I haven't done a compression test on my 83' turbo yet (haven't needed to), but I was wondering if the lower compression ratio would affect the pressure readings. The turbo engine is only 7.5:1 compression. The car has 193k miles and runs and drives well, but sometimes (usually when cold) blows a bluish\black plume of smoke from the exhaust when shifting (as soon as throttle is closed). Im pretty sure this is the turbo wearing out, and im not too worried beacuse the cat is gutted (oil cant damage plain metal). Will a compression check help deternime if this is worn valve guides or a turbo seal leak? I believe the turbo has around 50-80k miles, and it is non watercooled. This symptom is much less apparent (if ever) when using 20w 50 synthetic oil, instead of 10w 30.

(car runs and drives well, and still accelarates hard under boost)

(sometimes when in high vacuum the same smoke plume will appear, such as coasting down with throttle closed)

Thanks for any tips or advice,

Zack