I have a 05 G35 sedan 6 my with the rev up engine....what would be the best way for performance with a $800 budget? Also is there a plenum spacer for a G35 rev up engine?

 

Nitrous oxide.

 

I gotta stock engine I'm not gonna blow my engine for nitrous

 

You can do a plenum spacer (not sure how much it helps on the rev up engines though) and Art Pipes for that if you install them yourself. But to get the most out of mods on these cars you need a tune.

Look in the performance forums here, lots of good information in there.