Please comment on potential problems with custom downpipe on 1.8T...

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Mar 8, 2001 | 08:32 AM

evan

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Please comment on potential problems with custom downpipe on 1.8T...

Hey guys,

I'm replacing the exhaust on my 225 hp TT with 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 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 who is right? 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.

Your thoughts would be greatly appreciated.

Mar 8, 2001 | 10:12 AM

Adam S

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Clarification: What are "cat back pieces?"

Ususally a catback exhaust would include a downpipe. If you are going with high flow cats, you should modify the exhaust appropriately so it can take advantage of the increased flow. 3" may be overkill, on the A4 2.5" is usually considered plenty of diameter.

My understanding is that your stock ECU should adapt just fine without an ECU reset but I could be wrong about this. I don't see how you would get "boost surges" with increased flow from the exhaust, that just doesn't make any sense to me at all.

But then, I'm not the final word on this subject. :-) One thing I can tell you, my Borla exhaust required no resetting of the ECU or adaptation to give me increased performance.

Good luck!

Mar 8, 2001 | 10:28 AM

DustysA4

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I agree with Adam S..

On the A4 anyway, I believe that the cat is right after the turbo and there are exhaust out there that are turbo-back that include an improved cat. These would be replacing the downpipe too. Borla is one I believe.

Mar 8, 2001 | 02:09 PM

Audiboy

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Not on the TT. It has a short downpipe to a primary cat. The A4 has the cat bolted on turbo.

Mar 8, 2001 | 02:15 PM

Adam S

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How large is that short downpipe?

Seems to me that if it flows OK, then leave well enough alone. But if it's restrictive, replace it!

Mar 8, 2001 | 02:19 PM

DustysA4

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Sorry, didn't know it was different. I'm an idiot.

Mar 8, 2001 | 06:02 PM

Audiboy

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I haven't jacked up the TT yet, so I am not sure. I imagine it is the same diameter as the rest of

the piping which is about 2 1/4. However, what bugs me is that there are a lot of compressed points on the stock exhaust for clearance. Even on an aftermarket, they do the same. That takes the effective area around that point to that of a 2" or smaller.