Cobra4B

Just got done unloading the car at my dad's place... fought the car all day yesterday at the UTCC and got no laps in... car snapped a belt in the warm-up session. Thinking it was a bad PS pump I ran all over Danville getting parts and got it all fixed, then on the first lap of the last session it snapped another belt. Then testing it in the paddock it snapped a 3rd. The car has an odd accessory layout and has a long run of belt (a little over 2') accross the top from the PS pulley to the alternator pulley... this can get jump at higher RPMs and there's a belt slop limiter rigged to keep it from chucking belts. That limiter had broken off so I fabbed up a new one and thought I had it fixed.

Car was vibrating more than usual and oil temps were high... but it was 102 today and 100ish yesterday.... car was doing this yesterday as well. So I look down and see it was 287 (the car had never been over 272 before even in full boogie in 100 degree weather)... I'm coming down the front straight and think "If this hits 290 I'm quitting". Well as I lay down the power coming out of 2... "CLANK!!! ching ching ching...." Car shut-off and since I was tracking out I just turned left and drove right off.... here's the resultant carnage.





Funny thing is the car just made 341 rwhp w/ a 12.8:1 tune and reduced timing at Sneed's Speed Shop and was running better than ever. The bottom end was never touched and is the original stuff the car has had since being built in '97 or '98. Oh well it was only a matter of time spinning a stock 2-bolt bottom north of 6000 rpms.

-Brian

markj

nice, just like ive always said "if its got tits or tires, its gonna give you trouble!"

nftuatn

are you able to put a bigger motor in it? or do you have to keep the 302..iirc that is what you said it was or do you have to keep the 302 block for the class your running?

Fabrik8

Take pictures of the broken area when you get it taken apart. Sounds like it was a fatigue problem that was waiting to happen sooner or later.

Is that a cast factory crank, or something decent quality?

Cobra4B

For TTS or ST2 you can only have 8.7lbs/horse so we can make all the power were allowed to have with the 5.0. Plus I dont want a stroker for a RR motor. Will most likely do a Dart 4-bolt main iron block 306 w/ the same bits we had/have on there now.
It was a stock motor from an Ausie Ford Falcon sedan... Panoz bought a batch of them for the cars. I can only assume it was whatever stock bottom-end that came in them.

What locates the crank longitudinally? I.E. what would have worn out to allow the crank to start walking? On Friday, the few laps I made before belt chucked, the car was vibrating more than normal and I was having trouble shifting which I never have... mostly upshifting. So Im thinking that under acceleration the crank (and flywheel that is of course attached to it) would walk back and prevent the clutch from 100% disengaging.

In the pits the crank pulley wasnt wobbling and belt allignment was fine... Im near certain that all the issues I was chasing Friday were just symptoms of the crank walking on its way to snapping. I guess the walk induced vibration and the harmonics snapped the crank... not the RPM or use/stress.

JDM74

I'm pretty sure that #3 main bearing has a thrust bearing surface also. That is what would locate the crank fore and aft.

J. O. BUDS

thrust bearing on front and back of 3, see top of this photo

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Damn, well prime opportunity to build a new motor for it. If you do are you going to stay within the limits of your class or step up? No idea what class you are in, so no idea on what modifications are allowed.

Cobra4B

No... will just stay in TTS/ST2 rules at 8.7lbs/horse... the next step up is TTU/ST1 which is 5.5lbs/horse. Keeping a 500+ rwhp horse RR motor alive costs quite abit more than a 330-340 rwhp one.

Fabrik8

I'm guessing that RPM and stress was making the block and/or bearing caps twist, which probably make the crank crack from fatigue. The vibration problems and shifting problems were probably from the cracked crank, which is obviously not as stiff as a non-cracked crank. Usually things don't move around in relation to each other unless they're under load, so usually a crank doesn't walk unless things are misaligned under load (because the block is twisting or something else is happening like the crank journals are moving). Basically I think that the vibration was signaling that crank had already failed, even though it hadn't actually completely failed yet.