Flex pipe needed for my setup?

[dirtdiggler]

I recently built a 3'' ss exhaust for my turbo E36 and did not install a flex pipe and am second guessing myself.

3'' ss all the way back to single muffler using stock bmw rubber mid and rear mounts
Akg solid engine mounts and E21 trans mounts with cups
Cast iron turbo manifold

Having solid engine mounts and cast iron manifold I can't see anything flexing but I really don't know.
Has anyone had good or bad luck with a setup like this?

[328iFun]

The flex pipe is there for 2 reasons:

1. Exhaust expands when hot causing the system to move and shift. You want the exhaust to have room to move when this happens - especially at the exhaust manifold
- Unlike a traditional NA setup, turbo back-pressure at the manifold outlet causes components to expand even more - as its not flowing as freely. Temps are highest at the turbo and the down-pipe thereafter
2. If road debris hit the exhaust the flex pipe will give, preventing damage to the cylinder head, manifold and turbo

The same applies for the waste gate DP as well, more so for #2 as it doesn't always see exhaust gas

It's common practice to have a flex pipe welded in the DP right after the Turbo, wherever feasible - The closer to the turbo, the better.

[tunermt]

always recommend a flex section, BUT 2 of my cars don't have a flex section haha. As stated above the flex gives room for movement that comes from heat and driving. Having a flex in the DP is def a good way to add some "wiggle room" and remove a little stress/strain from the entire exhaust system.

Last DP i made for my E30 that could have used a Flex section but didn't get one... This one is made from some very thin wall material so it actually allows the entire thing to flex a bit, still not as good as a quality flex section though. https://www.youtube.com/watch?v=MigIxVR-oTc

[chikinhed]

I have only ever used a flex section on the wastegate recirc pipe. I have not had cracks on the down pipe.

[dirtdiggler]

Seems a little conflicting? but kinda wanna redo my downpipe anyways so might as well. The wastegate is open dump right now and its kinda annoying so I would like to dump it back into the downpipe where is the best place to do that?

Also I have seen two different kinds of flex sections what are the advantages or disadvantages of each

https://www.aceraceparts.com/product...nt=11002653316

or

https://www.aceraceparts.com/collect...nt=16342541380

[pbonsalb]

People have had issues with the unlined braided one, which is why they are not recommended for turbo exhausts. I used a lined braided one for a while on my 3.5” but it had literally no flex to it and was massive and scraped. I use the accordion style on my WG pipe, which ties into my 3.5” at about a 45 degree angle.

[Def]

People have had issues with the unlined braided one, which is why they are not recommended for turbo exhausts. I used a lined braided one for a while on my 3.5” but it had literally no flex to it and was massive and scraped. I use the accordion style on my WG pipe, which ties into my 3.5” at about a 45 degree angle.

Yep, the bellows style flex joints (accordian style as you termed it) should only be used on a pipe that is fixed on both ends. I've seen some people use them as flexes in a main exhaust and they are way too soft for that. WG pipes or downpipes where the bottom end is restrained against the engine/trans are great applications for them though.