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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.

Attached Files

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!

My question is how did the cylinder bores themselves break like that? I wonder if at the time of everything going on, the cylinder bores first began to crack and allow coolant in. The water being compressed was too much, snapped and bent the rods breaking the pistons and sending them flying into the head etc...but as in the pictures, there's also the indication of how hot they got (big end bearings). What a way to blow up. Not sure how the driver just only stated it just shut off, that was a c4 bomb going off in that engine bay.

My belief is that the bores broke in the aftermath. The pistons at BDC only have maybe 10mm of clearance to the crankshaft carrier. It would be easy for something to wedge between the piston and the crank carrier and cause the piston to cock sideways in the bore then have all hell break loose. I'm fairly certain this is what took out cyl 5 aftercyl 4s rod came apart.

Number 4 bearing ran out of oil. Is it the furthest from the oil pump?

There is a bit of myth that in the case of oil surge you would expect ALL bearings to suffer. It is not uncommon for the first bearing to suffer being the one furthest from the oil pump...It is the first to lose oil pressure/flow and the last to get it back. Not always the case but often can be.

Another "weird" blow up I had was when someone was over zealous with PTFE tape, some broke loose and partially blocked an oil way. Heard of similar with sealant.

Bad luck. I spent years chasing oil surge issues. Ended up fitting an accusump which kind of solved it but did cause other lesser issues. Eventually moved to a series that allowed dry sump which (one act of stupidity by ARP aside) solved almost all problems. I know people complain about the cost of dry sump, but it is less than the cost of a blow up.

I have a gen 2 Cayman with the seriously reworked engine. Porsche put a lot effort into the oil system (err...and IMS etc) for a good reason, might be worth looking at.

Yes, that makes sense. I've just never seen the cylinders break like that even when pistons and rods are in pieces. That is just insane! As Andre stated, I am very sorry for your loss

I usually ended up with bits of piston/rod punching through the block and ending up in all sorts of strange place. Embedded in the starter motor and being returned to me by a (laughing) trackside marshall are two that spring to mind.

The Toyota 4age was seriously not designed to go round a large number of right hand bends in succession, especially not at 8-9500RPM.

@Dennis, #4 isn't first nor last on the crank which is part of what caused some question in me. The first main looked good, then #1 rod looked good, the second main had some polish marks on the coating, then #4 rod, then the third main was completely toast. All the rest of the main and rods were mint. I hadn't split the crank carrier yet when I made my first post so I hadn't seen the mains.

We've now got a 3.8L S engine installed and last Thursday did a test day with me driving. I didnt reuse the aftermarket baffle box from the old engine, instead chose to keep the stock one and add an Accusump. I'm waiting on one more component to come in to fit the Accusump, so the test day was all stock oiling system. In the sharpest right turn at the track I got some drops in pressure, but not to zero. I was able to manage it by altering my line through the corner so we could at least get through the rest of our testing. Racing this coming weekend, will have the Accusump fitted by then.