My cousin picked up an LS motor from a friend of his. Motor ran real strong, no codes, etc. But the guy is kind of a tool.
After installing the motor, we check timing to find out it was at +23! WTF. He was alos running a P06 ecu on B18B and raced it plenty of times that way.
What can break from timing this far advanced? and running the incorrect ECU for so long? Any mechanical issues that would probably cause low compression readings? Bent valves? etc. Help.

 

Too much advance would cause a condition where the fuel is ignited while the piston is still too far down on it's upstroke travel. Could harm the bottom end.
I don't know if 23 degrees is really bad, or just kind of bad.

If the "tool" raced it, it's been abused. Low compression causes can be worn cylinders, bad/broken rings, bent valves, blown headgasket, holes in piston, etc. The comparative compression between cylinders shouldd be fairly even. As you may know, there are various ways to do compression tests to rule out certain possibilities.