Coby Wheel
BMW Garage BMW Meets Register Search Today's Posts Mark Forums Read

Go Back   BMW M3 and BMW M4 Forum > BMW F80 M3 / F82 M4 Technical Topics > Engine / Drivetrain / Exhaust / Bolt-ons / Tuning

Post Reply
 
Thread Tools Search this Thread
      04-24-2015, 08:15 PM   #1
mshab356
Go ///M or go home
mshab356's Avatar
United_States
266
Rep
1,279
Posts

Drives: 2015 F80 M3 6MT
Join Date: Jan 2007
Location: Los Angeles, CA

iTrader: (5)

Pros and Cons of Flex/No Flex section on Downpipes

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.
__________________
2015 AB/SO F80 M3 6MT | 19" Black M437 | KW H.A.S. | AutoTechnik Black Front Grills | BMW Performance Side Grills | Painted Front Bumper Reflectors | Gloss Black Exhaust Tips

GONE 2011 SG/Blk E92 M3 CS 6MT
Appreciate 1
      04-24-2015, 08:37 PM   #2
chillindrdude
Lieutenant Colonel
chillindrdude's Avatar
1118
Rep
1,724
Posts

Drives: anything with 4 wheels
Join Date: Feb 2015
Location: Virginia

iTrader: (1)

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.
__________________
My Trackable Street Car
2016 BMW M4 GTS
Appreciate 1
      04-24-2015, 08:58 PM   #3
apexit
Captain
United_States
212
Rep
972
Posts

Drives: 15 SO M3
Join Date: Nov 2007
Location: northern nj

iTrader: (-1)

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.
Appreciate 1
      04-24-2015, 10:58 PM   #4
mshab356
Go ///M or go home
mshab356's Avatar
United_States
266
Rep
1,279
Posts

Drives: 2015 F80 M3 6MT
Join Date: Jan 2007
Location: Los Angeles, CA

iTrader: (5)

Quote:
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... 😒
__________________
2015 AB/SO F80 M3 6MT | 19" Black M437 | KW H.A.S. | AutoTechnik Black Front Grills | BMW Performance Side Grills | Painted Front Bumper Reflectors | Gloss Black Exhaust Tips

GONE 2011 SG/Blk E92 M3 CS 6MT
Appreciate 0
      04-24-2015, 11:08 PM   #5
Chan
Private First Class
60
Rep
149
Posts

Drives: 2015 BMW M4
Join Date: Jun 2014
Location: North Miami Beach, FL

iTrader: (0)

Quote:
Originally Posted by mshab356
Quote:
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... 😒
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.
Appreciate 1
      04-25-2015, 04:07 AM   #6
emfo
emfo's Avatar
43
Rep
456
Posts

Drives: TT
Join Date: Mar 2013
Location: Tri-state

iTrader: (20)

Quote:
Originally Posted by Chan View Post
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?
Appreciate 0
      04-25-2015, 08:12 AM   #7
WERKE
President Directeur General
WERKE's Avatar
Canada
69
Rep
244
Posts

Drives: F82 M4/ E92 M3 ESS/F-type R
Join Date: Mar 2007
Location: Somewhere over the rainbow

iTrader: (1)

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

Quote:
Originally Posted by mshab356 View Post
That's exactly what I was thinking. Too bad the only DPs with flex section are like $1300-1400... 😒
__________________
Appreciate 1
      04-25-2015, 08:20 AM   #8
black335i
Captain
black335i's Avatar
162
Rep
722
Posts

Drives: M 4
Join Date: Mar 2008
Location: tx

iTrader: (2)

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.
__________________
2008 335i- FBO
2015 M4-exterior mods, JB4 E 85
2008 Zo6- intake
2015 Honda Accord
2020 Range Rover Sport HST
Appreciate 0
      04-25-2015, 08:35 AM   #9
edwinm3
Brigadier General
edwinm3's Avatar
United_States
677
Rep
3,594
Posts

Drives: F82
Join Date: Nov 2011
Location: Broward

iTrader: (26)

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.
__________________

Grigio Telesto M4, 6MT.
Coyote F-150
Appreciate 2
      04-25-2015, 11:32 AM   #10
Bowser330
Lieutenant Colonel
256
Rep
1,540
Posts

Drives: i3 REX
Join Date: Jan 2011
Location: Socal

iTrader: (0)

Quote:
Originally Posted by eurospeed
Quote:
Originally Posted by Chan View Post
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?
+1 links to budget friendly flex dps
Appreciate 1
      04-25-2015, 12:22 PM   #11
M3ANS55
Banned
62
Rep
145
Posts

