5-6psi is all the guys run on non-intercooled M90 setups.
Some run up to 8psi, not sure how well it goes though.
All they use is a RRFPR too, no electronic wizardy.
With a decent piggyback though almost anything (within reason) is possible.
But I'd highly suggest going aftermarket ecu if you want more than what an M90 can offer.
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