[Chris Gamlin]

Sorry e_peña, I missed this post before.
What RWC says is correct, you get much better combustion if the fuel is allowed to vapourise and mix with the air before it it ignited - petrol vapour burns much better than petrol as a liquid. Direct injection is being developed in road petrol engines (as James mentions, the Mitsubishi GDi engine is one) but i think it's main advantages are in emmissions and economy rather than increased performance.

The top of the inlet "trumpets" themselves (the curvy bits) are just there mainly to provide a smooth "laminar" flow of air into the throttle body (the "carburettor" bit of a fuel injected engine), which then feeds into the inlet manifold. Inlet manifold length and shape can be designed to tune the engine for high torque or high power, depending on what is required most. The inlet manifold design determines how well the air/fuel mixes at certain RPMs, and how fast it travels into the combustion chamber, hence affecting performance. A long inlet manifold with a rough internal wall texture will generally give high torque at low RPM, as this configuration allows the fuel/air to mix very well, which is good for torque. However, this can limit high RPM power as the mixture cannot flow quickly enough along the manifold to satisfy the needs of the engine at higher speeds. A shorter, polished manifold will not mix the fuel/air as well at low RPM, but will allow more fuel/air into the combustion chamber, as there is less resistance on the fuel/air travelling down the manifold and it has less distance to travel. This helps produce more power, especially at high RPM, as more fuel/air gets to the cylinders. Also, at high RPM, when the air is flowing into the inlet manifold that much faster, the fuel vapourises easier than at low RPM, so you don't have to worry so much about long manifolds and rough walls to make the fuel/air mix properly. Obviously in a road car the manifold has to be a compromise, but in a high revving racing F1 engine, you are willingly sacrifice low end torque for high end BHP.

Chris