Originally posted by hobieboy
dumb question - how does one tell knock sets in?
You usually hear weird sound from engine compartment that sounds like a bunch of BB's rolling around in a barell. It's pre-ignition in it's mild form. If you hear it, back on ignition timing / enrichen the fuel / lower the boost. Engine will hopefully survive if it catched early. It's audible enough to be detected by ear from inside of the car.
If you are very off-base on turbocharged engine (wastegate ruptured, very lean, fuel depletion etc.) it will sound like somebody took a hammer and started knocking on the engine. It's usually an expensive sound. Compression rings will often crack. Prolonged knocking of this magnitude ("prolonged" can be as short as 5-10 seconds) might end ut with holed piston. 911 pistons will mostly develop holes on it's edges, seldom in middle.