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#13: 02-08-2010, 12:26 AM
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Originally Posted by JoeFromPA View Post
And, secondly, are the stock pistons really that much of a problem or is it simply that the stock tune is very questionable & alot of tunes also have dangerous running conditions, leading to the illusion that the stock pistons are weak for both stock and tuned cars. Get me?
I get you, and I think you're on the right track.

On one hand, the stock pistons aren't great. In spite of our ECU's knock mitigation people still manage to kill stock pistons fairly often. Busted ring lands happen for other makes too but they seem to happen to Subarus more often. I think it was Boostin who mentioned that in other niches, people just listen for audible knock, and that approach works well enough. I think there's a reason that nobody tunes Subarus that way. Not successfully anyhow.

On the other hand, there are plenty of Subarus out there with crap tunes. I don't know exactly what models/years of factory tunes have this problem but I do know that it's been well documented in 07 STIs. They stay at 14.7 AFR way into boost, before switching to richer AFR. I don't think it's a coincidence that a lot of people at NASIOC believe that the pistons got weaker around the 07 model year, since the failure rate went up. However other people claim that there's no difference in the pistons, and I believe those people. We know the STI tunes changed for the worse, I don't see why people want to speculate that the pistons did too.

It's been seen in other models/years besides the 07 STI, I haven't paid close attention to which. I have not seen evidence that any LGT models are affected by that particular problem, though.

I tried to help a guy at NASIOC clean up what I thought was a simple problem with an 02 Cobb WRX tune, not too long ago, and eventually decided that the timing was just so messed up it would be better to start over from scratch. For starters, the car was pulling 2.3 g/rev and the timing table only extended to 2.1. I don't believe that all Cobb tunes are bad, since I've only seen one up close, but that one wasn't good.

I've also had random people send me logs enough times to know that there are people out there calling themselves tuners who have no business tuning.

I don't think there's any question that bad tunes - tunes that knock a lot - will kill pistons. So when people (usually at NASIOC) post up about their dead pistons, I like to ask what they were doing to monitor knock. In all but one case, the answer was some variation of "nothing, I just trust my tuner."

My guess is that if the tune knocks very little, the stock pistons will last a very long time. I'm hoping to prove that theory by running 400whp, logging a lot, knocking very little, and keeping my stock pistons for another 100k miles.

I'm also ready to buy a new motor though, just in case.
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