George, that would imply that the cylinder cracked before the bearing/rod/piston distruction OR something else broke that hasn't been found yet allowing the SC or boiling coolant to pressurize the crank case. Would boiling coolant produce enough steam pressure in the crankcase to push the dipstick out?

 

George I was sure that you knew just wanted it clear for the casual reader

 

Drill it like a Chevy; drive it like a rocket.

 

If the crankcase breather system was clogged or not functioning properly,
it could cause pressure to build up in the crankcase. That's another thing
that was unusual about that day. The engine usually blows out a quart of
oil per leg, but this time I only had a few drops in the catch can and the
dip stick measured no oil was used on the first leg. This is also an area
that I'm having Dave address with a new oil breather system.

 

As George is indicating, there are a number of things that were going on that could have contributed to the failure. Speculations should be sent to George written on the back of a crisp $100 bill. Better yet, just send George a donation at his PayPal account and put your guess in the comment box. George's PayPal is jorj7@sis125.berkeley.edu. George's bill for the rebuild is going to be a big one, all joking aside. There have been some generous souls that have stepped forward already.

 

Quick correction: George's Paypal is jorj7@sis125.berkeley.edu

All that time spent at Berkeley in the '60's catching up to you, Bill?

 

I think he was almost typing Berzerkly.

 

Engine gurus-- Would a grooved main bearing shell take care of some of these oiling issues? I haven't looked at a 928 crank, just have a mental image of oil flow in the crank from the discussion. A grooved main would give oil to the rods all the way around the rotation. Penalty is loss of load-bearing area, though. Dave/Mark/Greg?

 

When you do the drilling process, you also have the main baring grooved (one of them) by just a bit. So yes, it helps, it seems.

 

Jeez, I can't even spell Bezerkely!

 

It is very unlikely that just drilling the crank to redirect the oil will do anything unless the failure is from very high rpm. since the off angle oil route is only affected when the forces on the oil itself exceed the pressure outputted by the pump.

But the failure is at lower rpms.... The most likely "oil related" cause is oil starvation, and or aeration due to low oil level in the case being fed to the first bearing opposite the main feed into the engine.

We always ran an accusump while road racing and checked its function before every event as part of pre race prep.

marc

 

Since you have studied the forces on 928 engines, and have one of the more flawless records running the events, what do you think happed, if you were to guess?

mk

 

Hard to tell without seeing the bits and pieces.

I would start by wondering why the reading of the dip stick at the halfway point and it reading the same as when the event started....something wrong with this. Maybe there was water "lifting" the oil evel to a "no oil used" assumption, or fuel form a leaky injector causing the oil seal between ring and bore to fail. the later would be indicative of failed cylinder to piston lube.

I also carefully look at the main oil galley and see what is there for residue...have it tested for glycol/water. the output of the filter is directed at the main oil galley.

Also look at the other rod bearings....showing signs of failure ???

Lots of different direction to look...but these three might be a good start. rule them out is the goal.

A guess...at 12.5:1...washed down oil coupled with oil starvation due to high rpms and oil in suspension and in the head.

But just guessing...

Marc

 

Matt, the siphoning effect of the oil heading up to the rod journal can only help so much, at best it can create a zero pressure condition, a vacuum, but positive pressure is needed to force oil through the passages from the main bearing. A vacuum is not a force but an absence of a countering pressure.

About "cross drilling", as Jim Bailey eluded to, cross drilling is a bad idea. It used to be a buzzword but then engine builders noticed fancy cross drilled cranks failing at high rpms where old fashioned "Chevy" drilled cranks did not. It was because the oil in a cross drilled crank is subjected to greater centrifical force and at a certain critical engine speed that force becomes greater than the engine's oil pressure. Whether that's a factor in George's engine at the speed he was running it I don't know.

 

Mike, liquids are incompressible so a positive pressure in a tube has the same effect as a negative pressure (vacuum) from the opposite direction. Only the magnitude of the pressure difference matters (still assuming equal cross section/diameter). The greater mass with greater angular acceleration in the rod passages would more than overcome the opposing forces in the main passages. That's even without the 8bar of pressure from the oil pump.

BTW, there's no such thing as centrifugal force.