So, I was chasing down a misfire from the last autocross when I encountered this chunk of aluminum on top of my carburetor body (inside the K&N filter area). I thought I had a dead cylinder due to a clogged idle circuit but it appears that it is a bit more serious.
I think this is part of the head where the valve guide is fitted. These heads only have about 10K miles on them since the complete valve job. If this is what I think it is, what do you suppose happened?
Here's my theory: This course had several sections where the rev-limiter kicked in. It's the factory "spring in rotor" limiter but it seems fairly accurate according to the original tach. Also, the afternoon session saw oil temps near 250*F. Both of these scenarios have occurred before with no ill-effects. So, perhaps high temps (thermal expansion) combined with sustained rpm-limit operation (and possible slight over-rev) allowed piston to valve contact? Maybe this stressed the guide and broke the head casting? Bent valve = dead cylinder. Or, is it possible that this is metal fatigue?
I should note that the engine is a "2.5L" with 911S 74mm crank and rods with 85mm JE "2.2S" pistons. Rev limit is set to 7100rpm and heads are stock 2.2E heads with no porting.
Obviously a tear down is in order when I have time. Until then, she's sadly been pushed back into the garage and covered.
