[ScottSmith]

My N20 blew up pretty spectacularly a couple months ago at Sonoma Raceway. I finally got it apart. It appears the #4 piston is no longer there. Connecting rod and wrist pin are still there, along with all the valves. There's quite a bit of damage to the head, and the piston shards took out the oil pump on the way out the bottom. Incredibly this did not send a rod through the block.

The engine "seemed" to be running fine until I lifted off the throttle at around 6900-7000 RPM. At that point the oil pan cracked, spilling oil and sending my car into a spin at 100mph. Luckily this happened at the beginning of a turn instead of the end, so I was able to bleed off a lot of speed before hitting the wall.

What kind of failure causes this?

1. Detonation?
2. Interference?
3. ?? something else?
4. All of the above?

A quick google search seems to imply this is an interference issue, though the chain seemed fine and the chain guides didn't look worn, and definitely weren't broken. I figure if it was piston to valve interference, then the valves would have popped off - but maybe not?

When I got detonation before, I would see cracked ring landings on two cylinders - not sure why it was always two. Here, it's one fully destroyed piston, and as far as I can tell, the others are fine. I should go remove the rings from the remaining pistons and see if any chunks fall out.

This event did happen after I retuned, but my tuning changed involved adding boost at low RPM - 3000-3750 RPM. I am up to 21psi in that range on a stock bottom end with ARP rod bolts. This failure occurred at 7000 RPM, though I suppose the damage could have started earlier?

I do bang off the rev limiter quite a bit, though it didn't happen on this lap.

Main bearings and rod bearings looked great. This is after a year of racing with an accusump.

Anything else I should look at?

[DG330]

Keen to know what you're running to cause something like this? HP? Stage, mods?

[ScottSmith]

Originally Posted by DG330

Keen to know what you're running to cause something like this? HP? Stage, mods?

(Pinky to lips) 2 HUNDRED horsepower! Really it's not that much. I wasn't even running that much torque (under 300 ft lbs).

It doesn't use a stock ECU (Link G4+), custom tuned. If the answer is "your timing sucks" so be it, at least I can fix that. Lambda target under 0.80. No mass air meter, ARP rod bolts instead of stock, catless downpipe, custom exhaust. Motor internals are stock, though the motor is a rebuild from previous detonation. I used the thinner head gasket but didn't have the block or head decked. Maybe I reused a piston I shouldn't have, and it finally let go?