Here in Europe,these things are marketed as AIRPOWER.They simply increase the air turbulence before it reaches the inlet manifold.It strangles the air at low revs giving you an illusion that as you step on the gas you get more power!!!!!!This is only so because it strangles it in the first place!!!!After all, if your air-filter is dirty doesnt your car feel slower???
On Turbo-intercooler cars(like my 3.0D) they recommend fitting 2 of these,one before the intercooler(after the air-filter) and another after the intercooler(near the inlet manifold).A friend of mine installed this on a Golf Tdi-150 (against my better judgement!) and lost max boost pressure,and gained more black smoke(proving that the Airpower restricts the air-flow mass!) .He asked for his money back!Each Airpower here costs about 90 Usd!!!!!!!(no wonder they recommend 2!!!!!)Now for some quick effective DIY power gain tip!Try drilling a 2.0" hole in your airbox!I got more boost pressure at lower Rpm,but mine is a Turbo Diesel!On a 3.0-4.8 ,the air-intake sound will bring a smile to your face..........