Rod length? Piston speed?

[Marine One]

At TDC every piston has some dwell time where the crank is turning, but the piston is stationary.

In theory we want the piston in motion 100% of the time with no dwell at TDC because 1,2,3 deg of dead dwell time is 1,2,3 deg of lost power.

The longer the stroke, the longer the dwell time, the faster the piston speed and greater the piston velocity. With that understood, we find longer stroke engines have greater cylinder wear at top ring turn around.

Instead of a gentle transition at TDC, the piston rushes to the top, sits motionless for a few degrees, then rushes back down. Depending on ring design, some funky stuff can happen in that few degree time frame where the ring is flexing and eating away at the piston / cyl wall.

This is the same reason the effects of fine dust particles entering the combustion chamber show up sooner on longer stroke engines vs shorter stroke engines. Hope this helps.

[Marine One]

Just to clarify - "longer stroke" implies an increase in crankshaft stroke, an increase in rod length and a decrease in piston pin height (move the pin closer to the rings).

Dwell time, piston speed and piston velocity is increased with this combination of a true "long stroke"engine. Simply increasing the stroke of the crankshaft alone and leaving everything else the same will not change much of anything, including piston dwell at TDC. Hope this helps.