Rod length? Piston speed?

Let me ask this, If you do not have a short deck block, would there still be any advantage to having a shorter rod than a 5.7 for a stroke in the the 3.00 to 3.25 range running in the 8000-10000 rpm band?

[Adger Smith]

Maybe.. Depends on the head port/design.

[Ron Finney]

I thought the prevailing thought behind the long rod is to keep the piston at top dead center longer giving the fuel time to burn completely and create maximum cylinder pressure. The burn time for the fuel remains relatively constant even though the RPM increases, that is why you have to advance the timing as well.

[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.

[outlaw]

This is an old thread but the site is quiet and I’m bored. Says when changing rod length you should change cam profile. If deck height was no problem and you took a well sorted out combination then added a .300 longer rod what would be the difference in cam specs. Cam guys just make up a starting cam for reference and a combination you think would be a good combination for starting point. Lots of super smart people on this site suited for the question.
Terry