Ahh...

So the oil that goes to the rods goes through the mains and gets heated up. The excess is from the mains is driven into the crank. This remaining oil is then sent to the rods. So the rods get recycled oil to start with that is already hot. No wonder the rods are the first to fail. They get "hot" oil already and this oil must be pumped into the crank by oil pressure from the mains. So main bearing clearance is critical to mainting oil supply in the passages to the rods. And as RPMs increase it is harder for the oil to be supplied in to the main journal passages thus less oil is avalible to supply to the rod hole. That is why cross drilling may have only a partial effect. The oil restiction may in fact be oil from the galley on the side of the block going into the crank through the main bearings.

 

That's about the way most engines supply the rods. The real question remains why #2? If oil feed was an issue wouldn't the #4 (farthest from the pump) be the first to go?
Come on guys somebody has to have a theory. I'm just an old Harley mechanic and those rods were roller bearings with oil fed into the end of the crank.
What's the relationship on the 928 of the #2 to the #6? From what I've caught they have a similar problem BUT it occures on those TWO rods. What similarities or differences do the '44 and '28 oiling systems have?
M758 maybe a cross post over to the 8cyl guys.

 

Hmm,
I wonder?

Are the mains on our motors cross drilled (two holes)? Has anyone cross drilled the mains? This should create more oil supply for the rods as oil will get feed into the hole twice per rotation vs just once.

The 928 actually supplies two rod bearings from one main and I think has two holes in the main bearing (cross-drilled). Our's supply just one.

 

There's a groove cut in the center of the mains that alows constant flow to the rods, otherwise the rods would only get a squirt of oil every time the holes lined up.

 

The way Porsche numbers the cylinders, 2 and 6 are on the same journal. Those are the second cylinders from the front on each side.

For those interested, this was covered on the 928 forum. There are some nice pictures of the 928 crank showing the oil holes and passage directions.

928 bearing thread

 

Al yep you are right about the grooves in the main bearings. I had forgoten about them.

So would two flow holes into the crank provide more flow to the rod bearings? Or could it starve the Mains?

 

Having just read the six pages over on the 8cyl board I think I have a headache!

I have seen at least one dry-sumped '44 and am aware of others out there. Jon Miledge does them to the tune of about 5K

The discussion about stroker cranks in the '28s being "Chevy" drilled fascinates me but either I'm fairly dense or no one over on the other board actually explained it. The book "How to Hot Rod Small block Chevy's" apparently talks about changing the oil flow path in the crank, looks like a trip over to Barnes & Noble.

The final thing that I got from my trip over to V8 land was the comment "fed from a different main bearing" this seems to indicate that the feed to the main that in turn feeds the #2/6 journal (#2 in the '44) is somehow deficent which is the cause of the #2/6 rod bearing failure rather than an across the board failure as would be seen in a catastrofic loss of oil pressure.

 

Ok, so you don't have enough theories. Here comes another one. The local expert and racer told me that he experienced that many of the rod bearing failures came when an engine where overheated, stopped and then cranked over again while hot. Perhaps the bearing was grabbing an overheated crank?
Anyone had similar experience?
Bengt

 

Bengt,
I can tell you that most rod failure i have seen happend on the track at speed. Not after a restart. It does dovetail into an intersting issue brought -up a few months back about coking.

Just a little background.
If oil gets too hot (400F type temps) the oil can burn and form coke. (not coca-cola). Think of this stuff like soot or ashes from a fire. Anyway coke is well know in engineering circles to be really bad as It tends to clogg oil passages. These clogged or resticted passages have a reducd oil flow thus causeing problems.

One way to form coke is to heat soak. That means if the crank is 450F and the coking temp is say 400F. If the oil is flowing even at 230F then maybe it can increas to 250F before flowing to a cooler area of the enigne. When you shut off the motor the oil flow stops and oil that stuck in the holt places like a 450F crank may start to coke.


I am not saying it does happen because I don't know the real operatating temps of the internals of the engine nore the heat transfer rates.

 

Sorry but when I opened up the motor I had there was no sign of sludge/coke or anything else for that mater in the oil passages.

 

Nor in mine either. Passages looked very clean (with the obvious exception of the copper shavings).

 

Good!

So maybe we can toss the coke idea!

One down.. 540 left to go

 

I think the coke thing has to do with oil passages in a turbo (the turbo itself) So yes 1 down 540 left.

 

OK so when I get fired for spending all my time on the internet I'll have plenty of time to research this over the winter.

Sitting in the garage is the old crank that had the bearing failure (you can never tell what one of those might come in handy for) and three other motors. One of which is the one I pulled when it started using a lot of oil suddenly, one I've been using as a parts motor and a third that I have yet to pull down and ascertain it's condition. Of course there's also the one in the car that I rebuilt that had the bearing failure. With all of this hardware to look at you'd think that I'll be able to come to some sort of conclusion over the winter.
So in the middle of typing this I take a bathroom break and while I'm sitting there it occures to me that I've never herd of a 968 motor or an S2 for that mater blowing up! Now of course it may just be that I didn't get the memo but then again it does have a different crank. Think maybe the engineers at Porsche actually did fix the problem?

 

Al,
That's a great point, and with all of the S2's that ran in the Firehawk series, I'm thinking that there must be a decent explanation. What about the other racing series? Did the Cup cars have rod bearing issues?

-J