As some of you may know my car died at Combe yesterday. There's a video on YouTube now for anyone interested.
I had the head off today to reveal a melted #2 piston at the inlet edge. Now I had the car dyno'd on Saturday where the fueling and ignition were setup to get some more out of the engine. It ran perfectly on Saturday and put down 165hp on a standard engine with knackered bores (a healthy figure I thought).
I'm not very experienced with this kind of failure so looking at the pictures, could pre-det' or burning too lean cause this? I can't help but think it's the heat from lean-burning that caused it, although all other pistons are fine.
I was told the C.O was showing high 4% above 5000rpm and the timing was set to 15deg static (maxing at around 36).
I had an Imprezza that ran lean and it melted one of the pistons just like that...mine was caused by a leaking head gasket leaking to the outside where there wasn't a water gallery. Not the same as a sprint I know, but the principle is the same I guess.....
Come and see some pretty shoddy, slow driving of a really well prepared competition Sprint here!
Detonation on Sprint pistons usually causes ring land failure on the thrust side....so the piston looks ok from the top. but you find out by poking a feeler gauge
down the side. The sooty plug is the giveaway which one to look at. Yours is a bit more obvious.
Haven't seen one melted like this before.
Jonners
Note from Admin: sadly Jon passed away in February 2018 but his humour and wealth of knowledge will be fondly remembered by all. RIP Jonners.
I think it ran lean on that cylinder and when the power was on it got the hottest. CO readings are just an average of the total so can be misleading. Getting individual lambda readings from each ex port would be ideal but of course tricky.
I also saw an impreza do this. The guy melted a piston, had another engine fitted by a subaru "specialist" and that one melted a piston on the same cylinder even after mapping. I stripped and rebuilt that engine and also stripped the inlet manifold (often overlooked) The No3 injector was blocked with rust! The front of the car had been underwater in its life and water got into the fuel rail and rusted it.
In short even though it was mapped No3 was getting very little fuel and under boost where it needed it most it melted the piston.
I'm still unsure of the cause of the failure but the mixture at 4.8% C.O or 12.6:1 lambda shouldn't have been lean enough to cause it in my opinion. To me it looks like its got too hot and melted and all that can really cause that is a lean mix...
i did speak with the knowledgeable Mr. Osbourne earlier and mentioned my piston trouble to him. He asked what fuel I was using which made me remember i had 97 octane in for the dyno runs and 95 on the track day. Perhaps it actually was caused by detonation? Although nothing out of the ordinary was audible. Stripping the engine revealed deeply grooved bores, a broken ring in 3 and another broken ring and cracked ringland in 4.
The other thing is that the engine lost almost half its water on the day of the failure. On inspection of the headgasket there were no faults but the head is clearly warped because having fitted it to the replacement block water dripped out the head face. I can't help but think the failures are related? Could detonation cause enough heat to warp the head?
What star rated petrol was the Sprint spec'ed for? 95 RON being only 3 Star, is that safe even at std advance (unless one drives like a nun)?
Having lost a set of pistons to using premium unleaded on standard advance - bust the lands out -, I don't ever use less than 97 RON super, and prefer to use Tesco's 99 or shell's 100 RON super supers.
I don't remember which mark of 44D4 dizzy I had in it at the time, A, B, or C, and I'm told that makes a difference to whether you can use 95 RON safely. But I was also told to retard the ignition a few degrees as well.
Graham
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.
Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).
we live and learn and I have done my share of engine killing in the 55 years I have been a petrol head 
