Is it common for ECU's to maintain individual limit maps for each knock sensor?

Question

Preamble

I'm in the process of interpreting my BMW M5's ECU binary with the aid of some nifty scripting and rather informative forum posts.

One of the things I've found is that there are knock limit maps for each cylinder.

While this is somewhat expected since the car has 8 knock sensors, the thing that intrigues me is that each map is slightly different. I would have expected to see 8 identical maps, assuming that each cylinder endures identical conditions (limits are based on RPM and relative fill).

I'm trying to understand if there is a physical reason for this. There are a few sources of inter-cylinder variation:

• fuel rail pressure, though I don't expect a huge variation between individual fuel injectors
• intake air pressure, bearing in mind that this vehicle has individual throttle bodies for each cylinder

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Questions

• Is it common to have individual maps for each knock sensor on ECU's? I'm curious to know how other manufacturers do this as well

• What inter-cylinder variations could be significant enough to warrant a unique map for each knock sensor?

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Knock maps _{(for the inquiring minds)}

Answer 1

Yes, it is quite common. I initially said that it isn't, but I was wrong. I had another look at my Subaru's mapping and saw that there are more ignition timing tables than I first thought. Here's a complete list of tables (note the greyed out ones):

Factors that could warrant separate knock maps for each cylinder would include the following:

1. differences in ability to hold pressure (think of a leakdown test).
2. different heat profiles between the cylinders.
3. inability of the fuel injectors to keep up with demand under Open Loop conditions.

Most manufacturers of sports cars will have one table each that manages FLKC*, FBKC* and IAM* and will apply the safest correction value across all cylinders. In fact, only Cylinder #1 is actively monitored for knock and there is only one acoustic knock detector (a microphone basically) for all cylinders.

Fine-learning knock control (long term stored values)

Feedback knock correction (immediate response to knock event. Normally under Open-loop)

Ignition Advance Multiplier (the more this deviates from normal, the worse the state of your engine WRT knocking).

Answer 2

After consultation with a BMW forum, an expert in MS S52 reverse-engineering had this to say:

As for why BMW has multiple knock maps - it's probably just because they could. Their engineers probably also experimentally determined what the signals look like when the engine is knocking.