Mighty Shilling

not that I know of.

Laust Pedersen

I can’t help jumping in here.

I have read about the “oil starving #2 bearing” elsewhere and most people writing about it seem to imply that this is a Porsche design flaw. In addition to having a very healthy respect for German mechanical engineering, I think this is a case of not searching for or finding the root cause. If this is a design flaw, then it should be possible to consistently induce the problem on all 951’s.

A very like explanation (and the one I adhere to) is that the engines in question all for various reasons suffered from low or no oil pressure (low oil level, high rpm, high g-forces, open relief valve or any combination thereof). Since it is very unlikely and not necessary that the oil path to all the bearings have exactly the same flow characteristics, one bearing has to fail first. So maybe the path to bearing #2 has the highest restriction for example by a few percent, which will make it fail first, but be fully adequate under normal circumstances.

Having tossed my 1987 951 (standard bottom end) around Willow Springs close to redline and 1G (in at least 3 directions) without any lubrication problems, makes me believe that this is not a design flaw.

It is worth thinking what happens to the oil in the engine at high rpm. A higher proportion is temporarily adhering to the inner walls of the block, simply meaning that there is less in the sump. Add to that high g-forces (1G results in a 45 degree oil surface in the sump) low static oil level, then oil starvation can easily be induced.

My simple solution to this potential problem is to overfill the sump with 1-2 quarts before I go racing. The obvious concern hare is oil foaming from the crankshaft hitting the oil, but if at all (I do not know at what overfill level that actually happens) this will only happen at low rpm, where oil requirements are less critical. Additionally, standard oil additive packages have antifoaming components.

In short, I have full confidence in the standard crank design and oiling system.

Laust

Mighty Shilling

I've spun a bearing after being on the track. I added an extra quart to the sump with Mobil 1 15w50. It didn't spin right away, but a few weeks later while I was on a road trip. so, because of that, i had my crank cross drilled and a baffle put in my oil pan. Good luck man, hope it doesn't happen to you, it's not cheap.

jchaley

The oiling problem is not just a 944 problem. I have the early Upfixings and in a article on the 917 (What so great about a porsche 917?) they have a good section on crank oiling. They have a drawing showing a 908, 911 and 917 crank and the amount of oil received at each bearing. They discuss that Porsche tested to destruction the RPM and oil pressure at which each of the cranks would survive. The problem stems from trying to "push" the oil into the holes in the main bearings radially while the centripetal force is working against the pressure. The 917 had axial oil passages (the oil came in the center of the crank at each end). The 911, 908 cranks needed 90-100 psi to survive and failed at 9000RPM. The 917 crank needed only 34psi and had know damage at 10,300. In the 911, 908 cranks the center bearing were the ones that failed. So what does this have to do with 944 cranks? not a whole lot but it does indicate that the problem is not just a 944 problem and is probably related to both oil pressure and volume available at each part of the crank. It is interesting having both 911's and 944 turbos that the 911 world lives by oil temps with anything over 230 being way to hot. In the water cooled world you almost never hear about oil temps yet I know most water pumpers run even hotter(less oil pressure at high temps) but do not really look at oil temp as a problem. just a thought.

john

Bengt Sweden

Correct me if I'm wrong but, I believe the centrifugal forces are with us. The outlet at the rod bearings are farther from the center than the inlet. No matter how the channel goes the sum of the forces will throw oil to the rod bearings. If Huntley put the holes even further out they will get some more force, but I doubt that that will make a difference. What could make a difference is that the oil hole is closer to the surface area where you have the most pressure ( combustion stroke) and that the oil haven't had the same chance to escape sideways.
I am considering improving my own crank since it is out of the engine but I don't want to risk making things worse.
We would need quite good statistics to be able to verify Huntleys claims. If the failure only happens to one in thousand and only on old engines. Then the few cars with Huntley cranks (and new bearings) will not be enough to give significance.
Bengt

M758

Bengt...

