As some of you (like recent chatters) may know, I'm replacing the exhaust on my 225 with Milltek high-flow cats and cat-back pieces and am considering going the whole enchilada with a downpipe as well. Locally, I have a fabricator who will make me one, it would be a 3" diameter piece made from 321 stainless, mandrel bent.

So, I was talking to a local performance shop guy who has done excellent work for many Audi folks around here... this guy has a lot of years in the business and comes highly recommended, so I'm inclined to respect him. But he feels that if I replace the downpipe I may be asking for driveability problems with my stock ECU... like boost surges and the like. He says that the reason companies like MTM do kits with ECU, manifold, turbo, downpipe, cats, and mufflers is so they have an entire solution that works together properly without the computer wigging out.

Still, others I respect (like our very own BracketRacer) have told me that even my stock ECU with a reset should adapt to the new flow characteristics of the exhaust and the car should run fine.

So I'm opening this up to all of you for your thoughts and opinions. Should I just do the cats and cat-back and forget the downpipe for this reason? I'm not made of cash, so if I get a part custom built and deal with all the hassle involved and it doesn't work right, well, that would be a big waste of time and money.

All input would be greatly appreciated.

 

If you area going to go as far as to replace both the cats, you might as well just go the extra yard and replace the downpipe. And if bracketracer has told you that the ECU should adapt to any flow changes with a reset, he should know best being a former Audi tech, I would say go ahead. I will eventually do it, the local exhaust guru in my area of Utah won't touch the cats, so unfortunately I will have to go aftermarket if it ever becomes available.

Jay

 

<center><img src="http://albums.photopoint.com/j/View?u=1258227&a=9305837&p=30374533&Sequence=0&res =high"></center><p>the least restriction the better. The reduced back pressure should equalize surging potential. First I went with a race downpipe, but after examining the diameter of the stock cat, I had the stock cat cut open and gutted by an outlaw exhaust shop. If you don't have to worry about emission regs/testing, any of these two strategies would help your turbo. A 5 psi fuel pressure regulator could help ensure fuel mix under high psi induction runs. P.S. my 944 would hit 180mph and smoked two 930's on the highway on separate occassions before it met its demise. Had to add that in.

 

what are the potential HP increases w/ cat and downpipe swap?

 

couldn't you, in theory, increase the bar for the turbo with less restrictive exit?

 

Motronic is adaptive, whether it be for better or worse, the system has the ability to *learn* new situations based on enviroments.
A good example of an excellent ignition/fueling system that is not adaptive is Electromotive, it is only as good as the fuel, air, boost tables that are input by the tuner/programmer. Electromotive does not have the ability to compensate for air leaks in the system, bad gas or excessive boost etc.....

While I do believe that the ecu will adapt to the 3" downpipe, you must ask yourself if it is really necessary. I think even MTM is only using a 2.5" downpipe no? Are you planning on pushing more than 300 hp to the wheels? What are your future plans?

I think a well built (mandrel bent) 2.5" downpipe along with free flowing cat(s) and a good 2.5" cat back would be plenty big for most applications

Sean

 

I agree with hoTTub

 

IMHO! :P (but I could be biased..)

The downpipe has done wonders for my 180, but I am running more boost. The throttle response is MUCH smoother, and it pulls a <b>lot</b> harder at higher rpm's (yes...even on the k03!). At lower rpm's, the turbo spools up a little faster as well. Honestly, I think a 2.5" downpipe is more than sufficient for our small turbo's (k03/k04). My tuner told me that 2.5" it would be sufficient for cars with 350+ hp, 3" would be an overkill. And about the adapation, I have driven my car with both my stock chip and APR chip, it makes a whole lot of difference in both worlds, everything is so much smooooother.

Hope this helps!

 

Any links to a race only downpipe for the 225?

 

www.dtmautohaus.com lists a downpipe, don't know if its for 225.