Lemming

As most of you may remember, I destroyed two engines last year.

1st engine - lost power up top during a race. I assumed that it was the variocam so I pulled over (I was in the overall lead of a PCA race at Barber ). Sure enough, when I pulled the cam cover the bottom pad was missing and one tooth was gone from the intake cam. I pulled the oil pan to get all the pieces out and found that my #2 rod bearing had also spun but had not yet really damaged the crank.

2nd engine - This engine was purchased from APR, it had been freshly rebuilt, ported, polished head, new valves and I was told the bottom end was in great shape. The engine lasted all of 3 track days when the #4 rod snapped 1" below the wrist pin. Piston was still free floating and not bound in any way. The rod blew out the bottom end with enthusiasm (jack hammer style!).

There is no doubt that I flog these engines, after all I am racing. Last year I was regularly bouncing the engine off the rev limiter (7K, RSBarn stage II). This year I have decided to set my shift light to 6,500 in an effort to keep engines a bit longer. I have also gone back to regular motor mounts (from solid). For oil, I run a 50:50 mix of Redline 40wt racing synthetic and Rotella 15w-40, which is typically changed every weekend if I run an enduro, every other if no enduro. Oil pressure has not been an issue while on track that I have seen. Oil temps do get up to 245-250 at times, water temp is always fine.

So, let the speculation begin. I decided to post this since I think the issue may be the increased revs that I was running. I am also running the S2 flywheel and harmonic balance and have no more vibration than my 968 daily driver with the DMF. I also revamped the oil cooler inlet for better cooling this year.

Mike O

Your are Joe Btfsplk?



Seriously, do you have a picture of the broken rod? I'm sure the fracture surfaces are toast, but just in case one of them is OK it could offer a clue. For now I am leaning toward a flaw of some sort coupled with the higher revs. A nick in the rod might have been enough of a stress riser to initiate the failure. Were the rods new or just "in great shape"?

Lemming

No pics on this comp, will get some posted tonight. Rods were not new.

Eric_k

I'd mark it down to sh*t happens. They are unrelated failures, unless you consider racing the relation.

I doubt it was the solid mounts that caused the first failure - unless your oil pickup tube was cracked. That said, I run stock engine mounts to reduce vibration. Oil too hot?

Is it possible the second engine had some previous trauma before you got it? Why was the head redone? Maybe the #4 had previously been hydrolocked by a leaking headgasket.

Maybe you are just using up your bad luck....

Lemming

The head was rebuilt as a start to a 3.0L turbo project that was abandoned. Could be bad luck but what about RPM's? What RPM are you running to Eric?

Mike O

I was looking for some info on RPM vs crank loads and I ran across this. Its a little DEEP but there some easy to compare charts and analytical models around page 95. What it does show is that for this particular rod, the stress in the area of failure is relatively low compared to other locations. Oh yeah, I know its not a 968 rod and its the wrong geometry and its the wrong material, yadda yadda yadda....

JamesE

Do you run a dry sump? I've seen a 968 that died that was being tracked due to oil starvation, due to wet sump.

Eric_k

My rev limiter is at 7100. I try to stay off it and usually shift by 6800. But I have bounced off it plenty of times in the last 6 years. For enduros, I try to keep the revs below 6500. My engine is all stock except header, chip & fidanza.

Lemming

I'm running stock setup. Most of my videos show the oil pressure gauge and I've never seen it dip while on the track. I also tend to run on the high end for oil fill.

I have purchased a crank scraper that will go into a spare engine that I will begin building soon.

RajDatta

Tim, oil starvation aside, you should seriously consider getting your car properly tuned. I think that is what is causing your problems. A standard tuned chip might work in street applications but in your case there is little room for error.
Raj

Lemming

It is possible that my car was running a bit lean last year. Most of my dyno runs are on a dynojet, the shop closed at the end of last year (hopefully to reopen in a new location). In Dec. I went to a mustang dyno to check some changes and they showed that my car was running really lean. When I told them where I had been tuning, they said that dyno's a/f meter was off. I had to increase my FPR quite a bit on the mustang.

I do check my plugs often and they did not appear to be running too lean but certainly were not running rich.

PorscheG96

I think that your first engine failure was typical failure of the Variocam tensioner ramps and #2 rod bearing which could possibly be avoided by changing the ramps and rod bearings every season.

Do you know what year the 2nd 968 engine was? The early connecting rods, although forged, are weaker at the base than later 968 connecting rods so it's possible that you had early rods that failed. You can see the differences between early and late rod designs in the 968 workshop manual.

Lemming

Rod bearings were less than one year. Top tensioner was in good shape, tough to see the bottom so it might have been bad.

Second engine was supposed to be a later engine, can't remember if it was a 94 or 95.

Lemming

pics of blown out bottom end and piston as promised.

rob76turbo

Ouch!!