save fuel 
Looking on ebay i seen these little fan type things that fits in the air intake which forces more air at a much higher rate
meaning you can save lots of £££££ on fuel.
If you increase the volume of air, you have to also increase the volume of fuel.
On petrol, indeed, air/petrol ratio of 14.5 iirc, and modern ecus will ensure that.
14.7:1 - but only at idle & low engine loads, WOT and high load will be richer than that by some amount as 'best power' is (with current engines) somewhere around 12.5-13.5:1.
Generally the engine is going to be 'open loop' at high engine load too, so the O2 sensors are ignored and fuelling based only on engine load and the learned fuel trims (which are learned by the closed-loop low-load O2 feedback controlled operation).
Anyway.. you could theoretically shove more air in during 'open loop' and the ECU would never compensate; course all you'd do is get detonation and engine damage
