from the first cat back, thats just taking out the second cat what hp increase could i expect on a standard imp turbo?

 

not alot tbh, guess 5 or 10.

 

is it possible to get 15+ bhp increase from just replacing the back box and the middle section from the first cat? i have heard that the back box on the imp is very restrictive?

 

Problem is, every impreza is different, dont know why, but they are. Whilst the factory spec may be 208 or 215 bhp as standard, in reality most are kicking out 230ish as standard, see here http://dyno.scoobynet.co.uk/PEindex/uk.htm.

So you may believe you are getting more than 5bhp by jumping from 215 or 208 to 230 to 235, but in reality you are only going from 230 to 235 in most cases.

Have a look through the link above and work out the averages.

Steven