[olddog]

This is off subject a bit, but the 4 valve per cylinder modular engines achieve exceptionally flat torque curve, especially on the low rpm just above idle. They did this by only using one intake port and valve until the rpms were up enough to keep the port velocity high with both valves.

I only point this out as, one of many, short comings of two valve per cylinder head designs.

You need air volume to make torque. You need velocity if you are trying to pack the air into the cylinder at or past BDC. It is difficult to get low rpm port velocity and not end up with the small port restricting the volume at high rpm. To make power at high rpm you have to give up low rpm torque.

Big truck engines make moderate torque with large ports and small duration cams. They do not have good port velocity, and they are out of breath by 3000 rpm usually. In the small range they operate in, they have a relatively flat torque curve. You have to go to a diesel to get a flatter low end torque.