Check out this thread for some detailed information by Huntley on his cranks and their success along with some other stuff done to these cars.
https://rennlist.com/forums/showthre...threadid=95863

Here is pic of the huntley crank I found on his site that I annotated that shows oil hole for the rod bearings.

M758

I completely agree!

Bengt Sweden

M758, Interesting thread. I understand the racers have bigger problems than us who just visit the track occasionally. I believe that the most common Mobil 1 is the 5-40 or now 0-30. Good oil for Swedish winters but I guess that it would be wise to use a more viscous variant on the track like 15-50. A more viscous oil will have better bearing properties and the flow out sideways from the bearings will be reduced.
Oil pressure drop should not occur. The pump should have extra volume capacity so that the relief valve always flow back some oil while maintaining the set pressure when proper viscosity oil is used.

I still wonder how many of the failures are with old bearings? You always have some bearing wear and if you take an old engine to the track, oil pressure will be lower.
Bengt

jchaley

The centripetal forces referred to are not the ones from the mains to the rods(once the oil is at the center of the crank the forces help get it to the rod bearing). The problem is getting the oil in the hole in the main journal in the first place (spinning crank is trying to push oil out the main journal not in). The rod bearing fails because it gets its oil from hole drilled in the main journal . Main bearings do not have this problem.

john

M758

John intersting idea. I certainly makes sense if the oil to the mains is feed from the outside. I am not certain of this however. I would mean that added drilling of rod journals does nothing since the oil flow issue is not at the rod but at the source in the main bearing journals. Not enough flow in to the rod bearing supply passages. Hmm... For those with cranks out of the car where is oil feed for the rod passages? I know the rods a feed from the mains, but does the oil from the mains come from? Outside the crank through the mains or from the oil pump through the center of the crank and then out to the mains?


Begnt... I have seen failiures in 944 NA race motors will old rod bearings and with fresh (under 10 hr) rod bearings.

M758

Yep oil feeds from the galley through the main bearings in to the crank and then on to the rod bearings.

Bengt Sweden

John,
since the distance from the center of rotation to the inlet at the mains is shorter than the outlet at the rods. The outlet wins the battle of the oil and will suck the oil into the crank at the mains.
This is valid as long as the lead of oil is not broken. There is a limit to how much you can suck a liquid before it either cavitates ( steam forms due to low pressure) or dilluted gases is sucked out of solution. In this case that wont be a problem since there will at least be some positive pressure from the oil pump.

Bengt

M758

So again it seems that one critical item to making the bearings last is to ensure adequate unterupeted oil flow. So uncovering an oil pick-up tube or creation of air bubble in the oil due to either cavitation or churn from teh crank counter weights is bad.

Laust Pedersen

It is obviously critical to keep a good oil pressure and two things come to my mind, namely the oil viscosity and pressure relief valve.

Under normal driving and high rpm my pressure is 4.5 bar, but during racing the pressure drops to 3.0 bar (high rpm), so maybe the 5-50 (Castrol Syntec) oil, that I normally run, is a little too thin.

Until about a year ago I was experimenting with “Petromoly” (http://www.petromoly.com ) as an oil additive and one day after the car had been sitting for a couple of weeks I (fortunately) noticed, that the pressure was significantly lower than normal (about 3 bar at high rpm). I asked my mechanic to change the pressure relief valve, (heck what could a spring cost these days? Well $250 if it is a Porsche) and also suspected that the additive had created sediments, maybe clogging up the valve, so no more additive either.
That solved the problem although to this day, I don’t know the real cause: the valve, the additive or both.

The pressure relief valve is quite complex and maybe it has a functional weakness for small particles (dirt) and/or age. A recent thread “What are your oil pressures, at idle and at 3k rpm” indicates that the pressure relief valve needs to be watched.

Laust

TonyG

Laust Pedersen

What's your oil pressure when it drops? Sounds like it's too hot which would easily cause the oil pressure drop. If you have the stock oil cooler, and I don't know that you do, you can easily see 250F-260F on the track. That alone will cause a huge pressure drop... not to mention drastically lowering the engine life.

Just a thought.