12-12-2021, 10:56 PM | #1 |
Private First Class
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Shattered piston / engine failure
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? |
12-13-2021, 07:41 PM | #3 | |
Private First Class
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Quote:
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? |
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