Had a compresion check completed on my 05 GT, bought the car used last month have found out since that the previous owner had ran NOS. Compresion check revealed that compresion good on all cylinders except for cylinder #2 which was showing an 8 psi drop. Asking for opinions, should this be a big concern yet? It does smoke a little on start up but seems to clear out with no noticable oil consumption. CATs have been gutted, if I buy new ones will it affect the compresion issue?

 

8 psi should be within spec. If all cylinders were 160 and #2 was 152, that's less than 10% difference, which is fine.

Adding cats isn't going to make a difference. If you don't have an oil catch can on the PCV valve, then it will smoke on start up. Oil collects in the intake from the crankcase vent and gets sucked in and burnt.

 

+1

The S197 will tend to smoke a bit more if you start it after another previous cold start with a very short run time, such as starting it to back it out of the garage to wash it and then starting it a few hours later, for the same reason moosetang stated. The oil just sits there until it can pulled in burn off.