3.0 vs 3.2 case - differences

[chstephens5219]

I had a catastrophic engine failure this fall with my '85 Carrera track car which had been built out to a 3.5L pushing 300hp. I just removed it over the holidays and found a hole on the top side of the case...something gave way inside. My best guess is either lubrication or detonation. All I know is a loud "boom" followed by a large cloud of smoke occurred as I powered out of a turn at Mid-America. I will find out more this week as I remove the heads and cylinders.

Question, when I rebuild can I go with a 3.0 case or should I stick with a 3.2 case? What are the differences and pros/cons? Both would need the appropriate machining and dovetailing to accept the 100mm pistons.

Thanks in advance.

Craig

[Flat6pac]

Both cases are the same with the addition of sensor locators for the flywheel on the Carrera 3.2 and the oil drain plate was removed on the Carrera. The late 3.0 in late 83 were built on the 3.2 case which had already come into the system.
Bruce

[chstephens5219]

Thanks Bruce. Would a 3.0 case require work to accept the flywheel sensor or am I not understanding your point?

Craig

[Flat6pac]

The 3.0 case will not accept the sensors except in the noted exception of very late 83 model run.
Bruce

[chstephens5219]

Got it. Thank you for clarifying.

Craig

[Wayne 962]

If that was mostly a stock bottom end, I would bet dimes to dollars that it was a rod bolt failure. If yes, then use ARP or Raceware this time around for extra insurance.

-Wayne

[chstephens]

Thanks for the insight Wayne. I'll look closely for this once the case is split. With all of the probable damage, it's hard for a novice to know what gave first. The hole is at the top of the case between cylinders 2 and 3. Upon removing all of the valve covers, both rocker arms are broken on cylinder 6. I think the #6 connecting rod is suspect.

Craig

[Walt Fricke]

If you are not using the 3.2 Motronic sensors, not to worry about cases. There may be a few small differences (a different intermediate shaft end cover) easily dealt with.

To accomodate the bigger crank in the 3.2, Porsche made the rod bolts one mm thinner. Fine for stock motors. Not good for hot rods, as Wayne points out. Since the stock crank oils the rods from each end of the crank, the #2 and #5 rod journals are farthest from the source of pressure. If you put enough tension on the rod bolt to stretch it (and those larger pistons can't help in that regard), the oil has a larger clearance to escape from, and may have less pressure behind it. A couple of swipes of metal to metal without an oil interface, and poof.

I think you don't have to be an engineer to dope out rod bolt failure. First off, it will be in two pieces. As the bolt stretches it gets thinner near the weak point. Then it breaks where it has gotten thinnest. So if the bolt looks like it is necked down just before the fracture point, pretty good guess its strength was exceeded. The aftermarket bolts are made of pretty good stuff. I had a top end failure which left the rod cap bent - the bolts were stretched a bit and bent, but still in one piece and the big end was not able to thrash around. No consolation, as the small end of things can saw your case almost in half by itself. But stout bolts.

Of course, you could have dropped a valve, or had a piston pull in half. Those will lead to a hole in the case also. But if your cylinder is still intact externally, maybe rod big end failure is the best guess. And your broken rockers do seem to point to one cylinder. Not much solace.

Don't you just hate those loud booms? I'm also thinking that when I had a #5 rod big end failure, it was still the left case half that took most of the case damage. Maybe due to the crank rotation?

Walt