Originally posted by Don Shillady

There the advantage seemed to go to the longer rod and there is dyno data. Of more interest to me as a real puzzle is how the dwell time can be different for TDC and BDC as shown in the Isky site above. How can this be?
Don Shillady
Retired Scientist/teen rodder
Hi Don,

Piston motion is what changes with rod length and stroke variations. Peak piston velocity is the easiest way to explain what you want to know. To accurately calculate piston velocity you need to meld together a little trig and a little calculus and you can generate these neat graphs that let you see the difference in "dwell time" for TDC and BDC.

Without going through all that I will try to give you a visualization of what happens.
The piston will reach peak velocity when the connecting rod centerline is approximately at a 90-degree angle to the rod throw. In other words, a line that runs from the center of the piston pin to the center of the rod journal is at right angles to a line that runs from the centerline of the main to the centerline of the rod journal. In most engines this happens around 74 crankshaft degrees after TDC. From that point, the piston is slowing down until it stops at BDC. So, 74 degrees to peak velocity and 106 degrees till it finally stops at BDC is: 74 +106 = 180. You can imagine what a graphed curve of those numbers would look like.

Now, all you have to do is mirror that curve and you will see what it looks like going from BDC to TDC. Coming off of BDC it take 106 degrees for the piston to reach its peak velocity and only 74 degrees to finally stop at TDC. There you have it, the piston moves much slower across BDC that it does across TDC.

Clear as mud, right?

Hope this helps.