the loose side if it's an alloy block. The specific clearance isn't excessive but in an alloy engine block these clearances will grow as the engine comes up to operating temp. If the bearings aren't showing any sign of wear due to crank journal distortion I'd be inclined to try going a little tighter.

My real concern would be that that the oil pump just can't operate efficiently at the rpm you're operating at. If the pump is cavitating then you'll usually see a patchy finish to the surface of the bearing (see the link below). The solution here would be an aftermarket pump or gears for your existing pump that are designed for the rpm you're using.

The variation in hot/cold oil pressure is pretty typical and not something I'd be concerned about.


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