dave968M3

Short/long story: had low oil pressure ~121,000 miles on my 968, many DE wkends, 4 race wkends, so decidied to have engine rebuilt, oil pressure was ~3.5 to 4 bar before rebuild.
Rebuild to stock specs, oil pan baffles, oil pick up tube mods, headers and race exhaust, RS Barn lightened flywheel, RS Barn air box mod and chip.
1st time back on track, exhaust too loud, ran for <1 hr, engine overheated light came on at end of last time out due to fan not plugged in, car cooled down when fan plugged back in, ran well.
Oil pressure ran 4.5 bar when hot on track. Installed larger oil cooler and oil temp guage. Oil temps run 180, max 210 on track. Oil pressure still ~4 bar.
In Sept after ~12 total hours track time since rebuild, oil pressure now 4 bar 1st 2 laps, then dropping to 3 bar down straight at 6K RPM full throttle 130 mph.
In shop checked oil pressure with mechanical gauge 5 bar cold at idle and at 3500 RPM. recalibrated factory gauge.
Back on track oil pressure 4 bar at oil temp 180, 3.25 to 3.5 bar at oil temp 210 max.
The oil pressure sender was replaced, tried other oil pressure relief valves, now have a new OPRV.
Ran car on dyno, oil pressure stayed 60-80, hp 209
This past wkend on track, oil pressure at idle 4.25 bar, 52-55 psi, after 15 min idle oil temp 155 degree, 3.7 bar, 35 psi.
On track 1st lap 52 psi, 4th lap 44, 6th 41, then 38 to 42 at end of 20 min on track.
At home this morning, cold start oil press 4.1 bar, 52psi; after 10 min idle 3.9 bar, 40 psi, oil temp 140; when car revved to 4K RPM oil pressure increased to 80psi.
None of the usual causes of low or no oil pressure make sense, especially since I seem to have normal oil pressure idling and only low oil pressure when engine is hot on track, but not on dyno.
Except for a crack in the block that may have been there before the rebuild, not seen during rebuild, and so is still there?
Any thoughts???
Plan is to try replaceing OPRV 1 more time, am going to re license the car to run on interstate and see if problem persists. If still low oil pressure, may have to look for cracked engine block, unsure how to do this, or rebuild new engine and replace engine block???

Dave

MB968

Just a stab in the dark since I know nothing about how track conditions affect this engine. But, here are a few thoughtsl

If the drop in oil pressure is thermal related as seems likely from your description, then you are looking for something that changes with temp.
- You might use a calibrated pyrometer to check oil temps, oil cooler in/out temps, etc.
- The oil viscosity changes even for a multi-vis oil. I know specs on various oils are available. You might try switching to an oil that has a higher viscosity in the 200F temp range than the one you are currently using and see how it affects your results.
- Any chance the oil pump could be the issue. If the internals are a bit worn, you could be getting more blow-by when the oil viscosity drops (and maybe the gaps increase) with temp.
- Running 20w50 oil, my cold startup oil pressure runs a full 5 bar. If I've been running for a long time at highway speeds, and then stop, the idle oil pressure is about 2.5-2.75 bar. I don't exactly recall the oil pressure at highway speeds with high oil temp, and don't know exact oil temp as I have no gauge. Engine has ~160k miles, rod bearings recently plastigauged at ~.0018".

dave968M3

thanks
I'm running Joe Gibbs XP6 Full Synthetic Engine Oil, Viscosity Grade: 15W-50. "A 15W-50 version of our XP1 formula race oil, XP6 provides excellent bearing oil film thickness and outstanding cold start protection compared to similar viscosity oils."

quick968

Dave,

A couple thoughts/questions. You stated "rebuilt to stock specs". Ok, did you or your engine rebuilder replace the oil pump with a brand new one, or did he just re-use the old one and check gear pump clearance to housing? If the latter, there's your problem. You should use new or at least overhauled oil pumps with new gears.
That said, you are tilting at windmills here as far as worrying about low oil pressure. Rule of thumb generally is, 10psi per 1000rpm. However, even Porsche has SB's stating it's actually acceptable for the low oil pressure light to be on at idle as long as it comes off when the rpm's come up. The other factor to consider is your temps, which are perfectly normal. If there was insufficient oil pressure/circulation, your oil temps and coolant temps would climb steadily towards redline under track use. Yours are steady at 210 under track use, so you have nothing at all to worry about. Your engine is just fine, oil pressure is on the lowish end of normal, but still normal.

Per the Porsche Factory Technical Specification book, Pg 16, minimum oil pressure at idle is .6 to .8 bar, you are well above that spec. Minimum oil pressure at 3000 rpm is 3 bar, again you are WELL above both those minimums. What Porsche is saying there reading between the lines, at or above 3000rpm's, 3 bar is sufficient oil pressure under all conditions. Rest easy, your engine is fine.

I use to road race motorcycles when I was young and stupid. Our engine builders would go out of their way to set the engine up to be as "loose" as possible while still making sufficient oil pressure. Loose main and rod bearings, loose cam bearings. Tight engines made less hp, "loose" engines made more, and lasted just as long.

In 15 yrs of messing with these 968's, almost all of it on the track, I've never heard of a single instance of a block or head cracking on these normally aspirated engines. Not saying it hasn't happened, but I've never heard of it. 951's turbocharged to extreme levels, you'll see blown head gaskets, maybe a cracked head from over boost, but still no cracked blocks. And it'd have to be a helluva big crack to bleed that much oil pressure with the OPRV trying to hold it. Bottom line your oil pressure is fine, but if you still can't sleep at night, pull the oil pump and replace with new.

Cheers
Mikey

Paul Waterloo

When I read the numbers, none of it seems out of the normal to me for an engine in general. I have not run my 968 on the track, but my Alfa Romeo acts almost identical. Increase the ambient temperatures, and the running oil pressure under heavy loads reduces a little more.

Rob in Oz

I observed the same issue with my old 944T on the track, put it down to causes as described above. But - on reflection, have you considered the possibility of crankshaft aeration of the oil via oil surge during cornering and consequent drop in effective pressure?

Just a thought...

Rob

LTCMontana

Listen to Mikey. I have close to 7K miles of track miles on my 95 968. Never rebuilt. rebuilt head after broken belt at barely off idle. Almost always shows the numbers you list.

Make sure your crank bolt is torqued to proper specs. I believe it is way high... like 3 foot breaker bar tight. non-torqued usually equals very low or no pressure but it could be part of your issue

RajDatta

That would 155lbs/ft.

tamathumper

I don't think those pressures are too low. They're lower than a fresh engine, perhaps. Mine at 55,000 miles is about 5 bar when cold idling, then drops as low as about 3 when hot, but anything above 2 is fine IMHO.