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what factors will decide at what rpm an engine develops max torque?
Valve timing, intake length, valve curtin area/displacement, and port size. Experiments in the 1950's determined you could manipulate the torque peak (lower it) by increasing the length of the intake tract out to 33 inches, with the trade off of sharply lower high speed torque. Every intake length has an rpm (frequency) where a positive pulse returning to the intake valve will boost VE. This is the principle behind multiple length intakes.
If you want to understand torque, focus on the fact that torque is proportional to displacement. Regardless of the design or parts, torque output is limited to around 80 ft/lbs per liter. The only way to exceed this is to pressurize the intake.
Much of engine design history is distorted by artifical displacement limits and tax laws. We were racing 15 liter engines until the 3000 rpm limit was broken in 1912. Speeds rose so fast, displacement limits, first 5 liters, then 3 liters, then 2 liters, then 1.5 liters, all in the span of ten years to keep the drivers alive. The small displacement high reving engine for a road car is far from ideal. The ideal engine for a 4 person road car looks something like a 5 liter V-8 with variable cylinder deactivation, +35% EGR and a turbocharger. Two liter economy and 8 liter torque.
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Paul
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