is typically close to the knock point because. the system relies on the knock sensor as a ... Knock Sensors... Ideal conditions would have the spark occur when the piston reaches top dead center (TDC). The flame front would travel across the combustion chamber for a complete burn of the air-fuel mixture just as the piston returns down the cylinder. Realistically, the air-fuel mixture takes longer to burn. The difference is a fraction of a second and must be compensated for