There is a track car 02 sti getting around on here with 240kw+ sitting at around 200k too that's still hauling ass every time it goes out. Who knows when yours will pop, but I wouldn't bother rebuilding anything until it does. To that effect, I wouldn't bother rebuilding it when it does either. I'd just budget for a shiny new 207 short block and a refresh on the heads if required.

Turbo will almost certainly need to be bigger for 300whp. On 98 I would look towards a blouch 1.5 or td0518/20g level turbo. E85 would help...

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MY08 WRX 325hp/456nm/18psi E85