The PAIR (pulsed secondary air injection system)system uses the vacuum created when the exhuaust valve closes to draw air into the exhaust system to facilitate the complete burning of fuel. Instead of sucking air for the outside the modification sucks air from the crankcase supposedly creating a partial vacuum and aditional horsepower. The only actually dyno results I have seen (on a GSXR1000) showed a slight (less than 1 hp) horsepower decrease but the engine reved quicker.
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