So I had planned to do a resonator / 2nd cat delete on the GP today, as I have been underwhelmed by the sound level and muted burble/pop out of the stock system (I did the same thing on my '10 JCW and really like the sound). Assuming the system must be the same as the one on my '10, I ordered 2.5" pipe for the job... Fortunately I measured the pipe on the car before cutting it, because wouldn't you know it's only 2"...

Here's what I don't get: My '10 fJCW came with a 2.5" system with no 2nd cat, which is how I understood all JCW cars to be equipped. Why on earth would the GP, a supposedly higher performance version of the regular JCW have a more choked down exhaust system? Am I missing something?

 

N18 they added second cat. I removed the 2nd cat and res on my jcw. Sounds awesome.

 

What size pipe did you use? I only measured between the cat and the res, looking at the parts diagram online would almost lead me to believe the system is 2.5" up to the 2nd cat.

 

what are you missing? engineering, tuning.... a bigger pipe isn't better in all cases (and usually where it is better makes it worse, too).

presumably (because I'm not going to sit down and do the math...) since the GP2's powerband tuning is significantly different than the JCW (ie, torquey instead of peaky), they needed a different exhaust design to match.


Seems to me like removing or reducing the 3" thick layer of foam between the carpet and the floor might go a lot further toward getting the sound into the car... :-D

 

^ It could well be this. Just struck me as odd to find a (presumably) higher-flowing system on a less focused car. I wish the exhaust guys hadn't welded directly to the DP when they replaced my flex pipe, otherwise I just would have switched everything behind the DP on each car.

 

Just out of interest the exhaust system is different in the US to the rest of the world.

The US gets the second cat, and the system labelled 2 in this diagram. Europe, Asia etc get only the downpipe cat, and the exhaust system labelled 1 in this diagram, with no second cat

http://realoem.com/bmw/showparts.do?...02&hg=18&fg=10

I think the cat / silencer part is the only difference, the size of the pipe is the same

 

I had the exhaust with the single larger resonator on my '10 (#1), whereas my GP has the 2nd cat downstream as shown in #2. I did crawl under the '10 today and re-measure; the whole thing is 2.25", not 2.5 as I had thought. Still kind of disappointing that they added in the 2nd cat though.

 

I'm pushing 300hp at 26psi, with my 2.5" DP and a Catless JCW exhaust. I just went and added a 3" exhaust from DP back and I gained 1hp. Same dyno, same mapping, same weather, and fuel. AFR, hp, TQ, and boost were mirror images of each other mQubed Motorsport, Manic Tuning Dealer

 

Thank-you Mario - That is about as definitive of an answer as anyone could hope for.

Looks like it'll be a de-catted factory exhaust for me, and an extra $1k for my wallet!

 

I have an appointment to remove the resonator on Friday, should I remove the 2nd cat then? anyone just remove the resonator?

 

I would remove the 2nd cat, because that will give you sound and a little too end power. If you have have JCW exhaust than keep the resonator, if it's a stock S then you can ditch the resonator too

mQubed Motorsport, Manic Tuning Dealer

 

minie, would love to hear the exhaust once you modify. Are you going to MiniMeetsWest at the end of the month?
I will be there with GP0042.

 

FWIW, I removed both the 2nd cat and resonator when I installed my Akra DP... NOT recommended unless want your ears to bleed from drone on the highway. I ended up adding a 2.25 x 12" resonator back in a few inches behind where the stock one was to neutralize interior resonance in the 150 and 300hZ ranges (placement of the resonator does matter for this purpose, don't just slap one in there wherever) which also toned down the volume a bit. If you're not doing a DP you'll probably be OK without a resonator.

I will try to grab a video sometime and post it.