Is it legal in STX? I didn't see it under forced induction in the rule book.

 

Definitely not.
Nitrous is not allowed in any SCCA classes. That includes SoloI, SoloII, Pro Rally, Club Rally, Pro Racing or Club Racing.
And I don't believe individual regions are allowed to allow it for
"regional only" classes for insurance reasons.
There has been discussion to allow it in the past, but so far it hasn't been approved.

 

Thanks for the info