How does tuning create more emissions? I thought it just does not pass on a aftermarket tune because the scan tool will show a code, so you go back to stock.
A tune usually optimizes the engine to run even better and people pick up MPGs.
The only damaging thing I think of is not running cats. Run cats, like I say in my previous post.
There is a bit that goes into it that I don't understand fully, but the basic premise (from what I understand) lies in the timing. Aftermarket tuning moves timing further away from when the exhaust valve opens, which reduces the temperature of the exhaust gas enough by the time it reaches the cats to make a difference in emissions. Retarded timing can actually burn more of the fuel, and late enough in the cycle that the gases are still at a high enough temperature to increase the effectiveness of the cats. The fuel is just burned late enough in the cycle it just doesn't help much hp wise.
So if you change the timing, you technically need to have the car approved for emissions.