difference between Exhaust with decat and exhaust without?

[Reissv1234]

what exactly is the difference? does having a backbox exhaust with a decat sound louder or something im confused?

 

[Don]

a catalytic converter will sap a little bit of power as its an obstruction (so to speak)

a decat is a pipe with no obstruction...so would be louder and maybe 1 or 2 hp better off. On a 1.2 though it isn't worth the bother (could say the same about the 172/182 also)

 

[steve32c]

Ones an MOT failure the other's not