Evening all. Just brought a focus st 2.5 07 off my mate. The thing is an animal I love it But.... Its loud... I mean really loud.  Straight through 4inch no cat etc, thinking of a resonator to help with low rev noise... is it has been dyno with current exhaust onit. Would I mess anything up putting a resonator on it? 

It'll be fine. Just put a middle box on, that'll sort it out

What power is it running? I'd double check you have a 4 inch exhaust and not a 3 inch (which is what 99.9% of ST's run). I've only ever seen a couple RS' that have a 3.5 inch which are running insane power (enough to kill an ST after a lap round the track). 

There are loads of off-the-shelf exhausts for the mk2 ST/RS to suit everyone's needs. If you still want a lot of noise, but not straight pipe then go for the KMS section 18. Otherwise the Mongoose section 59 is a popular choice for those who like a milder sounding exhaust (still significantly louder than stock).

If you want the cheap option then perhaps an RS centre section would be the best bet.

If you do really have a 4 inch exhaust then ignore everything I said above. You'll have to get a muffler custom made from an exhaust shop.

Fit a cherry bomb mid section sure there 4” there for trucks but sure that would work not sure it would help the noice but would be funny as lol 😂 😂😂 never seen a rs with 4” pipe work going to need to find this and have a look 👀

The car is running 404at the wheels 99% certain it's 4" iv seen an mid section 4" to 3 " i can't stand the rahhhhh any longer 😂 

Rahh 

4 inch is way over the top for 400hp and probably means your exhaust is custom made. If you're lucky then whoever made it put the flanges in the same place as the stock exhaust so you can just bolt on a new rear section. Otherwise you'll have to chop and clamp it.

So if it is 4 inch would u just put a 3.5 reducer init? Or straight through resenator? 

The majority of aftermarket exhausts for the ST/RS are 3 inch (which is more than capable of handling 400hp). You said earlier you saw a 4" to 3" reducer in the mid section so can you not utilise that?