It stands for Knock Retard. "Knock" is when there'* too much heat in the intake charge, and "detonation" occurs in the cylinder. "Detonation" is when the air/fuel mixture in the chamber ignites before the piston is ready, and can cause damage to the piston, valves, etc.
"Retard" is what happens when the PCM detects knock through the knock sensors, and pulls your timing and spark to stop the detonation. Although the PCM is pulling your timing and stopping the detonation, damage can still occur if too much happens. 7* or more of knock retard can start to damage components if done too much.
That'* why we recommend opening up the intake, and going with a FWI (Fender Well Intake) so that the intake is constantly pulling in cool air. That will help with KR. Even then, in order to go much smaller than a 3.4" pulley, you'll need to start going with higher ratio rocker arms, headers for the exhaust and a high flow cat before going any smaller.
SC is me spinach!
Black 2000 SSEi
All stock so far, except for an oversized idler pulley.