[CBRBob]

The PAIR system injects fresh air into the exhaust to help burn any unburned fuel in the hot exhaust with a oxygen rich mix. When tuning on the dyno, most tuners use air/fuel (O2) sensors either in a port in the exhaust pipe or a 'sniffer' probe inserted into the muffler. With the extra air being injected, it biases the exhaust gas into looking 'lean', when the reality is it could be rich. By disabling the pair system, you would get a truer reading and be able to tune more accurately.