Just an idea. Newer ECU's (factory ones) tune A/F mostly off of the MAP and fron O2 sensor. Installing a piggyback ECU might not allow you to tune the car properly due to the fact that the stock ECU might attempt to compensate due to its "closed-loop" monitoring capabilities.....Wondering if anyone has ever hooked an O2 sim up to the first O2 sensor (the one which monitors A/F) for tuning purposes?

 

Pointless.

Although O2 sims are used on the exhaust system's secondary sesor after the cat so it doesnt try to adjust the ignition timing and to compensate for emmisions.