Pros and Cons of Flex/No Flex section on Downpipes

[mshab356]

Ok, I have heard a lot of mixed things about DPs without the flex section. I've heard that you don't need them and people have had no issues with DPs without that flex section because they don't produce as much heat as catted DPs so they don't contract/expand as much (among other reasons); then again I don't know how long these people have had their DPs installed for. I've also heard that they are needed due to that heat issue and/or rattling (if it doesn't have flex then the heat expansion/contraction and/or rattling can potentially cause problems down the road). I don't know what to believe.

Can someone who is experienced in this stuff, preferably an engineer or tech, let me know whether the flex section is TRULY needed or not? Would be nice to have a solid answer, as I know tons of people ask this question quite often. Thanks.

[chillindrdude]

My feeling is, if BMW didnt think they needed it on the stock DPs, it wouldnt have them.

For me, I have passed on ER DPs despite their low prices due to the lack of the flex sections.

[apexit]

Pretty much my thoughts exactly ^

If the N54 didn't have them and the S55 does it must be for a reason, even if I don't understand the why I felt more comfortable with the flex.

[mshab356]

Originally Posted by chillindrdude

My feeling is, if BMW didnt think they needed it on the stock DPs, it wouldnt have them.

For me, I have passed on ER DPs despite their low prices due to the lack of the flex sections.

That's exactly what I was thinking. Too bad the only DPs with flex section are like $1300-1400... 😒

[Chan]

Originally Posted by mshab356

That's exactly what I was thinking. Too bad the only DPs with flex section are like $1300-1400... 😒

I was about to pull the trigger on the Akra DP since it has a flex section and heat shielding but am now considering the Active Autowerke DP that will be coming out in a couple of weeks since it will also have a flex section but will cost under a grand.

[emfo]

Originally Posted by Chan

I was about to pull the trigger on the Akra DP since it has a flex section and heat shielding but am now considering the Active Autowerke DP that will be coming out in a couple of weeks since it will also have a flex section but will cost under a grand.

link to info about AA DP?

[WERKE]

ZIMA still at +/-1200$. Quality is top notch with ceramic coating.

Originally Posted by mshab356

That's exactly what I was thinking. Too bad the only DPs with flex section are like $1300-1400... 😒

[black335i]

I would get it with flex part. I would not cheap out etc. the car was designed to have flex portions on dps. Obviously there is a reason for this slight flex tyre in the area under certain condition.

[edwinm3]

If BMW decided to put flex sections on theirs it was for a reason. Have you seen the length of our exhausts? I wouldn't want all that stress of those heavy long exhaust pipes trying to move up and down, front and rear and nothing allowing it to do so, that's a lot of stress on the turbo flanges.
Flex sections will allow for some movement there taking a lot of the stress off the other (Very expensive) parts.

[Bowser330]

Originally Posted by eurospeed

link to info about AA DP?

+1 links to budget friendly flex dps

[M3ANS55]

I haven't seen a single reason as to why a flex section is better then not.. And before people go telling me about rattles and stock is this way, I know for a fact I haven't come across a single set of down pipes installed that rattled and since one person on these forums claimed they do now every manufacturer is doing it so they don't loose a sale... Perfect example of word of mouth being a catalyst even when probably installed wrong on that one car, causing the ripple effect... These down pipe manufactures aren't rookies, they line up with exceptional precision, when ebay down pipes come out and fit like shit then I'll understand the need for a flex section

[NorCal f80]

from what I've read the heat with catless won't stay trapped like a catted DP, so you are good flex or no flex. OEM has flex because it's catted and the heat stays in that area.

[tareemaa]

I had AR design dps on my n54, no flex pipe, no issues ever. This time around in gonna go with ZIMA dps. They do have a flex pipe, ceramic coated, and have yet to throw a cel. Price is very competitive to the "big" name guys. I don't see the reason to be paying 1000+ for dps for the name.

[-JLT-]

Originally Posted by tareemaa

I had AR design dps on my n54, no flex pipe, no issues ever. This time around in gonna go with ZIMA dps. They do have a flex pipe, ceramic coated, and have yet to throw a cel. Price is very competitive to the "big" name guys. I don't see the reason to be paying 1000+ for dps for the name.

isn't Zima down pipe cost over $1000 also?

[Chan]

Originally Posted by eurospeed

link to info about AA DP?

Sorry, the only info I received was over the phone when I called about their Active 8 tune. We ended up talking about CELs (the Active 8 does not cancel them) and DPs and they mentioned theirs is coming out in early to mid May. They said it definitely will have the flex section but they are not sure if they will add heat shielding.

