The Ford Focus is a small, sporty car made by the Ford Motor Co. and marketed in Europe and North America. While Europeans could buy a Focus in 1998, the compact Ford was not introduced in North America until 2000. Worldwide, the Focus has always been equipped with a four-cylinder engine; but even in stock form the Focus is no slouch. Some enthusiasts do not leave well-enough alone, though, and they modify their Focus cars to extract every bit of power and speed they can possible muster from the little car's 2.0-litre powerplant. One simple modification that boosts power a bit is removal of the air box resonator. This allows more air to enter the engine intake system, hence increasing power slightly -- but at the cost of louder engine noise.
Open the bonnet and locate the air filter housing near the driver-side inner fender liner.
Locate the small box attached to the air filter housing. This is the resonator.
Loosen the two screws on the air filter housing closest to the resonator with a Phillips screwdriver.
Remove the windshield washer bottle cap to allow more room to remove the resonator. Grasp the air filter housing and pull up on it slightly.
Pry the resonator away from the air filter housing with a large flat-blade screwdriver until it pops free. Tighten the two screws loosened earlier, using the Phillips screwdriver. Replace the windshield washer bottle cap. Close the bonnet. Keep the resonator for future use, if desired.
After you remove your air filter housing resonator, check your air filter element more frequently. The hole in the housing left from removing the resonator will let more air into the intake system, so your filter element may get dirty sooner, requiring a more frequent replacement interval.