I'm quite disappointed with the exhaust system on my GP2 and have been searching high and low for a suitable replacement. General consensus seems to be 2.5" is the way to go, although I can appreciate the argument for 3" as well (I already have the bigger JCW turbo and plan to run higher boost as well, so additional flow capacity isn't a bad thing).

My old JCW had the stock 2.25" exhaust with the resonator removed, and I loved the sound. Most of the aftermarket systems I've seen / heard have a resonator in-line and are not quite to my taste, which brings me to the original question posed in the title: I quite like what I've heard of the Quicksilver exhaust (probably due to it not having an in-line resonator), but I'm curious what the consensus on the 2.25" tubing diameter is... Thoughts? I know everybody seems to like the Akrapovic system, but I'm not keen on the sound and certainly not into paying almost $2k for something I'd have to modify anyway.

 

I went down this road, trying to find a good tone with out the coffee can effect.. I ended up with the thought of - I wanted best performance...so
Least amount of back pressure possible / big pipe with little muffler

I'm sure you will get some chime in from others...

If it was my daily driver then I might have gone a different way...

One thing I can say is the videos on YouTube with a go-pro camera don't sound like the exhaust in person. ... to me ..

 

isn't the stock exhaust 2.25"?

I have the Stratmosphere DP back exhaust, its 2.75" the whole way. Good luck finding one, only a few were made and I think they are out of business now. Every tech and mechanic that gets near my car starts drooling out their ears. I can send you some pics if you wanted to have a shop copy it. The muffler itself just appears to be a large diameter, long straight through similar to a Magnaflow, and it deletes cat #2 and the resonator.

when swapping between the 2 exhausts, i notice that the factory 2.25" pulls harder just off the line up until about 2500 or so, and when there's no boost (which is exactly what I'd expect), whereas the 2.75" exhaust turns into a rocket from that point on. The car is otherwise stock aside from a panel filter, noisemaker delete and Forge hot side charge pipe.

 

Why don't you consider doing an Akrapovic Downpipe? That will not only increase the sound but also give you performance.
http://www.waymotorworks.com/akrapov...6-r57-r58.html

I've used the Akrapovic Exhaust with the stock downpipe on my GP. But from reading what you are looking for that isn't what you want. I did it to get rid of the drone from the JCW and love it. It may work if you do the downpipe too, but before you spend all that I would try just the downpipe first.

 

I actually just picked up a lightly used Akra DP from the marketplace yesterday - my understanding is the DP is where most of the performance gains come from anyway, so I had always planned to throw one on the car. The stock exhaust is a bit too subdued for a car intended as a track-day special imo, so the goal for any change behind the downpipe is mostly about getting a bit harder-edged sound and more pop/burble out of the car without making it obnoxious. I re-measured the stock system yesterday, and it is indeed 2.25" - so the easiest and cheapest solution would be to just cut out the 2nd cat and resonator, which I know will give me the sound I want... I just don't know if 2.25" will end up being a bottleneck as I turn the boost up.

 

Back pressure is the enemy. Here are a few post for turbo exhaust theory ..


Now looking at the exhaust, or post turbine the larger the exhaust, the larger the pressure differential can become. The increase in area of the exhaust, gives the exhaust gas much more room to expand. Hot gas has only one goal, to expand as quickly as possible. The goal pre-turbine is to focus the energy into the turbine to carry as much energy as possible. As the exhaust expands the energy dissipates, so the goal post turbine (i.e exhaust) is to have the largest area possible for the gas to expand. Looking at the immediate exit of the turbine housing, the downpipe, the exhaust gas is traveling at a very high rate of speed. The exhaust gas is expanding rapidly, and is in a very turbulence state from being flung from the turbine. At this point having a 3″ downpipe becomes critical since the exhaust gas is both in a turbulent state, and is expanding. In turbulence, the expansion of an area the turbulence is forced to become more laminar (although this doesn't happen very quickly). Also the increased area allows the gas to expand rapidly, allowing the energy in the exhaust gas to dissipate quickly and letting the pressure created by the exit from the turbine housing to drop. Essentially you are creating a greater pressure drop.



Downstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you can. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. Here, the idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine. Remember, a turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure. This means the turbine is able to do the most amount of work possible (i.e. drive the compressor and make boost) with the available inlet pressure.

Again, less pressure downstream of the turbine is goodness. This approach minimizes the time-to-boost (maximizes boost response) and will improve engine VE throughout the rev range.

As for 2.5" vs. 3.0", the "best" turboback exhaust depends on the amount of flow, or horsepower. At 250 hp, 2.5" is fine. Going to 3" at this power level won't get you much, if anything, other than a louder exhaust note. 300 hp and you're definitely suboptimal with 2.5". For 400-450 hp, even 3" is on the small side.”

 

My main concern with a 3" system is that, when paired with a 100-cell DP, it will be far too loud for civilized daily use. But then 2.5 is not a huge increase over stock, and I do see some folks here putting out decent numbers with a stock JCW system minus the cat and res. My guess is I'll probably see more benefit from going to a larger volume FMIC and just modifying the JCW exhaust, versus blowing $1k+ on a 2.5" system and no IC upgrade - right?

 

if you are worried about sound, run a larger resonator. If im honest in a daily driver you want sound but you just dont want the drone between 2-4k rpms. If you are actually running a catted system a nice sized resonator should keep the car quiet enough during normal driving but will give a nice sound. It should be said that unless you change the cat, exhaust system are merely for sound, so talking about diameter i personally feel is unnecessary unless you are de-catting the car completely which would allow for more aggressive performance gains.

 

Firstly: Yes I do agree with the explanations of the least restriction after the turbine housing the better. No ifs buts or maybes :-)

We measured the backpressure before the CAT and after the CAT when running standard power on standard UK spec exhaust (only one cat in the downpipe) and found the CAT was the restriction, there was no restriction after the CAT (180bhp ish)

I am now running the same standard exhaust but with Milltek de-cat downpipe replacement, no other mods to the exhaust. It sounds great on boost and quiet on cruise.



We measured the backpressure of this exhaust setup when I was running 235bhp and there was no issue at all. No backpressure of any amount worthy of discussion.

SO if you aren't going to run say more than 235bhp, we have proof that my current exhaust is not creating backpressure measured via an actual test, not a guess or a formula. Standard with no cat or cats.

I am now running 260-270bhp, on a JCW turbo with billet inlet wheel on the exact same exhaust setup. Standard with no cat in the replacement downpipe. We will be measuring the back pressure next week when we do a few other tests and are happy to share. Currently we don't "think" it is creating any power issues at all and no heat issues as it has run HARD for a good few months at this power. Of course it's now time to measure rather than guess again.

Our guidance is that 2.5" exhaust will be good for about 280bhp.

3" exhaust is overkill for most modified minis. Thought a 3" downpipe leading smoothly to 2.5" might give the ultimate best solution as you have that initial big chamber for the hottest gas to expand into.

Cheers and I hope the experience helps.

Steven RW

 

I entirely agree. The restriction is the CAT not the rest of the exhaust for most peoples sub 250bhp power.