Oil Pressure falling at sustained high revs

[Super_Rich_Bernie]

This is on an 1300 cc Alfa Twin cam motor in an Alfa, however blatchat tech advice is much better than Alfabb tech advice..... Appreciate any engine gurus thoughts on this.

 

The engine was rebuilt with new pistons and bearings about 15,000miles ago, and runs up to 6500rpm smoothly.

 

Oil is 20w50 Selenia HPX, semi synth - as recommended by Alfa for these engines, and used in my 1750 Alfa with no problems.

 

Oil pressure normally behaves as I would expect, falling as the car warms up, dropping back at low revs, but sitting at >60psi @ 4000revs with hot oil (where the book says it should be). The oil gauge is a capillary one, not electric.

 

 

However the car is geared to 16mph per 1000revs (4 speed box), so on the motorway I'm typically running beteen 4500 and 5000 revs - with bursts up to 6000 revs. When I do this I see the oil presure start to drop after a few minutes fast (ish) cruising.

Oil temp is not increasing dramatically (I have checked with an IR thermometer, as well as the crappy electric guage).

 

e.g... join local A3 in 70 limit with car already warmed up, cruise @ 5000rpm - oil pressure slowly drops away from 70psi bottoming out at 50ish. Go into 50mph limit, cruise @ 3500rpm, oil pressure slowly gets back up to normal. Get to the traffic lights, use 6000 revs through the gears, pressure stays up.

 

Any thoughts?

 

Sustained high revs causing the oil to froth? Crankshaft whipping it up?

 

or Oil pump? I'd expect problems with that to be apparent as soon as revs rise?

 

Bearings? - again I'd expect clearance issues or wear in these to cause an immediate drop

 

Should I swap to a 15w40 oil? Original spec was 40 grade oil in summer and 30 grade in winter or 20w40 if you could get the then new fangled multi grades.

 

 

 

many thanks

 

 

Jonathan

 

[Johnty Lyons]

Carefully check your level sounds like windage to me.

[Klunk]

Does the engine have a pressure relief valve on the oil pump? If it does, can it be altered?

 

 

Edited by - Klunk on 11 Jan 2013 17:10:09

[Super_Rich_Bernie]

Johnty ... The sump level is at the max on the dipstick. It is conceivable that the dipstick isn't the original or right one, so I will try re filling it with the correct amount and see where it comes too. Inclined to put something like 15/40 castrol in it instead of the thick 20/50, since the original spec called for 40 grade, not a 50.

 

Klunk... Yes there is a relief valve on the oil pump, though getting at it is a sump off job, and sump off looks like engine out. It appears to work as it should when the engine is cold though. I guess if it isn't oil level or grade related I have to face up to the engine being pulled anyway!

 

Thanks

 

Jonathan

[Johnty Lyons]

Relief valve is ared herriing it only blows off excess pressure does not cause reduced pressure ar revs.

I suggest exact amount of oil recommended, and see where that comes on dipstick,sounds to me like you are aeriating the oil.

Just my opinion mind quite happy to be proved wrong.

[Super_Rich_Bernie]

Johnty... top marks, and huge thanks.

 

I drained 8 litres out, book spec is 5.5 litres. The right amount barely shows on the dipstick - clearly the wrong one!

 

A short test run saw the pressure rock solid at sustained high revs. I need to give it a longer run to make completely sure, but looks like this was the problem. I think i could notice a bit more urge too.

 

Jonathan

 

 

[charlie_pank]

You might need to think about why there was so much extra in there - is it to combat surge in cornering? The alternative would be to fit a windage plate above the pickup so that even though the oil was being 'frothed' above the plate, it would be fine below it - this is the method used in my BEC to 'avoid' the 'need' for a dry-sump.

[Super_Rich_Bernie]

Charlie, I think it had that much in there simply because the dip stick is the wrong one and it was topped up to max mark.

 

The car has a colourful history and the engine has various repalcement bits, including the cylinder block, so its not so surprising. The last owner had two engines let go, the first one spitting bits out when he took the car to Monza, and the second running main bearings.

 

I did a full circuit of a roundabout with no adverse effect. Cornering forces are limited by skinny 145/15 tyres.

 

There were/are other problems too, such as huge venturis in the weber carb (now sorted) and a distributor which if advanced enough to run sweetly at low revs causes pinking under load at higher revs. (Next job on the list). Hopefully when its sorted the engines life expectancy will improve along with performance.

 

Jonathan