Cheburator

Well, it had to happen. 155 race miles today and she sounds like there is a little dwarf with a BIG hammer somewhere inside the block. She still runs but I have not driven it as I don't want any further damage. I am hoping that the rod will be OK and that I will get away with just oversize bearings where needed.

Now, how it happen given that I have a drysump? I honestly don't have a clue.

One thing I noticed that suddenly the oil pressure went down to 3 bar at full throttle say at 4500rpm. If I kept going to the limiter the oil pressure would not go higher, however if I lifted off, oil pressure on the gauge will climb to 4.5bar. WTF? Only happened during the first race of the day once the engine was hot. In normal road driving it was up at 5 bar and then the big end(s) went. Any ideas? I did have enough oil in the tank before you ask. What would cause the pressure to climb higher even as I have lifted off.

Any help will be greatly appreciated.

Thanks,

Alex

Kiln_Red

My advice would be DO NOT RUN THE CAR AT ALL until the failed conrod bearing is addressed. The more revolutions the worse. You need to check the rod and crank for visible scarring and check to see how out-of-round they are. If within tolerance, fit an oversized bearing and consider yourself lucky.

Imo000

There is nothing to check. If you spun the con rod bearing, both the crank and the conecting rod will have to come out. The connecting rod will have to be re sized and the crank re ground.

Kiln_Red

Machining the crank is not always an option if the damage is too great, though. The rod is not definitely reusable either. The first thing I would be looking at is whether they're reusable or not. Beats packing your crank down to the machine shop to let them determine for you if it's scrap or not when you've access to the WSM and the appropriate tools at home.

AO

That sucks. Was talking to DrNick the other night and he mentioned there is a video of his old orange car (with drysump) going around a track and you can see the oil pressure fall off. He couldn't explain it.

Only thing I can think of is if the sump was filled with aerated oil?

Leon Speed

I was going to say either an obstruction in the oil passage or an oil pump problem, but I don't know your dry sump.

Cheburator

I hear you, but I have a five US gallon tank at the back, which should allow the oil to de-aerate, plus I was running aroun 11ltrs of oil in the system. Moreover, the pan has a rudimentary scraper in it, which should help combat aeration...

Can someone explain the odd pressure gauge behaviour? Why would the oil pressure climb as I lift off? I think the answer to this is the key to why the bearing went in the first place... I am using a two-stage external scavenge pump and the engine oil pump on the pressure side.

As with regards to the bearing journal - why would the rod be in need of attention. Usually it is the crank journal which need grinding after such a problem. We will drop the pan and get the micrometers out to check journal roundness. I keep thinking that I might as well do all bearings when we open her up - "cheap" insurance so to speal....

slate blue

First of all Alex sorry to hear of this problem and thanks for being brave and posting it. Unfortunately this will all cost money, my suggestions are to make sure you have enough rod clearance. Now this may not have been an oiling problem, it may have been a clearance of rod bearing problem.

So what clearance were the rods set at? I suggest 2.5 to 2.8 thou. Now if they were in this range and every bearing was checked not just a sample, lets move to the next issue, which may be capacity to remove the frothy oil from the sump. Do you know that the oil tank is actually full? There is a lot of different sensors available for this. If this is a potential problem have a look at Gill Sensors, they are in the UK. In my dry sump tank the sensor monitors the level of the oil and the temp. I think the development is about taking out the guesswork.

As to the changing oil pressure, what about metal from the bearing getting caught in the bypass?

On the rods, a trick is to put them in special vice grips and have them honed, this changes their shape, eggocentric which is what modern high rpm bearing are like from factory.

Good luck

Greg

Cheburator

Every single bearing in the engine was new and plastigaged before instalation. I went one step further and plastigaged in a x-pattern in order to perform a rudimentary check for journal roundness. I don't remember the actuall values off the top of my head, but I do remember that all journals, both on the crank bearings and rod bearings were right in the middle of the WSM specification, which was good enough for me.

Yes, I check the oil level in the tank before I go out with a dipstick with marks on. Thus I know I had 11ltrs in the tank before the race started. The engine does not burn or eject oil since both catch tanks were dry when I checked them following the bearing failure.

Does not make sense - pressure would not rise on lift off if metal from the bearing was trapped in the bypass. I think it is something to do with oil temperature and viscosity - the oil pipes are routed very close to the headers who glow red hot... Or flow restriction on the pump side...

Mike Simard

What odd pressure behaviour. Is the crankcase vented or sealed?

Emboss

Checked for oil pump cavitation?

danglerb

The only thing that I can think of that relates oil pressure to throttle position is the "slosh" withing the oil pan or other systems. When accelerating stuff tends to slosh to the rear, and when you lift off it would tend to slosh forward.

When its apart maybe the situation will become more clear?

Cheburator

Vented via the original vents on top of the passenger cam cover, which go via a T-piece to a common pipe, which in turn ends into a catch tank.

How do I check for for oil pump cavitation?

No sloshing, as the tank I am using is a Moroso tank. Correct me if I am wrong, but these guys know how to design these things in order to avoid surge and to help with oil de-aeration.

Someone with a bit more knowledge of fluid dynamics should be able to explain why pressure would rise on lift off. When the engine was cold, pressure always has been at 5 bar as soon as you go past 2rpm. It would only drop off as it got hot

danglerb

One other thing I just thought of changes with load, force on the piston, which is force on the bearings.

slate blue

By Cheburator

I checked the factory spec from the GTS tech spec booklet, it quotes for radial play a range of 0.027 to 0.069 mm I went down the middle and converted to imperial and that comes out at less than 1.9 thou. That is tight for a race engine, or 1 thou less than optimal, it doesn't need much to pinch the bearing and game over.

A scenario could be, engine a little hot, oil viscosity breaks down and then metal to metal contact and then trouble.

Fair enough, 11 litres is a lot of oil, Greg Brown said he saw a 928 engine running on a dyno and it emptied the dry sump at certain revs, as soon as the revs came down the tank filled back up, hence my question. This also takes me to the quality of the oil supply to the pump. You got me thinking given the pressure fluctuations.

Maybe the fluctuations have to do with aerated oil, as the revs come down the aeration subsides. In another thread the Porsche 928 late and early pump were measured for capacity and it was stated that the pump (used in your car, i.e after 1980 MY) had a capacity of 3.95 gallons per 1000 rpm. I presumed US gallons were used when I converted to litres.

At 7,000 rpm it would be almost 1 3/4 litres per second, I know you may not be revving that hard, just making a point. Now if the dry sump pump is getting overwhelmed with all this oil the engine crankcase and heads will be getting fuller and fuller with frothy oil. So your oil tank no longer has 11 litres in it, it may only have a few and then it is getting deluged with lots of frothy oil and some of this is coming back to the engine oil pump. The scavenge pump needs a far greater capacity than the oil supply pump due to the mixing of the air and oil.

Have you got any pics to post of the tank and its method of removing the air? Also a dynamic oil level reading would be handy in this case which is the why I raised it initially. Just for additional info some pumps separate the air and oil before sending back to the tank. I would imagine for this very reason.

Best

Greg

P.S

If there is metal in the engine remember to clean out the lifters, lots of debris gets caught in them.