355hp RWHP and 400lbs of torque from a 3.4 conversion
"The RUF 3.4 liter Boxster S conversion will yield 310hp and has the same reliability and maintenance intervals as a RUF 3400S and 996 Carrera."
"The RUF 3.6 liter Boxster S conversion comes in two versions. The standard 3.6 liter engine will yield 330hp. The "X51" version of the 3.6 liter engine will yield 355hp and uses a GT-3 ECU instead of a 996 Carrera ECU. Each has the same reliability and maintenance intervals as a RUF 3600S and 996 Carrera."
"The RUF 3.8 liter Boxster S conversion will yield 365hp and has the same reliability and maintenance intervals as a RUF 3800S and 997 Carrera."
You can attain 500hp (not sure if this is RWHP or not) but I believe that is only with a very beefed up TT setup.
Threaded Mode
