I always thought I knew the answer, but a recent discussion has me guessing now.

Regarding B or D series motors, or any 4 stroke motor...........

When someone says that ignition timing is at the stock 16 BTDC (Before Top Dead Center) at idle, are they saying that the spark is igniting when the piston is 16 degrees BTDC on the compression upstroke, or 16 degrees BTDC on the power downstroke. I would think the latter is 16 degrees ATDC (After Top Dead Center)

In other words, when does the spark ignite the mixture? On the upstroke BTDC or the downstroke ATDC?

I've always maintained that the spark fires right before the piston is at TDC on the upstroke, and by the time the explosion is complete, the piston has passed TDC. The other guy says that my explanation will cause the motor to be destroyed.

Who is the retard?