You may have read of the dangers of knock to your engine, and to your car's performance. While it's hard to quantify just how dangerous knock is, it's generally agreed that over 5º knock retard (KR) is the time to start letting off the throttle.
The PCM engages KR very quickly; the result is knock is usually not heard. I've personally observed spikes into the 12º range without hearing a thing. But most of us think we can feel
the difference, right? It's true, retarding the ignition timing results in loss of power. Somebody somewhere once said that for each degree of timing pulled we lose approx 2hp. If you're knocking 10º, that's 20 hp, surely a noticeable difference. But what about 5º or 2º KR? Can you really feel that?
I did a couple of WOT test runs last week. Both were under nearly identical conditions: same stretch of road, both launched from a stop, both days were dry and 75ºF, same 94 octane, same mods, same 3.4" SC pulley. The only difference was the tune. One tune didn't command enough fuel (moderate KR), the other tune came pretty close to optimal (minimal KR). Here is a graph of the two runs:
Looking at the curves, you can see vehicle speed and KR for both runs. Pink (mph) and red (KR) correspond to the "bad tune" run, while the teal (mph) and blue (KR) reflect the "good tune" run. Notice KR for the good run is under 1º for the most part; KR for the bad run is a near sustained 2º with spikes close to 5º and 7º.
Did the car feel any slower? Not really. Was KR slowing the car down? Probably, but only a tiny bit, as can be seen from the two mph curves (teal is good run, pink is bad). One can argue I had a better launch on the good run, so if I'd launched as well even with 5-7º KR, it's likely the two runs would be very close to a tie. The performance difference would cause small differences in track times, or could change the outcome of a race by a nose, but I don't believe I'm losing tenths in the 1/4 mile from moderate levels of KR.
I think these results are interesting, because I used to think that if my engine was knocking 7º, I could expect a sag in power that would slow me down enough to feel it in the seat. If you look closely at the pink mph curve, it does show up barely (teal line is slightly straighter), but just barely... you would never notice this otherwise. A change in head wind could cause a bigger loss in velocity than what we're seeing here. Severe KR levels in the 15º range are another matter. I believe knocking to this degree could be felt, but things should never be allowed to get to that point.
What did I learn? Basically that KR should be minimized more for engine safety reasons than for performance reasons. Knock is still a bad thing, but all else being equal, you must have a lot of it to slow you down. Other things, such as launch technique, wheel traction, even a 10ºF difference in ambient temp seem to be the more important factors affecting the car's acceleration performance.