Originally Posted by t-ray

First, in naturally aspirated form, the car runs too rich. There's a bit more power left on the table just by leaning the car out a bit. I've seen HR's run afr's in the 11's in the heat of the summer, bone stock. I know my wife's car does that(07 g35 with the hr). The factory ecu probably does that to manage heat - fuel cools the combustion chamber.

And because fuel keeps internal temps down, it can help prevent premature detonation. And that's the biggest fear when running forced induction - compressed air is much hotter than air at ambient pressure. If the combustion chamber becomes too hot, the air/fuel mixture in the chamber can combust prematurely - which can cause all kinds of nasty things.