this was part of an answer on yahoo answers about whether an intake increases preformance and mpg or not. what do u guys think of what has been said here


Once in the engine there is a knock sensor (newer vehicle) in place to see if the the air fuel mixture is too lean(air more than fuel) or rich(fuel more than air).
Leaner gets more power but increases temperature drastically to the extend that fuel ignite even before spark is created which is when the piston travels up in your engine creating a "metal knocking" sound when they collide (Very Bad) This can destroy your engine faster than you think.

 

There is only one knock sensor on the engine, so it doesn't detect air/fuel ratio in each cylinder. It detects engine vibration from knocking. The air/fuel ratio controlled by the engine computer from multiple sensors (map, ect, O2, etc.). The knock sensor could have some part in how the engine computer adjusts the air/fuel ratio.

When the knock sensor sends a signal to the ECU, the ecu will change ignition timing to prevent knocking from happening.

A new intake should allow for more air into the cylinder. Your engine computer should add more fuel to get the air/fuel ratio correct. Now I don't know if the engine will knock by adding equal amounts of air and fuel.

You should measure the gain in hp and/or gas mileage. You could use a dyno before and after you install the intake for hp. Gas mileage is easily tracked, so do that before and after an intake install. The knock sensor should be able to be scanned when doing the dyno.