Disclaimer; Attached are
MY quick random thoughts and my synopsis is believed to be +99% accurate. You are welcome to disagree and debate.
HP is a calculated number; (RPM * Torque)/5252=HP
Clearly, as rpm increases, so does HP, assuming torque is constant. If torque is decreased, a HP number can still be achieved with increased RPM. Now, for me and my street car, torque it where it’s at IMO.
Here is a synopsis;
Let’s say you have a de-stroked engine, and the head FLOW is the limitation (Think RPM/stock untouched heads). The performance loss on a de-stroked engine can’t be made up with RPM. The performance can be increased with boost, but like everything else, there is a break-even point. (Not sure but probably in the high 50 PSI range with the right fuel)
If these stock untouched heads are operated by a high rpm cam (I am sure nobody would do that right?
), a short stroke (Less CID) may be a better match because the smaller cylinder will fill better. Maybe not to 6K, but you get the idea. In other words; the smaller CID runs more efficient, with the same sh** heads, and can therefore be run at higher rpm.
We understand that torque is a function of “force x lever”; Shorter stroke, shorter lever. Force is result of the pressure applied to top of the piston. (Lb/in^2 * in^2 = Lb)
Clearly, a 4” bore has higher total force for a given combustion pressure over a 3.8” bore. The 4” bore also appears to have increased mass flow, not because of valve size, but un-shrouding. Obviously, the increased CR for the same chamber is a plus as well. (Look up the thread Boost vs Back pressure)
A linear calculation model for simplistic approach comparison;
3.8” vs 4.0” bore piston area is 45.36 vs 50.27 in^2. At the same stroke length; (50.27 - 45.36 in^2) = 4.91 in^2. This is ~11% increase in torque, or ~12.5% HP increase.
A 3.40 vs 3.55 stroke will result in 4.4% increase in torque, resulting in ~5% HP increase.
So do you increase area (Bore) or lever (stroke)? I think bore wins every time, up to the break-even point.
Obviously, some cars will be quicker with de-stroked engine but the contributing factors are not limited to the CID or RPM. It is entirely possible that the converter now slips less at the increased rpm/lower torque, or the cam and ports are now a better match.
Obviously, it’s complicated, and one size never fits all . . . . .