Broken Parts:Tale of the two piece Crankshaft

WOW, this is one of the more extreme failures I have ever witnessed from an M96 engine.... Its not too often that a crankshaft shear on an engine that has 7 main bearings, but this one damn sure did!



The engine has the 3.6 X-51 package and was making 325 RWHP and had seen TWELVE THOUSAND track miles prior to this failure. We had initially thought the engine had broken a rod due to the material that came from the oil sump, but as soon as the engine arrived at our facility a five minute inspection found the crankshaft to be in two pieces!

The material the M96 crankshaft is made of is powdered metal, it's what most modern engines use for crankshaft and connecting rod materials; I am less than impressed with this material to say the least.. I can't believe that a component with such mass could break so extremely.



I feel that this failure was attributed to by a couple of things-
1- The engine was "upgraded" to a lightened flywheel. This new flywheel was installed onto the existing stock engine without being balanced to that assembly. This created an imbalance in the rotating mass AND it did away with the factory dual mass flywheel.

2- The dual mass flywheel was removed to alow the single mass lightened unit to be installed. This eliminated ALL MEANS OF HARMONIC DAMPENING!! The crankshaft was forced to absorb ALL harmonics from the engine and transaxle when the dual mass unit was removed..

So- adding the light weight flywheel was a double negative, not only did it create imbalance, it also eliminated the harmonic dampening of the dual mass arrangement.

Due to this I feel that adding a lightweight flywheel to any existing engine is not a wise decision, and that they should only be added when the entire rotating mass can be balanced and indexed to accomodate the lightweight unit. This means engine disassembly, so I'd only add one of these when doing one of our performance upgrades so the entire assembly can be precisely balanced.

This X 51 will be going back together with our 3.8 package applied using LN Nikies cylinders. It will see some head work upgrades along with severe duty valves and our hi-rev spring package. We'll be upgrading the rods from the stock powdered metal units to LN Billet connecting rods and we'll also be applying many oil system mods to this one. Since the crank needed to be replaced, a Flat Six Innovations Billet Chromoly Crankshaft is being used (instead of another crappy Porsche powdered metal unit) and is in the process of being made now. I feel quite certain that these mods and materials will eliminate failures in the future when coupled to our balance and assembly procedures.

Target for this engine is 425HP at the flywheel N/A

 Update:

After this article was written in October of 2008 we experienced a rash of similar issues related to broken crankshafts in the M96 engine as well as the M97 engine.

In one instance the owner of the car had experienced FOUR broken crankshafts in a period of eight months with his 996 track car. These four failures consumed a total of 4 different engines, the one constant that remained between all of these engines was the re-use of the same flywheel, clutch and pressure plate assembly. That assembly was of "lightweight single mass" nature, just like the one we have illustrated and discussed within this article.

 

Other crankshaft failures have occured on the street and during DE track events with the 3.4 and 2.7 liter engines used in the 911 and Boxster respectively. It seems that the majority of the broken crankshafts we have been exposed to share one common denominator; the use of the "single mass, lightweight flywheel" package.

Failures with the newer and larger displacement 3.6 and 3.8 liter engines is most common, due to their longer strokes, higher performance levels and heavier pistons coupled to shorter connecting rods.

As of the update of this article in September of 2009 I have not experienced (directly or indirectly) a failed crankshaft from an engine using a dual mass flywheel.

Is the purpose of this article to "slam" the lightweight flywheel? Not at all.
We merely have more direct experience with these engines and their modes of failure than most anyone and feel it is imperative that as much information about failures is shared. I have given you my story, and some pictures; you can do your own research and decide whether a lightweight flywheel is best for your Porsche.

Jake Raby
President, Raby Enterprises Inc.