Wednesday, November 12, 2008

Brittle Pistons on 2008 STI?

We've heard of an alarming number of 2008 STI cars that suffered piston ring land failures. Once the ring lands break, the engine loses power. Repair involves removing the engine and rebuilding it. Many of these failures were on completely unmodified cars. Subaru issued a recall on the car to reprogram the ECU in an attempt to remedy this problem, stating:
During a quality review, we discovered that under certain harsh driving patterns, specifically when repeatedly or continuously operating the engine above the “red-line” (RPM above 6,700), abnormal combustion could occur resulting in serious internal engine damage in the form of broken piston ring lands. This abnormal combustion condition is the result of an “over lean” fuel mixture caused by a combination of fuel cut activation and high turbo boost.
I'm not convinced the problem is ECU-related. Most of the failed pistons show no signs of detonation, although I admit that doesn't prove it's NOT detonation that caused the failure. Various theories have been advanced on the Subaru message forum communities, including:
  • Crankcase oil is contributing to detonation which results in piston failure
  • New materials used for the piston castings are more brittle than older ones
  • Solid (not sodium-cooled) exhaust valves create in-cylinder hot spots which lead to detonation.
Personally, I'm leaning toward the "brittle casting" theory. It just seems like the pistons are more fragile than in the older Subaru 2.5 liter engines. Given the general trend towards cost-reduction, it would not surprise me if Subaru changed suppliers and that the new supplier used a slightly less sturdy metal formulation for the pistons. The bad news for us if the problem is caused by the actual piston construction, is that we will not be able to fix the issue just by re-tuning the car, or by any bolt-on modification. Only an engine teardown and piston replacement would solve the problem.

This problem hasn't surfaced on our shop STI (yet), but we're keeping our eyes and ears open.

Thanks to the guys at Turn-In Concepts for the picture above.