Will an exhaust swap upon a standard turbo allow the engine to breathe more easily and free up more horsepower, in the same way as a normally aspirated car? Or, does an exhaust swap have to be matched to other mods? I can see that a de-cat system should remove a lot of back pressure but will this mean messing around under the car before an MOT to swap back? What sort of power gains can be made if any?

 

A freer flowing exhaust will help, on a stock car there is no need to go mental, you can loose one cat easily keeping you legal (A full de-cat is not only an MOT fail but illegal to drive on the public roads as well).

You can go for a de-cat downpipe it will have a better effect on spool up (marignail though), alternatively just changing the centre (with its cat) and rear box is easier (MUCH easier) and will give the same power benefit, all assuming you have a classic!

If you decat the centre you have a choice of with or without a silencer, if you want stealth keep a silencer (or get an STi centre which has a helmholtz resonator) if you want more burble go with a straight pipe.

If you go full decat most people swap in the cat centre section for the MOT, which should only take 20 minutes and is good enough to pass as long as its healthy!

Ebay is often a good source of cheap exhaust bits, search for subaru exhaust AND Impreza exhaust as some sellers list better than others!

Simon

 

Thanks for that Simon.

Two questions

1. any rough idea of power gain for changing the mid section for a decat?

2. What on earth is a helmholtz resonator? LOL

Mike

 

Most claim 10-15bhp, so the real gain is probably about 7bhp...but it will sound like 20!

MODE: Anorak on

A helmholz resonator is a tuned volume or length (so works best at certain frequancies) with a single entry from the exhaust, most back boxes include one, on the STi its achived by a small break in the pipe through with the entry being swaged down slightly to create an overlap with the exit thus minimising flow loss, name comes courtesy of a German scientist.

Hermann von Helmholtz - Wikipedia, the free encyclopedia

Helmholtz resonance - Wikipedia, the free encyclopedia

Modern intake pipes on many cars include mini HH resonators that are often nicknamed 'donkey dicks' due to their appearence, in fact some Land Rovers have 7 just to tune out the frequancies they don't want you to hear!

MODE: Anorak off

Simon

 

Thanks mate....will check out Ebay

Mike