Everything listed above is correct. I have decreased mine with my EEC Tuner, and as Hissin and Grady said it can be removed too by powershifting or removing the spout and locking down the distributor at full timing at the track. In doing so you take away the computers ability to alter and change the timing advance/retard during the run. I've always been told that the EEC uses the TPS as its only reference to determine if a shift has been made, and thus the reason why powershifting is supposed to remove it.