Bruce,
I don't have a cooling system yet, so I have always run the engine for short periods (-5min) I have noticed, as it's inside my shop, it puts out a lot of smoke, grey stuff. I figured it will clean up when hot and broken in.
Today, as I have an extra head plug, I decided to experiment. I chucked it up in the 4 jaw by the wrench flats, and cut .050 off the seating shoulder. I installed it and started it up and loaded it. The exhaust was now clear. Ambient Temp -5C. The shop was smoke free.
I monitor the plug and when I put my fingers on it and don't want to keep em there, I shut it off. It seemed the plug got hotter more quickly. This is a good thing.
I have no idea what the compression ratio is, someone with more brains will be able to figure it out, but I am going to break in the motor and then experiment in cutting til I get a Knock.
Gerry