Recently I learned that a number of things such as the cam profile, compression ratio and the intake manifold runner length and volume can all effect the power curve of the engine.
Here is an example of one.
Intake manifold runner length is the linear distance from the inlet port (the face of the head) to a common point shared by all cylinders. Depending on the throttle position this could be the open atmosphere (if each cylinder has its own throttle butterfly) or a plenum (if they share a throttle body).
The intake runner volume is the volume of that section of the inlet system, ie the cross sectional area multiplied by the length.
The very short answer to the final question is that short, wide inlet runners move the torque curve up the rev range whereas long, narrow runners move the curve down the rev range. BUT - there is a whole lot more to it than that. The effect is caused by 'pulse waves', waves of relatively high and low pressure in the runner.
The opening of an inlet valve causes low pressure at the engine end of the runner, as the engine sucks air out of the runner. Air starts to flow down the runner into the cylinder until the valve shuts, at which point all that air (travelling at high speed) crashes into the shut valve and creates a relatively high pressure 'slug' of air. This is reflected and starts to move back up the runner until the valve opens again, at which point it heads back towards the port.
If the dimensions of the runner are calculated well, the high pressure 'slug' or 'pressure wave' will make its way through the port before the valve shuts again, when the next 'slug' is created. The dimensions are critical to the operating speed of the engine. If the runner is short, the reflected pulse might fall out of the end before the valve opens to suck it back in. If the runner is too long, the larger mass of air may react too slowly to perform well at high engine speeds.
This is the reason car manufacturers have invented variable or twin length inlet manifolds, which change the dimensions of the inlet runners at a certain RPM to give improved torque across the engine's rev range.
There is an excellent feature written by Dave Walker, an engine tuning guru from the UK. He performs A-B-A tests comparing inlet runner lengths on a 1600cc engine in an amateur racing car.
...and here is an interesting print out of torque curves, comparing a 40mm length with a (ridiculous, but interesting) 330mm length.
The short inlet runners...
...and the really long ones.