Drives: BSM F80 m3
Join Date: Dec 2014
Location: Bloomfield NJ

iTrader: (0)

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
Appreciate 0
      04-25-2015, 12:27 PM   #12
NorCal f80
Lieutenant Colonel
326
Rep
1,648
Posts

Drives: M3
Join Date: Apr 2014
Location: Nor Cal

iTrader: (0)

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.
Appreciate 1
      04-25-2015, 12:50 PM   #13
tareemaa
Bimmer Fanatic
United_States
260
Rep
1,544
Posts

Drives: M3
Join Date: Mar 2011
Location: Chicago

iTrader: (7)

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.
Appreciate 0
      04-25-2015, 01:24 PM   #14
-JLT-
Brigadier General
-JLT-'s Avatar
2914
Rep
3,892
Posts

Drives: '20 X3M COMP, '18 X6 50i
Join Date: Dec 2006
Location: CHITOWN

iTrader: (0)

Quote:
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?
__________________
-JLT-
Appreciate 0
      04-25-2015, 02:04 PM   #15
Chan
Private First Class
60
Rep
149
Posts

Drives: 2015 BMW M4
Join Date: Jun 2014
Location: North Miami Beach, FL

iTrader: (0)

Quote:
Originally Posted by eurospeed
Quote:
Originally Posted by Chan View Post
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?
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.
Appreciate 0
      04-25-2015, 02:06 PM   #16
tareemaa
Bimmer Fanatic
United_States
260
Rep
1,544
Posts

Drives: M3
Join Date: Mar 2011
Location: Chicago

iTrader: (7)

Quote:
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.
Appreciate 1
      04-25-2015, 02:13 PM   #17
CaryTheLabelGuy
Colonel
CaryTheLabelGuy's Avatar
United_States
2572
Rep
2,816
Posts

Drives: 2016 M4 ZCP
Join Date: Mar 2015
Location: Jacksonville, FL

iTrader: (0)

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.

Last edited by CaryTheLabelGuy; 04-25-2015 at 02:19 PM..
Appreciate 5
      04-25-2015, 02:52 PM   #18
Bowser330
Lieutenant Colonel
256
Rep
1,540
Posts

Drives: i3 REX
Join Date: Jan 2011
Location: Socal

iTrader: (0)

Quote:
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
Appreciate 0
      04-25-2015, 03:00 PM   #19
mshab356
Go ///M or go home
mshab356's Avatar
United_States
266
Rep
1,279
Posts

Drives: 2015 F80 M3 6MT
Join Date: Jan 2007
Location: Los Angeles, CA

iTrader: (5)

Quote:
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.
__________________
2015 AB/SO F80 M3 6MT | 19" Black M437 | KW H.A.S. | AutoTechnik Black Front Grills | BMW Performance Side Grills | Painted Front Bumper Reflectors | Gloss Black Exhaust Tips

GONE 2011 SG/Blk E92 M3 CS 6MT
Appreciate 0
      04-25-2015, 07:52 PM   #20
Loboost
Enlisted Member
15
Rep
42
Posts

Drives: AY M3
Join Date: Mar 2015
Location: USA

iTrader: (0)

Quote:
Originally Posted by CaryTheLabelGuy View Post
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.

Last edited by Loboost; 04-25-2015 at 07:54 PM.. Reason: Missed a word
Appreciate 1
      04-25-2015, 08:15 PM   #21
edwinm3
Brigadier General
edwinm3's Avatar
United_States
677
Rep
3,594
Posts

Drives: F82
Join Date: Nov 2011
Location: Broward

iTrader: (26)

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

Grigio Telesto M4, 6MT.
Coyote F-150
Appreciate 3
jacobe92555.00

      04-25-2015, 08:21 PM   #22
NorCal f80
Lieutenant Colonel
326
Rep
1,648
Posts

Drives: M3
Join Date: Apr 2014
Location: Nor Cal

iTrader: (0)

Quote:
Originally Posted by edwinm3 View Post
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.
Appreciate 0
Post Reply

Bookmarks

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off



All times are GMT -5. The time now is 04:53 AM.




f80post
Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2020, vBulletin Solutions Inc.
1Addicts.com, BIMMERPOST.com, E90Post.com, F30Post.com, M3Post.com, ZPost.com, 5Post.com, 6Post.com, 7Post.com, XBimmers.com logo and trademark are properties of BIMMERPOST