right, this is were im stuck, surely the more power comes from more air being forced into the engine, and as the size of the cylinders stay the same volume, the only way of having more air in them would be to increase the pressure it is under, ie if you have a 2.0 engine then at atmospheric you would have 2.0l of air (not quite 2 litres i know but close enough) regardless of how much air you can flow up to the inlet valves, the only way of actually forcing more air into the cylinder is by increasing the pressure??

sorry if this is a bit babbly,