my buddy has a 98 gst. its all stock with 70,000 miles. when he lets the motor rev up to and over 4500 smoke comes out of the exhaust. i was thinking 1 of 3 things. bad headgasket, bad valve seals, or bad piston rings. my plan is to do a compression test then pull the heads off and check the head gasket, check the back side of the valves for oil buldup or residue, and check the cylinders for scratches. if there is oil on the back of the valve and not just on hte face then the seals are bad, if there is scratches on the cylinder walls then its bad rings, by looking at the head gasket will tell me if it needs replacing. any of you guys have any other suggestions or tips.this is just what i came up with i know if there is something im missing you guys can help.thanks.