The timing gets retarded because of knock. Depending on the algorithm used the PCM will continued to pull timing until knock stops, and it then may slowly feed advance back in if it detects no knock for a certain amount of revolutions. Also if there's heavy knock it can make long term timing changes where it won't run as much total advance to start with, but you wouldn't see that unless you also monitor what the timing advance is doing in addition to knock retard.
How much depends on the severity. A few degrees may be normal depending on the fuel you run and operating conditions (hot weather, etc.). Double-digit retard and generally bad things are going on in the combustion chamber. But I'd bet some of the big turbo guys trying to use a piggyback will also see some larger knock retard numbers as well when the PCM is trying to run a ton of advance but the fuel can't support it.
|
