Why does torque increase with revs?

Torque doesn't just linearly increase with RPM, it's a curve. There are several reason why it is a curve, but the main one is efficiency. Think of your engine as an air pump. When torque is at it's peak, that is when the engine is most efficient at moving air. Now because of the size and shape of the intake and valves, and the timing of the cams, the engine can only be most efficient in a relatively narrow rpm range. So as the engine starts off at a low RPM, torque builds as it get's closer to it's highest efficiency, and then once it's past it's highest efficiency it starts to drop off again.

There's really a lot more to it than that, but that's the simplest I can think to make it at the moment... Time to hit the library, son!

Back in Ye Olden Days we called the pickup in torque near the midrange "getting on the cam" but we also thought it was "groovy" so I don't know what the young whippersnappers call it today. The term reflected one of the factors in the torque rise from low to mid rpms - the cam profile for all but the most sedate engines have some overlap, when the exhaust valves have not closed and the intake valves are already opening. That overlap is to improve flow at higher rpms, resulting in greater combustion pressure at the mid and upper rpm ranges. The combustion pressure is what produces torque, which is rotational force at the radius you mention. Torque is not a characteristic of the machine but a result of the pistons putting force on the crankshaft.

A lot of it has to do with fluid dynamics, and the engines ability to "breath" (valves, cam, heads, exhaust, and intake design). Torque isn't simply the length of the throw on the crank, but the pressure applied to it.Air (which is scientifically a fluid) has mass. Anything with mass takes a second or two to get moving. There fore, at low RPMs, the engine isn't breathing as efficiently. less air flow = less pressure = less torque.

You're forgetting the torque is measured at the rear wheels. The ring and pinion gear ratio can increase torque if you install a pinion gear with less teeth and a ring gear with more teeth. The drive shaft transfers the twisting motion from the transmission to the pinion gear. It (drive-shaft) doesn't have any thing to to with the engines ability to make torque or power through the drive-shaft to the pinion gear.

The compression, cylinder heads, induction system and camshaft regulates torque an horsepower.

’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.

Toque always increases with added speed until torque and horsepower curves peak. To prove it, hit a watermelon with your fist at a very slow speed and then back up and do it again as fast as you can punch with your fist, and see the difference. Or look at a ballistics table and check to see the power of a projectile as the speed increases. It's a common law of physics.

......but how do steam engines and electric motors have maximum torque at zero/low rps???