Well, here is the death and destruction (Parental Guidance Advised) !
As mentioned, engine failure occurred at Sears Point Race track, exiting the "carousel", or turn 8. Prior to the track weekend, the car was tuned on a 4-wheel dynojet, all a/f ratios were spot on, EGT's were acceptable and timing was not too aggressive, especially considering the 100oct. fuel.
Exiting the turn, acceleration fell off abruptly passing 4500RPM. I immediately lifted the throttle, reapplied throttle and heard a bad rattling sound - looking up into the rearview I saw a cloud of white smoke out the back of the tail pipe. I shut off the ignition and coasted to a stop at the side of the track.
All morning long the motor was running VERY strong, dyno tuning the week before showed over 300 HP at the wheels, so this was a little disappointing. I suspected a broken timing belt or loosened crank pulley. Back in the pits I confirmed the timing belt was intact, so I began pulling spark plugs. It didn't take long to pull out #1 to see what had happened...the exh. valve head had broken off and spent a good few seconds bouncing around the inside of #1 cylinder. Oh well, what could I expect when my150HP/liter motor was pulled out of a junkyard with an unknown history? It held up for over 3 years, enduring over 30 track days, drag racing, commuting, and tuning experiments, one after the other. I had definitely gotten my money's worth. It really is a testament to the strength of the 10v MC motors. Aside from the abuse the piston had taken (and held up quite well), the rod appears useable, and the block is literally undamaged. This was a rusty junk yard motor that I got for almost free, making 400 crank HP even after likely more than 200k miles.
In all opportunistic honesty, I wasn't very disappointed. It was actually very interesting to see what had happened. Fortunately I had, by that point, began a 20v project, and this was a really good excuse to accelerate that project into fruition.
The exh. valve failure is actually quite common in the 10v motors. The exh. valves are hollow, filled with sodium that absorbs heat and transfers it out of the valve head. Though this greatly improves the heat range of the exh. valve, it also has a toll on physical strength. Fatigue over continued useage often causes failure of this in high mileage motors, especially those that have seen track time or extended times of high RPM.
My advice to anyone building a high output 10v motor is to ditch the stock sodium filled valves in favor of an aftermarket Inconnel or high temp alloy valve. New Sodium valves sell for over $35 each, so the price is really not an issue. Though these may cost up to $40 each, its affordable insurance to protect a $3k motor (if one is doing a performance rebuild of the motor). In my instance, it has cost me a junkyard long block and 7 months of downtime to get this new 20v motor built.
Stay tuned for updates on the 20vt motor, this will be an all out HP motor - o-ringed block w/copper headgasket, ported head, aggressive cams, lowered compression, 2.3l, etc. I'm shooting for 500 crank minimum, this motor should easily be able to do that.