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So even if you go to the longer rod your really not gaining any piston speed or loosing any piston speed.
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Well.....yes and no.
Over all -the piston has to move from TDC to BDC in 1 -360degree rotation of the crank right? No matter what the rod length or stroke length is.
1 crank revolution = 1 stroke cycle.
BUT...it's the length of that rod that determines how quick it's going at what part of that revolution/cycle/stroke.
A Short Rod is slower moving to and past BDC and faster at the TDC range.
A Long Rod is faster at BDC range and slower moving at TDC range.
Look at the 2 files attached.
The first shows 2 rod/piston combos. Both should have the exact same compression due to a piston with a different compression height making up the difference of the short & long rod choice.
They're identical overall right?
The second picture shows the exact same piston/rod combos -90 degrees past BDC.
They are at
identical locations of the crank revolution/ cycle/stroke.
But...the long rod combo has moved the piston up further in the cylinder.
Why?
They will both reach TDC at the exact same time. But the long rod combo has moved the piston up further...faster...in the stroke cycle.
That...is a part of piston speed that the formula you shared doesn't take into account....The speed of the piston at different parts of the cycle.
So- overall average speed is the same for both. But the longer rod accelerates faster where you need it.
It's believed to increase cylinder pressure, reduce exhaust escaping back into the valve area during
the power stroke...it generally does more of what you want when your valves are opened...and when they are closed.
(a side note: Look at that rod angle!!! The short rod combo has a lot more force being exerted on the cylinder side walls...more force, more friction, more pin wear, more rod fatigue etc.)
-now I'm dizzy
