Low Oil Pressure New 2.8SS

[Walt Fricke]

Well, I finally got my 2.8 short stroke motor together, with suitable clearances, and put it in the car.



The bad news was that the oil pressure quickly ran up to 60 PSI, but then stopped there.

It has a GT3 oil pump, and pressures should be at least 80 or 90 in the 5,000 to 8,000 RPM range. But they weren't - did not budge above 60.




I took the motor out and put the 2.8 long stroke back in for the Hallett race. But before I did that I removed the main pressure setting spring and shimmed it up with two washers (amazingly you don't lose a lot of oil and have a mess doing this, at least I didn't). I only made a quick buzz up (in my garage in my residential neighborhood, not on a chassis dyno as previously), but the shimming didn't seem to make any difference.

With the engine on the stand I pulled both pressure springs. I have the long one with the guide tube vertically for the pressure setting, and the short one horizontally for the relief system. Both pistons are the proper new type with a recessed band, not the old ones with the holes. I am quite sure that this 1976 930 case had the oil pressure modification cast into it. The pistons didn't just drop out, but a magnet had no problem pulling them out. I don't see any problems with their bores, and measuring things indicates that their springs push both of them up to their respective stops.

The springs I am using came from my parts bins. I only had one long one, but four short ones. The short ones are all about the same length, and on my modest spring tester (a Summit Racing hydraulic load gauge and a small arbor press) seem to all have roughly the same spring rates.

I ran the engine with the same oil pressure gauge and sender that I used on my 2.8 long stroke, which produced fine oil pressures up to my redline of 8,200 RPM with a 3.2 Carrera pump. I think this sender may be a bit off above 100 psi (it tends to peg), but otherwise gives believable pressures and has done so for two years now.

My other hypotheses, none comforting, were these: I left out one of the rubber cylindrical seal rings (especially the pump-out one). But review of my photos shows them in place, and if one got knocked out befoe I put the other case half on I likely would have noticed this.

I wondered if perhaps one of my oil squirters had squirted out of its hole. There has to be a more elegant way of installing these than staking for a race motor where you might want to remove them frequently. But when I drained the oil I didn't find any mashed aluminum bits, or little spring or ball bearing bits.

I didn't go overboard measuring clearances on this motor (crank was in spec, though on the low side, which seemed OK for a race motor). But would that show up as a pressure vs RPM curve which rose steeply, and then went dead level?

Which is why my suspicions are focused on the pressure setting system, even though I haven't found anything obviously out of whack there.

Any ideas?

[MBEngineering]

HI Walt
was this only cold oil pressure? poss' a crank plug has come out and I see you have being working on the inter-gear with a new drive gear, the end plug is in OK ?? or as you have pinned the cases, one of the inter-gear bearings has come out on assembly, usually one of the inner ones if the cases have been taped together. and could you please put a return edge on the bottom of your engine mounting bar, before it fails on you with all the holes in it.

regards mike

[cgarr]

Walt, The pressure relief valve spring should be the one that's 88mm long for the bypass mod, the old one was 70mm

[MBEngineering]

HI Walt
just noticed the sender you have in the engine, is it only a 5 Bar sender???try swapping out the one from the other engine and re-checking

regards mike

[Walt Fricke]

Mike

It is a 10 bar sender, per its markings. Were it a 5 bar sender I'd be worse off for pressure, though perhaps more convinced something drastic is wrong inside. This sender works properly on my long stroke 2.8. Here is a graph of its resistance. I posted this separately, but nobody who had made similar measurements chimed in to comment on whether my figures were close to theirs or not.



I'm going to buy yet another new pressure sender and measure its resistances to see how all this stacks up. But the old sender worked fine when reinstalled on the long stroke and raced., so I don't have a lot of cconfidence that this is a measurement problem.

I was going to use a mechanical gauge, but didn't have all the parts to adpat a 1.00 M10 hole to an 1/8" NPT male end gauge from my stock of gauge heads retired from air pressure or plumbing uses. If I decide to reinstall this motor to recheck anything after making some suggested change without a teardown, I'll be sure to have the mechanical setup handy.

I'm reasonably sure that the end plug of the IS shaft is in place, not just because of my confidence in my workmanship (I am certainly capable of some big blunder despite experience), but also because it is staring you in the face when checking chain wheel parallelism.

I've not thought about the interior IS shaft bearing getting knocked out during installation of the 2d case half. A teardown ought to show that easily enough.

The crank was unplugged, cleaned, and replugged by a crankshaft shop with Porsche (as well, of course, as other) experience. They did the extra hole in the center main for rod oiling, but were so conservative they only drilled one hole, not one through to the other side. I suppose something might not have been done properly with the plugging, though there were plugs in all the holes.

Craig - I'll pull the long spring again and measure, but it is for sure a lot longer than the 70mm one in the relief system. I think that for good measure I'll get new springs, but my experiment with some shimming was disappointing, as that should have compensated some had the spring shrunk, so to speak. My understanding is that springs don't lose rate even if they are permanently twisted. That's the case with torsion bars - you reset and they behave as they should (with perhaps some loss of range, or with springs travel before coil bind).

