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.
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.