The car been dynoed and tuned last week, but just have the exhausts system fitted, the car is now slower is it necessary to RE-DYNO again?

 

Any time you make a change to your configuration you need to re-dyno.

WHen you had the I am assuming stock exhaust on, you needed different fuel maps to counter the restriction, now you may either be too lean or too rich since the exhaust is flowing better.

 

The system was replaced from 2" to 2.5".

Thanks