> Why do people install these parts? Do they improve performance somehow? Yes. Most turbocharged cars come from the manufacturer with an enormous amount of lef

> Why do people install these parts? Do they improve performance somehow? Yes. Most turbocharged cars come from the manufacturer with an enormous amount of left-over power. Many "Stage 1" (no hardware, only software) calibrations for a modern turbocharged car like a VW GTI will produce a 30-40% gain in peak torque. This is for several reasons: * Emissions. A catalytic converter in some ways works as an oxygen capacitor - it "fills" with oxygen while excess O2 is present (for example, when fuel is cut on deceleration or when the car chooses to run lean-of-stoich) and "discharges" while converting excess hydrocarbons to water. There are other extremely complicated principles at play like catalyst light-off and heating, but this is the basic concept. * This means that an OEM calibration is a careful balance wobbling between lean and rich of stoichiometric operation (called "catalyst stimulation"). However, this is not the ideal condition for producing power. Due to the flame propagation properties of gasoline-air mixtures, gasoline engines produce optimal torque at a mixture condition rich-of-stoichiometric (this is called, counterintuitively, Lean Best Torque or LBT). And turbocharged and high-compression gasoline engines are extremely vulnerable to unstable combustion conditions (detonations and preignition) which are also alleviated by running a richer mixture. The most common way to increase power on a turbocharged car is to change the air-fuel lambda target from 1.0 (stoichiometric) to around 0.84 under full load, then to increase boost to the safe maximum from the turbocharger (or beyond) and timing to the knock threshold. * There are also obvious warranty and longevity concerns. Running a turbocharger at 70% of its shaft speed maximum will make it last longer than running it at 120% of its shaft speed maximum. Putting 450 ft-lb through a transmission designed to handle 300 ft-lb will break it.