exhuast flow problems?

[sicknote]

ive just fitted in a new...ish backbox to my 318i and ive lost some power

the old one was falling off so had this second hand one fitted to the back half

but when id finished i found out the front box on the front part of the exhaust had tiny holes in it so i sealed them

instead of getting some power back i seem to have lost some

is there some sort of airflow problem it seems to be on properley im just not sure what else i can do

any help appreciated

[Ant]

is it possible that where you pugged up the holes you've forced some into the gas stream and caused a restriction ?

unless the replacement box has collapsed internally that is ?

is there less "puff" @ the back now ?? and a quieter tone