,87 A/R on the exhaust side.......dyno i saw showed 560 @ 20PSI.....though that was using a 90 MM piston. If you have a built motor you can run 28 PSI of boost on these turbos...something you CANT do on the 600's or the 535 to ANY degree of safety!!!!!!! The 87 housing will add lag....im bumping up the compression to make up for it but sounds like your motor is already together so...that may not be an option. Anyway...the extra housing was told to me by Mike Kojima, Jim Wolf, Clark Steppler and Mike Smith to be a 80 RWHP increase over the .64 housings stock (2530's)!