Ok so I have a jdm gsr that I turbo'd(stock other then arp headstuds)Just finished it and took it for a tune. Car was running good! Made 320 on 8psi. But then after some more pulls it made 240. Then made 320 again. Was going back and forth from what the tuner told me over the phone. Well neways car would smoke like a bitch after wot sometimes then it wouldnt smoke at all. All cylinders were fine except #4 made 75 psi. But other then smoking sometimes car made power and sounded normal! Wtf piston rings? Well any ideas its getting tore down this weekend....

 

Perform a cylinder leakage test. It will help pinpoint the problem, that way before tearing it down you already know the issue.