[Pinscher]

I suppose it's just for additional support from where the tube is being shrunk down, but I'm not sure.

Thanks!

[Joebie]

The ribs allow for the pipe to flex slightly when the motor moves relative to the rest of the exhaust system or the frame of the car twists and flexes over bumps.


Or maybe its ribbed for her pleasure, idunno.

[feuer]

Heat expansion too.

[Pinscher]

well, that's pretty thoughtful, though is it over kill on consumer cars like 335?

[gurusan]

No, it's to avoid cracking welds. They are or aren't necessary depending on the materials and design.

[Pinscher]

understood, thank you!

[Joebie]

Originally Posted by Pinscher

well, that's pretty thoughtful, though is it over kill on consumer cars like 335?

Not at all, its completely necessary. The engine moves around in the engine bay more than you probably realize, if there was no flex in the exhaust, things would fatigue and crack almost immediately.

[Emilime75]

Originally Posted by Joester

Not at all, its completely necessary. The engine moves around in the engine bay more than you probably realize, if there was no flex in the exhaust, things would fatigue and crack almost immediately.

Then again, the VRSF downpipes, which are likely the most widely used, don't have those ribs or any other flex prevention...and I don't recall ever reading about one cracking. The exhaust is hung by rubber hangers, after all.

[Joebie]

Originally Posted by Emilime75

Then again, the VRSF downpipes, which are likely the most widely used, don't have those ribs or any other flex prevention...and I don't recall ever reading about one cracking. The exhaust is hung by rubber hangers, after all.

Interesting, maybe they're not quite as critical as I thought.

[IllSic_Design]

It's not 100% necessary but as said it is there so there is a little flex in the down pipe as to reduce fatigue on the exhaust system/turbo/manifold. They likely did it because BMW did it, and to some people that makes them feel better about the purchase.

[musa]

Originally Posted by Joester

The ribs allow for the pipe to flex slightly when the motor moves relative to the rest of the exhaust system or the frame of the car twists and flexes over bumps.


Or maybe its ribbed for her pleasure, idunno.

Ding, ding, ding, bingo, we have a winner!

[Emilime75]

Originally Posted by musa

Ding, ding, ding, bingo, we have a winner!

Which part? Because we've already concluded they are likely not necessary.

[feuer]

Originally Posted by Emilime75

Which part? Because we've already concluded they are likely not necessary.

Who said they are not necessary? It would not be there if it was not necessary. Car manufacturers would put regular straight pipe there. It would be cost saving. All have it from factory. Either before or after the catalyst.

[feuer]

Originally Posted by Emilime75

Then again, the VRSF downpipes, which are likely the most widely used, don't have those ribs or any other flex prevention...and I don't recall ever reading about one cracking. The exhaust is hung by rubber hangers, after all.

Look up images from n55 or b58 downpipe from said supplier. If they did not put flex pipe on n54 that does not mean it was not necessary. Could mean they could not do it considering the higher cost and lack of space on the n54. Also look up OEM and high end aftermarket downpipe for s55 from Akrapovic or Zima and post photos here. They have purpose. If people decide to ignore it that is separate issue all together.

[Emilime75]

Originally Posted by feuer

Who said they are not necessary? It would not be there if it was not necessary. Car manufacturers would put regular straight pipe there. It would be cost saving. All have it from factory. Either before or after the catalyst.

Originally Posted by feuer

Look up images from n55 or b58 downpipe from said supplier. If they did not put flex pipe on n54 that does not mean it was not necessary. Could mean they could not do it considering the higher cost and lack of space on the n54. Also look up OEM and high end aftermarket downpipe for s55 from Akrapovic or Zima and post photos here. They have purpose. If people decide to ignore it that is separate issue all together.

I said it wasn't necessary. If it was we'd know about it by now, considering how many aftermarket downpipes without flex prevention have been installed over the years.

[feuer]

Originally Posted by Emilime75

I said it wasn't necessary. If it was we'd know about it by now, considering how many aftermarket downpipes without flex prevention have been installed over the years.

So all of those engineers across all manufafturers are clueless?

[Emilime75]

Of course not, but they also like to over engineer the shit out of things. Are you telling me that you think everything BMW has engineered is peachy and perfect? How many examples of problems can you tell me about caused by downpipes without this?

[NightStalker]

well I deff learned something new lol

[feuer]

I agree. Far from perfect. I believe isn't such thing. But the flex section has purpose. Whether someone ran some down pipes with out flex section for some time without detriment is another question. As someone who works on engines and has seen many torn flex pipes from worn out engine or transmission mounts can imagine what would have happen to exhaust manifold or turbo housings if flex section did not fail first. Exhaust systems now last the lifetime of the vehicle. They are well designed.

[Joebie]

Also worth noting that the stock N52 headers have no flex section in them, yet the stock N54 downpipes do. Hmm.

Maybe something to do with the presence of turbos?

[Chris@VargasTurboTech]

Just a flex section, can be a point of failure on aftermarket pipes, certainly will increase mfg cost. It certainly has a purpose, doesn't mean you can't skip it with few consequences.

These motors move around quite a bit, especially with increased output. It's useful to not get too caught up in component-component matching but look at the system as a whole. I.e. don't worry about dp flex section design utility while you leave your worn out oil-soaked soft stock engine mounts installed.

Chris

[Pinscher]

Man, my threads take on a life of their own, lol.

Okay so maybe someone should not have sloppy engine mounts if they are going to have a Downpipe without a flex section?

Is that the complete lesson here?