[tareemaa]

Originally Posted by M3ANS55

I haven't seen a single reason as to why a flex section is better then not.. And before people go telling me about rattles and stock is this way, I know for a fact I haven't come across a single set of down pipes installed that rattled and since one person on these forums claimed they do now every manufacturer is doing it so they don't loose a sale... Perfect example of word of mouth being a catalyst even when probably installed wrong on that one car, causing the ripple effect... These down pipe manufactures aren't rookies, they line up with exceptional precision, when ebay down pipes come out and fit like shit then I'll understand the need for a flex section

I agree 100%. I think people were installing the wrong and not clocking them properly and so fitment would be off, thus rattle and contact to the frame.

[CaryTheLabelGuy]

This topic has been beat to death over and over.

The stock DPs have cats. Those cats get literally red-hot to do they're job and they retain a lot of heat, which in turn causes lots of expansion at operating temps. This is the reason for the flex section. It has nothing to do with exhaust weight or movement. The stock DPs will expand faster and contract slower than the rest of the system. This is also why there is a TON of heat shielding on the stock DPs, due to the heat retention.

Aftermarket DPs don't have cats and therefore don't retain heat like the catted stockers do, which means the expansion is minimal between the downpipes and the rest of the system post DP.

Think of the flex sections as expansion joints, like a bridge. Those expansion joints are simply not needed in the aftermarket DPs.

[Bowser330]

Originally Posted by CaryTheLabelGuy

This topic has been beat to death over and over.

The stock DPs have cats. Those cats get literally red-hot to do they're job and they retain a lot of heat, which in turn causes lots of expansion at operating temps. This is the reason for the flex section. It has nothing to do with exhaust weight or movement. The stock DPs will expand faster and contract slower than the rest of the system. This is also why there is a TON of heat shielding on the stock DPs, due to the heat retention.

Aftermarket DPs don't have cats and therefore don't retain heat like the catted stockers do, which means the expansion is minimal between the downpipes and the rest of the system post DP.

Think of the flex sections as expansion joints, like a bridge. Those expansion joints are simply not needed in the aftermarket DPs.

I think that about sums it up

[mshab356]

Originally Posted by CaryTheLabelGuy

This topic has been beat to death over and over.

The stock DPs have cats. Those cats get literally red-hot to do they're job and they retain a lot of heat, which in turn causes lots of expansion at operating temps. This is the reason for the flex section. It has nothing to do with exhaust weight or movement. The stock DPs will expand faster and contract slower than the rest of the system. This is also why there is a TON of heat shielding on the stock DPs, due to the heat retention.

Aftermarket DPs don't have cats and therefore don't retain heat like the catted stockers do, which means the expansion is minimal between the downpipes and the rest of the system post DP.

Think of the flex sections as expansion joints, like a bridge. Those expansion joints are simply not needed in the aftermarket DPs.

This is the answer I was looking for. Thank you. Everywhere I searched there were conflicting answers. Glad I found a concise explanation.

[Loboost]

Originally Posted by CaryTheLabelGuy

This topic has been beat to death over and over.

The stock DPs have cats. Those cats get literally red-hot to do they're job and they retain a lot of heat, which in turn causes lots of expansion at operating temps. This is the reason for the flex section. It has nothing to do with exhaust weight or movement. The stock DPs will expand faster and contract slower than the rest of the system. This is also why there is a TON of heat shielding on the stock DPs, due to the heat retention.

Aftermarket DPs don't have cats and therefore don't retain heat like the catted stockers do, which means the expansion is minimal between the downpipes and the rest of the system post DP.

Think of the flex sections as expansion joints, like a bridge. Those expansion joints are simply not needed in the aftermarket DPs.

While your explanations seem like they are correct do you have a engineering source that can confirm your explanations? I am not trying to start a argument but would like to learn and a technical reference source explaining this would validate your comments.

Also as mentioned earlier the rest of the exhaust is prone to movement along with the engine twisting clockwise and counter clockwise along with moving up and down at the same time depending on load acceleration or deceleration.

It would seem that some flex on the exhaust piping after the turbos would be required if not something may break not immediately but possibly later.

It just seems to me there are other factors and dynamics happening on the exhaust system besides expansion. Have a look at some the various exhaust systems with a camera mounted at the rear of the vehicle and look how much the exhaust tips move in and out during acceleration and deceleration.

[edwinm3]

There is a reason why high end aftermarket exhaust manufacturers like eisenmann, Akra, etc (including OEM) used flex sections on DP's.

[NorCal f80]

Originally Posted by edwinm3

There is a reason why high end aftermarket exhaust manufacturers like eisenmann, Akra, etc (including OEM) used flex sections on DP's.

because most of us want peace of mind is my guess. I talked to one manufacturer and that's what they told me. They said it's not really needed for catless but people like to compare to OEM, so they added it.