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Opinions on a failure

Practical Engine Building

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I've had an engine failure at the track and I'm looking to build the best understanding possible of the cause(s). Fair warning: this will be a long post as I intend to include every detail possible.

The patient is an M96.05 code 3.6l from a 2005 Porsche 997 Carrera. Flat 6, DOHC 4v, fully aluminum with Nikasil coated bores. The car is a dedicated race car being used in an amateur endurance racing series. The engine had about 30k street miles before being installed in the car prior to being put under my care. Since then it had done 40 hours of track duty including 14 hours at Daytona (which is particularly hard on engines).

I elected to refresh the engine over winter, and do some reliability upgrades. The engine was torn down and inspected. It was in quite good condition, with the only issues found being worn rod bearings which had the ever so slightest bit of copper visible in the right light. Main and rod journals were in spec, rod big ends were round. Bores had no scoring which is a common issue on these.

It was reassembled with OEM bearings which I had coated by Calico Coatings. Stock main, rod, and head hardware replaced with ARP. Stock oil pan baffle (plastic w/ rubber flaps) was replaced with an EBS Racing unit (stainless with hinged trap doors and more coverage). Camshafts were replaced with Schrick units. Oil used is Amsoil Dominator 20w50 synthetic racing oil.

The engine was broken in on my Dynapack with steady state holds at various RPM and throttle positions between 2000rpm and 5000rpm. It was then street driven about 15 miles. Then it went back on the dyno for tuning where it was run through about 20 ramp runs 2000rpm-7100rpm. The stock rev limit is 7200rpm, and the power was still climbing at the end of the run so we turned it up to 7500rpm limit, and ramp runs to 7400rpm. Peak power ended up being 7150rpm.

Next the car was loaded up and taken to Road Atlanta for an event. The engine was warmed to 95C water and oil temp in the paddock before being sent for its first laps. It ran 3 laps before the engine failed and the car coasted to our pit. The driver proclaimed "it just shut off with no warning", as they do ;-).

The car has an AiM MXG data logger which was in operation, as well as a camera running in the cockpit. While the driver certainly was too aggressive on his warmup laps running the engine right to the limiter exiting pit lane, the engine was up to temperature and no overrevs were recorded.

Upon disassembly I found a catastrophic failure concentrated around cyl4 and cyl5. Head has been impacted, valves bent, pistons broken, cylinder bores broken, rods broken on those two, while the others seem unharmed. #4 rod bearing is welded to the crank, and the crank and rod are both blackened from the heat. All 5 other rod bearings, including cyl5, look like brand new. Cam journals are perfect, no scoring. I have yet to extract the crankshaft carrier and open it to inspect the mains.

It appears that the #5 piston and rod failure are due to debris from #4 getting between the #5 piston and the crank carrier before BDC and causing the #5 wrist pin to be pulled out of the piston, followed by the piston being forced into the head on the upstroke and that rod bending then breaking when the piston reached TDC.

It is my belief that #4 rod bearing is the root of everything, but I'm left stumped as to why it failed. It is the second rod from the nose of the crank, and #1 rod bearing is in good condition. The data logs do show a dip in oil pressure in righthand corners, but if that is the cause of the failure I'm left wondering why the other bearings are tip-top.

Any input on further checks, or opinions on the failure, are appreciated.

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I'm very sorry for your loss - That is a hell of a mess! From what I can see it does appear that the failure has originated from the big end bearing, and the amount of heat produced suggests that this probably didn't happen instantly but 3 laps could easily be enough for it to occur. It can be a little hard to diagnose the exact mode of failure at this point and it's always confusing when you've got one damaged rod bearing and all the others look perfect. This could come down to clearances or dirt/debris ingress during assembly although it sounds like you're probably all over that. Beyond this my guess would be that it is related to oil surge, particularly given you'd changed sumps. While you could reasonably expect oil surge to affect all of the bearings, in just 3 laps you may find that the it was the luck of the draw and it was only #4 that was under compressive load when the surge occurred. I'm sorry that I can't be more specific but that is my current suspicion.

Thanks for your thoughts!