I’ve been searching for an answer to this question on the web for 3 hours. Thought I would find it quickly, but, noooooo. I’m trying to understand why crankshaft torque falls off at low RPM in normally aspirated piston engines. I think I understand the high end of the torque curve OK. At high RPM, torque drops off primarily from intake and exhaust flow restriction. Volumetric Efficiency declines because the engine can’t breathe any faster. Mechanical efficiency also declines at higher RPM, due to increased frictional resistance. This further reduces torque at the output. As RPM declines from high levels these limiting factors diminish, allowing torque to increase. Unfortunately, the description above doesn’t seem to explain or apply to the low end of the torque curve. As RPM declines below the rate where the engine has peak torque, intake and exhaust flow resistance as well as frictional resistance should continue to decrease. Also, with variable valve and ignition timing, it seems like it should be even easier at lower RPM to generate higher peak pressure in the cylinder and focus it on the optimal crank angle. These things seem to suggest that torque should continue to increase as RPM falls to near zero. Instead, torque and combustion efficiency both decline at lower RPM. Why? Any direction to a good reference that explains the basic science of low end torque drop off would be appreciated.