Follow up on oil pressure questions from 1-2 years ago...

[bpu699]

Posted a thread a year ago, before my engine rebuild (being done due to pitted cam, leaks)... A year later, half done...

My 930 oil pressure peaked at 4 bar, or a bit over, at 3000 rpm. Going any higher on the RPM scale doesn't budge it further. Idle 1.5 bar or so, hot.

I previously shimmed the pressure spring, no changes. Oil restrictors, slight change at idle.

Feedback I got was its probably wear and tear, and the oil pressure is otherwise good (peaks at 60 PSI), or 10 psi per 1000 rpm. Car had 60k miles.

Well, being on the track, I really wanted to see 5 bar like other were seeing, not 4.


I am now finalizing my long block, and buttoning it up. All the old bearings were in spec, with slight wear. Oil pump looked fine, no wear/scratches. Basically, internal all look good, in spec. I replaced the rod bearings with Porsche ones, coated the main bearings (looked new). New clearances in mid spec or so, similar to the old (There wasn't much wear to start with... couple rod bearings had slight wear).

But, I still don't have an explanation of why my peak oil pressure prior to rebuild was 60 psi. I couldn't find any issues, unusual wear, failed oil pump, missing gaskets, torn O-rings, etc... Internals looked awesome for a car that was occasionally tracked, and previously modded by the prior owner (running higher boost prior to my ownership)

Now 60 psi is fine and meets criteria for normal... but I thought the motor should generate way more psi than that, and then the relief spring blows it off...

Any other thoughts? Only other things I can think of is, "Its normal, stop worrying about it," or, "Look for external causes...."

[otto_kretschmer]

I've always followed Smokey Yunick's 10 psi/1000 rpm hot rule. If you rebuild your own engine and keep all the bearing gaps per the factory specs, and the oil pump is in good shape and you get around 10 psi/1000 rpm I would say "good enough" and move on to my next problem.

[stownsen914]

I think I remember the other thread and don’t recall whether you were able to confirm that your gauge and sender are good. If it were me, I’d consider calibrating or swapping in spares to see if the readings change. Another option is to put a tee (temporarily) and use a mechanical pressure gauge to compare it to your gauge.

You confirmed that your rod big end bores and crankcase bores are in spec?

[bpu699]

Originally Posted by stownsen914

I think I remember the other thread and don’t recall whether you were able to confirm that your gauge and sender are good. If it were me, I’d consider calibrating or swapping in spares to see if the readings change. Another option is to put a tee (temporarily) and use a mechanical pressure gauge to compare it to your gauge.

You confirmed that your rod big end bores and crankcase bores are in spec?

Used a mechanical gauge, was about the same...

What I don't get, is how the pressure peaks so quickly at 3500 or so RPM, and then plateaus. Most every thread I saw on here with low oil pressure, also had low idle pressure. Mine has fabulous idle pressure.

When the motor was apart I checked the pressure release every which way, to make sure oil wasn't by passing it... looked great...

Will make sure the chain tensioners aren't somehow bleeding it off...

[bpu699]

Originally Posted by otto_kretschmer

I've always followed Smokey Yunick's 10 psi/1000 rpm hot rule. If you rebuild your own engine and keep all the bearing gaps per the factory specs, and the oil pump is in good shape and you get around 10 psi/1000 rpm I would say "good enough" and move on to my next problem.

The oil pressure was technically fine. At the motor and wear surfaces looked good, so clearly lubrication was fine. The cam spray bar on one lobe was partly clogged with coke, that the one with the slight pitting... and even that's minimal.

Anyone know what the max oil pressure the 911 motor can produce is? I assume its way over 150 PSI... with 60-70 psi needed to run, and that's where the oil blow off valves come in.

Maybe the issue magically fixes itself once the motor is back together and running. The oil pressure was normal, and work great, even on the track.

But still, would love to see 5 bar...

How many of you guys see 5 bar on your gauges?

[Rosco_NZ]

Did you check the springs are in correctly ...?? There are longer & shorter ones I think.

[dannobee]

Originally Posted by bpu699

What I don't get, is how the pressure peaks so quickly at 3500 or so RPM, and then plateaus.

Because the oil pressure is regulated to that pressure. That's how the spring and valve work. If you shimmed it and it didn't come up, just how thin was your shim? And did you shim the right spring?

On our nascar dry sump engines, 60 psi was pretty much the given. Some engine builders would reduce it to try to gain a few hp, but it was "normal."