What puzzles me is how pressure (including when hot - I had this on the chassis dyno) seems to be normal at idle, and build normally and quickly to a sharp "cutoff" at 60 lbs, after which it stays exactly the same. I have no experience with a big leak in the oiling system, so I don't have a feel for any characteristics of the pressure/rpm curve it would generate. I had the relief valve blow out of a pressure tensioner once, but that had zero effect on oil pressure that I recall.

My expectation would be that if there were an opening which is not supposed to be there in the oil pressure system its effects would be felt throughout the RPM range, including idle oil pressure. Seems to me that the oil flow needs ought not to increase much with RPM (unlike, say, the fuel flow needs), if the pump can keep up at low rpms, the extra flow at high rpms should compensate.

I am aware that focusing on the pressure regulating system has an air of wishful thinking about it. I've already pulled the nearly completed motor down to a short block when I found I needed to deepen the intake valve pockets. On the other hand, it is still clean, and a complete motor so I don't have to futz around with the odd missing fastener or other bit. And the heads can come off as a unit with cam in place and valve lash set, etc.

Mike - I remember your posts about reinforcing the engine carrier cross piece. I saw one break on a real RSR - the driver went several laps of a 1.5 mile track with sparks flywing from the exhaust dragging. But this one has been in service for over 10 years. And the one which broke looked like the holes were punched and somehow some stamping drifted over to the remainder, causing a stress riser. One has to accept a few risks when trying to cut weight for racing.

If I knew how and had the tools, I'd do FE analysis on this piece. The old method of lightening things was to cut holes, and then more holes. And when the piece finally broke, replace it with one which has one or two fewer holes. Not what most of us think we can afford in money and lost opportunities for fun racing.

[bpu699]

Subscribed...

I posted a similar question years ago.

My 930 does the same thing. Great idle pressure of 2 bar plus. At 4000 rpm it hits 4 bar, and that's it. 5000 rpm still 4 bar. 6000 rpm still 4 bar. I too can't understand why if there is wide tolerances, idle pressure wouldn't be low and increase proportionally from there.

My 930 hasn't had a rebuild though. Runs great. Great leak down. Oddly.

If you figure it out I would love to know...

[Walt Fricke]

If I never planned to rev it beyond 6,000 I think I would be OK with what I have. But I'd like at least 80 psi at 8,000, and that's not happening.

[Walt Fricke]

Bruce said I should have a 70mm spring and an 88mm spring.

So I measured.

RELIEF (SHORT) SPRING: My relief spring is 68mm long, same as the three others in my parts box. So right there perhaps I can do something if 2mm might make the relief spring open too soon.

I went through the smattering of electronic catalogs which I have, and I find that this spring, with the same part number, is the same from early on (I didn't bother to go back before 1974) for every two piece crankcase 911 until at least a 2008 GT3.

Porsche technical references give its wire diameter as 1.8mm, and its opening pressure variously as 8 bar/113.7 psi. Others give pressure at 52mm at 104 Newtons, and at 46mm at 138 Newtons (2.4, 3.0). One reference has it as 69mm, but several others give it as 70mm.

PRESSURE (LONG) SPRING: Mine is 86mm long. Porsche literature variously gives this as 87mm, or 86.6mm, and the 964 manual states that it is 89mm long. For a 3.0 it has a wire diameter of 1.5mm and its pressure at 50.5mm is 80.4 Newtons. For a 3.2 its opening pressure is 6.2 bar, +/- 0.8 bar.

That is as far as I was able to get dimensions or specifications for the pressure spring. However, while the same pistons continue to be used (today, as a reasonable guess) in the two piece cases, the spring has gone through a variety of different part numbers:
930 107 531 00 was the part # in the SC and 964 (thus probably the 3.2 also). For the 993 it is 930.107.533.00. For the '05 and '08 GT3 it is 996 107 127.80. Looks like three to choose from.

And at some point a spacer ring part number 964 107 532 00 or 532.81 was added. It looks like it goes under the support tube, which would do just what the washers I tossed in to - increase the base spring pressure, and thus raise the set pressure.

So it looks like I will at a minimum get a new releif spring (and maybe shim it up - Mike conjectured that perhaps the relief system might be causing this).

And perhaps purchasing these various numbered pressure springs, unless the current one supercedes all earlier ones, in which case I just need one.

[BURN-BROS]

Walt,

It's definitely where you are looking. Any other issue would be causing low and slow to rise oil pressure.

[Walt Fricke]

New short relief spring arrived from Pelican. It measured out at 71mm, a mm longer than specified.

I'm thinking of making this one externally adjustable, using a screw through the plug and some kind of flat slug to push the spring tighter while still inside the hollow plug. I've got something like that on a VW bug motor. Shouldn't leak much.

The new pressure spring new measured at 86.5 or so (I didn't haul out a precision caliper), but I now know I can pull it and shim it with the engine in the car and full of oil (I think the relief spring plumbing would let much of the case sump out under those circumstances).

