Ignition Timing

The (p,v) diagram opposite shows the point in the compression cycle at which the spark was fired. This
point is before the piston has reached top dead center
(tdc) and it allows the combustion event to be well
underway before tdc is reached. This gives a maximum cylinder pressure (and work output). This ignition timingcondition is about 20 degrees of crank rotation before tdc, i.e. 20 degrees btdc.

From, Reynolds & Perkins, McGraw Hill (1977)

If ignition is delayed until tdc, the pressure rise due to combustion will be counteracted by the pressure reduction due to the downward motion of the piston after tdc. Less work can be done under this condition. Delaying the ignition 20 degrees after tdc will result in an even smaller maximum cylinder pressure and less output work.