I knew some drag racers that were perfectly happy seeing 30 psi at over 8000 rpm.

[stownsen914]

Did you check your piston squirters while the engine was apart? A stuck one can bleed a lot of oil. Also it might be with checking that you have got the right combination of parts for the pressure relief valve. Porsche made a few different versions over the years

[bpu699]

Originally Posted by stownsen914

Did you check your piston squirters while the engine was apart? A stuck one can bleed a lot of oil. Also it might be with checking that you have got the right combination of parts for the pressure relief valve. Porsche made a few different versions over the years

2-3 of them were stuck open...

Would that really do it???

[stownsen914]

Stuck squirters usually cause more of an issue at idle when they’re supposed to be closed. Idle oil pressure doesn’t seem to be your issue. Perhaps others more knowledgeable than I will chime in here on whether oil pressure at high RPM could be impacted by several squirters stuck open ... I rebuilt an engine once that had a squirter that had failed. When I sprayed brake cleaner into the back side of the squirter, it really gushed out of the squirter opening. I don’t know if that was flowing more than a properly working squirter with the little ball rattling around inside to create the oil mist, but I remember being impressed by the volume.

[ed mayo]

There are early and late springs, difference is length and wire diameter. Your spring wire is 1.4 mm, early spring wire dia. is 1.8 mm. Try a pair of early springs, that should get your pressure up. I think the break is about 76 or so.

[Walt Fricke]

So you replaced the bad squirters, right? That sounds like a likely reason shimming the springs didn't do much. As long as you are sure you have the right springs and other parts for the two oil pressure control systems, you should be good.

If you want to make your max oil pressure adjustable, easy to modify the plug end by drilling and tapping, and using a bolt to raise the spring base. But I doubt you'll need to do that.

[Walt Fricke]

Here is how to make adjusters. I had an issue somewhat similar to yours with a newly built race motor, so I made these adjusters. They didn't solve it entirely. I now suspect it was premature wear of the rod bearings which may have been the cause. But something like this can be useful if you want to fiddle (which none of us should have to do, although Porsche did something similar with the 2.8RSR motors).

[bickyd]

As a reference for comparison. 1988 Carrera 101000 miles
The car warm or cold when running north of 2000 rpm is always 5. At idle and warm 2.5 plus. I would tend to think this is average.
Dan.

[acar24]

I missed what year you are working on but if it is early with the sump plate make sure it has the updated screen and installed correct. also possible crack in the pick up tube. good luck. LA

[bpu699]

Originally Posted by Walt Fricke

So you replaced the bad squirters, right? That sounds like a likely reason shimming the springs didn't do much. As long as you are sure you have the right springs and other parts for the two oil pressure control systems, you should be good.

If you want to make your max oil pressure adjustable, easy to modify the plug end by drilling and tapping, and using a bolt to raise the spring base. But I doubt you'll need to do that.

Yep replaced all the squirters...

This rebuild was supposed to take a year...so far it’s been 2... sheesh...

Racers supposedly run with bearing clearances loose, yet I assume have fabulous pressures...

I dunno... guess I will see when it starts up...

[bpu699]

Originally Posted by bickyd

as a reference for comparison. 1988 carrera 101000 miles
the car warm or cold when running north of 2000 rpm is always 5. At idle and warm 2.5 plus. I would tend to think this is average.
Dan.

88 930...

[bpu699]

Originally Posted by Walt Fricke

Here is how to make adjusters. I had an issue somewhat similar to yours with a newly built race motor, so I made these adjusters. They didn't solve it entirely. I now suspect it was premature wear of the rod bearings which may have been the cause. But something like this can be useful if you want to fiddle (which none of us should have to do, although Porsche did something similar with the 2.8RSR motors).

Neat contraption there... slick it up, patent, and sell .

[bickyd]

Originally Posted by bpu699

88 930...

If your referring to a turbo... nope. Just a naturally aspirated. Lousy pic but this is my pressure at temp and idle.

[Miguel Antonett]

Here is the other side of the spectrum.. I just put a new clutch and fixed few oil leaks on my 78 930, including the oil pressure switch on the top by the breather... all good but noticed the oil pressure is higher than it used to be... it pegs 5bar at 4K when hot, it used to peg 5bar at 5.5K sooooo I really don't know how high is getting. Btw, idle is 1br hot. Would you guys be concerned about it? How high is too high?