Grey smoke at high rpms

[Ender]

...technically, backfiring is ignition of fuel in the intake as opposed to the exhaust. Afterfire is what this is. It's fairly common, especially for RX-7's. Deceleration of the motor, even hitting the clutch and letting the revs drop only enough to shift, can let the motor to run rich enough to afterfire (if you want me to elaborate on why, I can). Since you said this mainly or even only happens when you really get on it it makes me think even more that it's normal afterfire (however afterfire can harm things. you could blow exhaust gaskets or in a horrible case maybe harm a turbo or engine internals if the afterfire is powerful enough with the ignition point close enough to the block*). If you could get video that would be great, but I would urge you to use a camera mount or passenger to record for you. Smoking a little during afterfire would not worry me, but if it's quite a bit of smoke then you may have degenerative internal issues that should be addressed. That said, I agree that it would be prudent to take a compression test.


*...damn, I just realized terrible afterfire might have been what did my old motor in. The apex seal groove was hogged out, the seal came loose during startup and locked the motor. The groove could have been smacked open by all the afterfire I was getting between the block and turbo...