[davidimurray]

Generally - you cannot make extra power/torque by tuning runner lengths, but you can move the position of the peak on the performance curves. The theory is that as the valve closes a reflected wave is produced back up the intake. When this wave meets a large volume change it is again reflected. If you can time this reflected wave with the normal pressure wave generated by the inlet stroke and the addition of the two provides an extra charge of air into the system - hence a peak. The same applies to exhaust system. If you look for Helmholtz or organ pipe theory you can do the quite simple maths behind it. Generally, the longer the runners, the lower down the RPM range your peak will be,

To get more power you need to increase the volumetric efficiency (VE) of your engine. Hopefully I'm not teaching people to suck eggs, but the VE is the relationship between the cylinder volume and the actual amount of fuel/air mix you can get in there. You can only burn as much fuel as you have air for, so you need to get more air in the engine. You can do this with a turbo and have a VE greater than 1. You can also improve the efficiency of the your intake system so that their are less frictional losses for the air coming in - in its simplest form porting, but the same principal should apply to your intake system as well.

Now of course when you really get into it, the two affects above actually combine. As your losses are based around the air velocity travelling through the system, but you have to size your diameter for a particular velocity. So in an ideal world you would have variable length and diameter intake runners!!