You have to reset the ecu so the car will relearn the a/f maps.
While it is similar the air is gonna be colder now with a diffrent pressure @ whatever rpm.

 

Bingo...we have a winner.
I actually disconnect the battery before i do anything under the bonnet. I can't tell you how many fuses I've blown because I didn't take the time to make sure I did it after installing stuff. I've fried a brand new HU that way. I'll never make that mistake again.
-Phil