The ignition value we can log with MUT4 is shifted left 8 times and then divided by 90 (decimal). 231 minus the result is stored as a value that is later prepared for ignition.

Knocksum is added to this value.

Therefore, we have knocksum*90/256 = 0.35 degrees of ignition retard per knocksum as people suspected. I can't see any other funny business.

The offset suggests that the spark firing is scheduled from 61 degrees BTDC. Notice when you log the MUT 06 with the engine off you get 61 degrees BTDC timing. All starting to hang together

 

So exactly what type of event triggers the low octane fuel and ignition tables?

 

Octane value interpolates between high and low. Subject to the coolant temperature being warmed up, load being sufficient and no faults, the octane value will increase when the knock sum is 0-3, stay the same at 4-5, decrease when 6 or higher.

So knock sum of >=6 starts you moving towards low octane.

 

I have noticed when a scan device is plugged in and the car not started you also get 61 degrees. Same on my permanent mounted Blitz R-VIT display.

FYI, from memory the actually crank trigger event is at 76* based on what almost all of the aftermarket ECU's use as their reference point for it. So they may be allowing 15* in which to allow time for timing calculations.

The big question is, does the system actually retard the timing in 0.35* chunks? I have always assumed it did. And is timing itself in the same increment. i.e. does the ecu interpolate between the 1* map resolution?



So from what the latest discoveries are saying, we all should be re-evaluating the term "knock sum" and change it to "knock retard" and use the correct timing values.

 

The calculation and lookup from the maps is to 1 degree accuracy. It gets multiplied up before the knocksum is added. So we only get the higher accuracy towards the end of the calculations. Depending on how near the final reported timing is to rounding to the next degree, this would explain why people feel that 1 degree being pulled is inconsistent between knocksums of 1,2,3.

There is a cam angle sensor pulse for every cylinder event (ie twice per engine rotation). Interrupts or compare match timers on the rising and falling edges coordinate the spark, fuel and knock measurement events. More digging to do obviously.