Well I know the basic stuff, even I'm not the hi-fi engineer but this is the concept: Suppose you are blowing air through a straw. The air will come out from the other end with greater velocity in a thin straw. If you blow through a hollow tube/pipe then the air coming out at the other end will be much slower.
Same applies to engines. A 2V engine with a single intake valve will have a smaller area than 2 intake valves of a 4V engine. So higher intake flow velocity for a 2V. A 2V engine has around 1/3rd combustion chamber head area covered by the valves, but a 4V head covers much more area, hence smoother and quicker breathing. A 4V design also has a cleaner and more effective combustion (not very clear why) and this coupled with lower flow velocity of the AF are probable reasons for better FE.
All this stuff was in Fluid Mechanics and I hated this subject. So, if you find something horribly wrong with the above, pls excuse. Although I'm reasonably sure I've got it right.
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