[PFM]

Walt,

The screw idea has been around the VW world for a long time, for your deal use it to get a measurement and then build a shim to set the spring at that height for a cover without the screw hole.

Still want to find out what is going on with this one.

[304065]

Walt,

The only detail you haven't mentioned is the plug (photo dark where it sits)- there are three types
Modern plug with hexagonal head
Interim plug with screwdriver head and groove
Original plug with screwdriver head

Which are you using and what seal ring thickness?

I have measured the depth between the original plug and the later one and it's different-- I was hoping to use the original plug (for originality, naturlich) but found that the depth of the recess on the wet side is greater in one or the other (I can't remember which).

All of which is interesting commentary but your trick with the shims should have shot the pressure through the ceiling. This would compensate for a fatigued spring I suppose, and your test with the hydraulic gauge should reveal if the installed one were a quitter. Anyway, you have a brand new spring which was presumably manufactured to spec.

It's unlikely that I'm barking up the right tree here given that the springs, pistons and plugs are usually changed as a kit but I thought I would ask.

Watching with intense interest here.

[304065]

I also vote for the mechanical gauge for this kind of diagnosis. I have had good luck buying Wika gauges from this place: Welcome to Wika Gauges From F.N. Cuthbert Inc.

There are plenty of off the shelf gauges and they will even make you one that reads in BAR if you want! Mechanical gauges are most accurate in the center of the range, so a 0-120 psi gauge would work fine (10 bar from your existing sender is 145 but I don't think you would even go that high). These come in back mount or bottom mount with a variety of threads and a variety of face sizes. The face size doesn't make it more accurate it just makes it easier to read!

[cgarr]

Here is an adapter I made for the gauge: Its a 10x1.0 bolt and nut, drilled out and a pipe fitting brazed to the top for the gauge. I used this in my vw.

[confused]

Originally Posted by MBEngineering

could you please put a return edge on the bottom of your engine mounting bar, before it fails on you with all the holes in it.

+1

that stock mounting bar with holes drilled into it is very prone to breakage.



I have made a reproduction with a return on each side for $250

[RoninLB]

BA wrote that these part numbers stayed same into the Carrera 2 and Carrera 4

[Walt Fricke]

John

Here are the three types of plug with the dimensions in question. I hadn't paid much attention to the fact that the earliest plug had a deeper hole, or that the groove headed plug was functionally the same as the later hex head. I chose the hex head because of its relative ease of removal. You can see the pipe wrench marks on my grooved head plug. The pipe wrench works great with everything up on an engine stand, and the oil lines and tubes removed. If one had the special screwdriver socket with a thick blade with the right curve, and the surrounding tube which fits over the outside of the plug, no doubt that style would be as easy to deal with as the other. It has a lower external profile to boot. Maybe weighs less (I should weigh these, since I am a weight fanatic).

[Walt Fricke]

Ooops - hit reply rather than add picture.



The copper washers I fished out of my sealing washer box are 1mm thick, which is the same thickness as the washers recently arrived from Pelican, which I see are also the same washers as those for the crankcase drain plug (which, in turn, is the same thread and size as the oil pressure plugs).

I'm awaiting the arrival of later springs (at least different part numbers) for the 993/GT3 engines to see how they may differ in anything other than nomenclature.

I have concluded that I need to order two more plugs and two of the tubular spring guides. My plan is to drill and thread a hole through the plugs, and then counterbore the outside so that an Allen head machine screw will fit down into the counterbore at least some. Then take a guide tube and, for the safety side, shorten it and fill in its bottom end. That will provide something for the screw to press against. For the pressure setting side the tube can remain full length.

Adding the (shortened) tube by itself will compress the safety spring a bit (half mm?).

Also coming is a spacer which Porsche installed to compress the springs further and, one presumes, boost the pressure. I'll be able to measure its length, and that should serve as a kind of guide for experimntation.

[Walt Fricke]

Confused

How much does your lightened engine support weigh? I may summon up the energy to pull mine off and weigh it. If really motivated I can pull the spare stock one out of the floorwell in my trailer (off at the storage place I rent) and weigh it for comparison.

If yours, with the second return, weighs less, that would be nice to know. You were much bolder than I in hole spacing. I don't recall any of the engineers who have FEA programs applying that technology to these pieces, though that would be interesting. I just used the Mark 1 eyeball and tried to keep the remaining widths top, bottom, and sideways roughly the same.

While the one in my picture was done by a machinist using I know not just what, I found that standard bimetal hole saw toothed tools worked fine with a drill press (maybe I did that just with a hand drill before I acquired a press). Just not as many choices in diameters as milling tools would provide. Finishing the cut from the reverse side reduced the edge cleanup as well.

You probably know what the old factory and pro racing philosophy was: Keep drilling/punching/cutting holes until something failed, then back up a step or two on the replacement or repair. A bit risky for most of us. In my case, the risks (of which I was fully aware, having seen a failure) have paid off in the sense that nothing has broken.

As they say, others mileage might vary.

[911 tweaks]

confused...by chance do you have any BEFORE & AFTER weights for the engine carrying bar...??
Thx, Bob