Ignition timing and knock, explained with a simulator

The spark plug fires before the piston reaches the top. How far before, measured in degrees of crankshaft rotation, is ignition timing. Fire it earlier — "more advance" — and the burn is further along when the piston starts down, so more of the pressure does useful work. Up to a point.

There is an optimum, and then there is a limit

Past a certain advance, the burn is fighting the piston on its way up, and power falls. That point is called MBT — minimum advance for best torque — and it is the most timing the engine can use. It is a property of the engine's geometry and the mixture, not of the fuel.

Separately, there is the most timing the engine can take without knocking. That one depends heavily on the fuel, on compression, on charge temperature, and on boost. On a modern turbocharged engine on pump fuel, the knock limit usually arrives before MBT. That is why octane matters: better fuel moves the knock limit up, toward the timing the engine could actually use.

What knock is

The mixture is supposed to burn from the spark outward, in a controlled front. Knock is when the last of the mixture, squeezed and heated by that front, ignites on its own — all at once. The pressure spike rings the block like a bell (you can hear it) and hammers the piston crown. A little, occasionally, the engine tolerates and the ECU corrects for. A lot, sustained, breaks ring lands and melts pistons.

The shape of the risk

Knock does not arrive gradually. It is close to nothing until the limit, then climbs steeply. Two degrees over the limit is not twice as bad as one degree over; it is several times as bad. This is why a calibration is written with margin, and why "just add a couple of degrees" is the most expensive sentence in tuning.

See it

Pick the EA888 in the simulator and leave everything at stock. Now advance the timing one notch at a time. For the first few notches, power creeps up — you are approaching MBT. Then the knock gauge wakes up, and after that every notch costs more knock than the last while power stops improving and starts falling. Switch the fuel to 93 and watch the whole thing move to the right: the same timing that knocked on 91 is clean. That is what octane is for.

Then try 87. The stock calibration itself is now over the line. That is what happens when a file written for one fuel meets another.

What a tuner does with this

Finds MBT, finds the knock limit on your fuel, and stays a safe distance below whichever comes first. The distance is the margin, and the datalog is how they check it is still there.