C6.5 3.0T Flex Pipes Keep Blowing

[QTwo]

Hi,

I have an 09 3.0T with an ongoing issue with blowing out flex pipes. The left one has been replaced 2 times and now has blown again. I have just taken the car to the local Lou's Custom exhaust and they cut out the old and weld in a new. Concerned with what could be causing the issue to reoccur. Any ideas? Is there that much of a difference in the quality of flex pipes? Used to be my daily driver, now one of the kids is driving it.

[LionKing]

Are they racing it everywhere? Lol
But yes, quality goes into it

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[QTwo]

Any ideas on where to get better quality flex pipes other than the stealership? Would need the pipe OD size and length. I found a thread on the A8 site and people were all over the place about what would fit and proper sizing.

[LionKing]

Any ideas on where to get better quality flex pipes other than the stealership? Would need the pipe OD size and length. I found a thread on the A8 site and people were all over the place about what would fit and proper sizing.

I'm actually unsure if Audi sells just a flex portion. You can always upgrade the downpipes. But you would be getting them specifically for a S4 and then having an exhaust shop modify them to work.

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[MTL4]

That converter pipe is stupid expensive and has crazy lead times if you want one. I went through the same thing myself when I first got my Avant (which is why I got it so cheap).

Here's what the stock one looks like, they are #1 (4F0-254-555-MX) and #2 (4F0-254-506-FX) in the photo:


So you can see Audi does not sell just the flex portion at all (bad design not to make that replaceable IMHO). If you want to fix it properly you need to TIG weld in a new section of flex pipe to fix it because it's all part of the upper converter pipe. The pipes are also 409 stainless so they need to be TIG welded (stainless MIG, stainless Flux Core or stainless Arc is possible but very hard to do a good job with it since the tube walls are quite thin). The key to getting a good flex piece to repair the pipe is to make sure it has a smooth interior portion like the stock pipe has. I found an exact match on eBay for them (they are made by JY Autoparts) but it I got it right before Covid and it looks like they might be a victim of supply chain issues because they are MIA now. The seller I used on eBay was "pricematchautoparts" so you might want to reach out to them with photo below (this is what mine look like). I think they were around $25 for the pair plus shipping.

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I was able to TIG weld mine on and I can tell you they do fit (2.25" x 6") but you need to modify the pipes you're attaching to by making slits with a cutting wheel lengthwise on the edge of the pipe to allow it to go around the flanges. I tried using an exhaust pipe expander but stainless does not like to stretch like mild steel does so I gave that idea up early and just made slits on the pipe to expand or reduce the size as needed. Then you simply push the ends in and TIG weld up any seams created by the cuts (pipe needs to be purged with Argon while doing this or the stainless will cook/sugar on the inside, creating a weak area). My welding was not as pretty as I would have liked (the TIG rods I used were too thick, I suggest to use 1/16" 309L instead) but it has been rock solid in terms of a fix and you'd have to look closely to know it wasn't stock at all.