13. Optimization of the exhaust system: Catless downpipes, secondary race cats
As already mentioned previously, I initially intended to only use upgraded downpipes from ar design with high-flow cats (300 cells from Magnaflow). However, as time went by and I started reading more and more positive reviews about the GIAC stage 2 flash, I decided to give this flash a try, as I thought it to be a considerable improvement over my current Evotech flash. Now, the GIAC stage 2 flash is intended to be used with a more or less free-flowing exhaust system, as it runs with much higher maximum boost (up to around 18 psi) than the stage 1 (which is more or less identical in this respect to the Evotech flash, i.e. 14 psi). Such increase in boost of course means the turbos need to do more work to create that boost, and if there was much backpressure they need to work even harder to overcome it. In order therefore to minimize backpressure and danger to damage the turbos, it is advised to remove more or less all catalysts out of the exhaust system. For me that meant replacing the catted downpipes through catless ones, and also removing or replacing the secondary catalysts which are located right before the mufflers.
Of course, I could also have left the stock catalysts in place - but a friend of mine (E92Fan) had done this and as a result toasted his stock catalysts after some time, as they could not withstand the high exhaust temperatures resulting from the catless downpipes (and the tune, of course).
So, why not remove the secondary catalysts altogether? Well, I had heard some reports about smelly exhaust fumes that result from this measure, and I also feared that - in combination with my Bastuck exhaust and catless downpipes - the noise level resulting from this would be too much to bear on a daily basis. As a consequence, I chose to replace the stock secondary catalysts with high flow metal catalysts that minimize back pressure but still filter the exhaust gases to some extent and muffle the exhaust noise somewhat.
As far as the downpipes
were concerned, my choice was fairly easy: I had had excellent experiences with the ar design catted downpipes, both from a customer service standpoint and from a fitment and quality perspective. I therefore chose to also order and install their catless downpipes. This time, however, I also went for the option of a ceramic coating
; as mentioned above, I had experienced some heat radiating into the driver cabin which may have resulted from the downpipes - and even if not, it is obvious that the turbos create lots of heat in a very limited space that should transfer as little as possible to other engine components.
Here's a picture of the downpipes before the install (sorry, I don't have pictures after the install yet):
For the replacement of the secondary catalysts
, the choice was slightly more difficult. I knew the company HJS Motorsport
which has an excellent reputation and produce first class catalysts for lots of motorsport vehicles; many car manufacturer and reputed tuning companies use their products. They have two products that could fit our cars:
Universal, 200 cell metal catalysts (article number 90 95 0040) that cost 370 EUR (+ VAT) - as you'll need two of those, it sets you back around 880 EUR if you live in Germany. They have the advantage that you could have them TÜV approved if you so desire, as they comply with current emissions regulations.
Motosport race catalysts with 100 cells and platinum covering (article number 90 95 0058 - WRC 1112/10 PE) which cost 560 EUR (+VAT), making it around 1500 EUR for both including VAT. With these, you will not be able to pass emission regulations and therefore they cannot be TÜV approved. However, I would imagine that if you have catted downpipes in addition to those (not catless ones as I have), you should still be able to pass emission testing.
As I knew that I would install catless downpipes anyway, emissions testing was not really a factor for me. The price tag of the 100 cells caused some hesitation, though, but in the end I went with those in order to maximize performance and because I knew that Tony had successfully installed them in his car already.
Here's a schematic drawing of the secondary catalysts with some data in it:
The install went smoothly, as expected with ar design downpipes the fitment was excellent. The stock secondary catalysts needed to be cut out and the new ones had to be welded in place. They are very unobtrusive and look almost identical to the stock ones, so that unless you look very closely and know exactly what you're looking for, you cannot detect the change. I kept the stock cats in case I should need them to pass emissions testing, or should I sell the car (which is unlikely to happen in the foreseeable future).
After having the catless downpipes along with the secondary motorsport race cats installed, I immediately felt that the car pulled harder, revved more readily up beyond 5000 rpms and felt more responsive. There was an additional edge to it that I hadn't felt before. Unfortunately I can't provide you with exact numbers, as I didn't dyno the car again right after the install. From my very subjective butt-dyno I would say it added around 10-15hp to the car, but that's just a guess. The driving pleasure has certainly increased!
Another very significant change was the sound. The car now sounded much more aggressively, in particular during a cold start-up or when revving it beyond 3000 rpms. When I first drove with it to the underground parking lot of my company, I just revved it up (very slightly!), and the alarm of one of the cars went off. Wow! Really awesome. My garage also told me that from the outside it sounded really great, the best sound they had heard so far of a turbo car. I can second this opinion - when I drive in a tunnel with the windows rolled down it is outright addictive. I'm now looking forward to summer to do that more often! However, the noise is NOT intrusive in the cabin - if I roll the windows up, it's slightly louder than before but not annoyingly so, no perceptible drone and long stretches of driving can still be done quite comfortably.
Problems / disadvantages?
None - aside from the price tag obviously, if you take both the downpipes, the secondary cats and the install together. Fortunately, I managed to sell the catted downpipes to a member from a forum in Germany (for his 135i), and he's quite happy with them.