Pros and Cons of Flex/No Flex section on Downpipes

[CaryTheLabelGuy]

Originally Posted by Henry655

I have spoken with a fairly prominent motor engineer here in the West of Scotland about this and they seem to completely disagree with your views.

I must say I was already a little bit sceptical given the fact that Akrapovic (and others) have flexible sections on their downpipes even though there are no catalytic converters present.

Anyway, in the opinion of the chap I spoke to (40 years motor engineering), the ONLY reason for the flexible sections is to allow for movement in the exhaust during driving, and that the flexible section has very little to do with the expansion of the catalytic converter under heat.

I'm not going to argue with the manufacturers/retailers/end users of downpipes which have no flexible sections who disagree, all I will say, is that on a car in the price bracket of the M3/M4 there would be absolutely no chance I would consider downpipes which did not have flexible sections if I thought there was a potentially safer, or more durable option out there.

Why risk it?

Show me an instance where NOT having a flex section causes an issue on the F8x?

[tom @ eas]

Originally Posted by Henry655

I have spoken with a fairly prominent motor engineer here in the West of Scotland about this and they seem to completely disagree with your views.

I must say I was already a little bit sceptical given the fact that Akrapovic (and others) have flexible sections on their downpipes even though there are no catalytic converters present.

Anyway, in the opinion of the chap I spoke to (40 years motor engineering), the ONLY reason for the flexible sections is to allow for movement in the exhaust during driving, and that the flexible section has very little to do with the expansion of the catalytic converter under heat.

I'm not going to argue with the manufacturers/retailers/end users of downpipes which have no flexible sections who disagree, all I will say, is that on a car in the price bracket of the M3/M4 there would be absolutely no chance I would consider downpipes which did not have flexible sections if I thought there was a potentially safer, or more durable option out there.

Why risk it?

Akrapovic most likely has them for homologation (BMW Safety vehicles come to mind).

The remaining areas of the exhaust system are suspended by poly hangers, which still allow plenty of movement.

[hotrod2448]

Thermal expansion cat vs no cat doesn't seem likely enough of a difference to be the real determining factor in flex versus no flex. I'm sure it's used as a reason quite frequently but, I'm not sure it's the actual reason if that makes any sense.

The length expansion of a 36" long stainless tube would be roughly 0.4" at 800°F. It's about 0.25" at 500°F. I can't see most exhaust manufacturers doing the r&d to know if that 0.15" is really going to be critical or not. More likely they just haven't seen many problems as a result of not including the flex section and have thus come to the conclusion it's just not needed.

Personally while I like the idea of giving all that thermal expansion a stress relief point, I would worry more about engine and exhaust system movement. Motor mounts typically aren't solid and engines can move around more than you might think. Same with heavy exhausts that are hung with flexible rubber bushings. I'd rather have the pipe be able to allow some misalignment than directly transmitting a bunch of stress into the turbo/etc...

[Henry655]

Originally Posted by CaryTheLabelGuy

Show me an instance where NOT having a flex section causes an issue on the F8x?

If you re-read my post you will see that I never said that I had an example to show you, I also never said that it was a sure thing that having no flexible section would cause problems.

What I said, is that an experienced motor engineer disagreed with your views on the catalytic converter expanding with heat being the reason for requiring flexible sections on the downpipe.

And the view I gave, was that on an expensive car such as an M3 or M4, I wouldn't be comfortable contradicting the advice of an experienced motor engineer, when my options are as follows:

1. Use downpipes with no flexible sections - may or may not cause a problem - who knows.

2. Use a downpipes with flexible sections - highly unlikely to cause a problem.

Can you see my logic?

Also, out of interest, do you stand by your assertion that the reason for the flexible sections on the factory downpipes is purely to allow the expansion which occurs due to heat?

[Henry655]

Originally Posted by tom @ eas

Akrapovic most likely has them for homologation (BMW Safety vehicles come to mind).

The remaining areas of the exhaust system are suspended by poly hangers, which still allow plenty of movement.

Yes, the hangers on exhausts do allow for some movement, I agree with that.

But, why would you restrict the movement, if it was possible to have downpipes which allowed for more?

Your suspicions over BMW Homologation may be true, but we don't know that, unless I have missed it.

What we do know is that Akrapovic make downpipes without catalytic converters, but do still have flexible sections in place.

[Henry655]

Originally Posted by hotrod2448

Thermal expansion cat vs no cat doesn't seem likely enough of a difference to be the real determining factor in flex versus no flex. I'm sure it's used as a reason quite frequently but, I'm not sure it's the actual reason if that makes any sense.

The length expansion of a 36" long stainless tube would be roughly 0.4" at 800°F. It's about 0.25" at 500°F. I can't see most exhaust manufacturers doing the r&d to know if that 0.15" is really going to be critical or not. More likely they just haven't seen many problems as a result of not including the flex section and have thus come to the conclusion it's just not needed.

Personally while I like the idea of giving all that thermal expansion a stress relief point, I would worry more about engine and exhaust system movement. Motor mounts typically aren't solid and engines can move around more than you might think. Same with heavy exhausts that are hung with flexible rubber bushings. I'd rather have the pipe be able to allow some misalignment than directly transmitting a bunch of stress into the turbo/etc...

Agree completely with this.

If the flexible sections are available on the downpipes I can't see any disadvantage to having it rather than not having it

[CaryTheLabelGuy]

Originally Posted by Henry655

If you re-read my post you will see that I never said that I had an example to show you, I also never said that it was a sure thing that having no flexible section would cause problems.

What I said, is that an experienced motor engineer disagreed with your views on the catalytic converter expanding with heat being the reason for requiring flexible sections on the downpipe.

And the view I gave, was that on an expensive car such as an M3 or M4, I wouldn't be comfortable contradicting the advice of an experienced motor engineer, when my options are as follows:

1. Use downpipes with no flexible sections - may or may not cause a problem - who knows.

2. Use a downpipes with flexible sections - highly unlikely to cause a problem.

Can you see my logic?

Also, out of interest, do you stand by your assertion that the reason for the flexible sections on the factory downpipes is purely to allow the expansion which occurs due to heat?

Don't agree with the "engineer"? Yes or no.

[Henry655]

Originally Posted by CaryTheLabelGuy

Don't agree with the "engineer"? Yes or no.

I said I wouldn't be comfortable contradicting the advice from the engineer. That means I agree with him.

I tend not to be involved in arguments, I just try to state the facts as I know them, and when I'm unsure, I ask. Would you say anything I have said is not factual?

Also, you never answered, are you standing by the earlier comment you made about the reason for the flexible sections being to allow the downpipes to expand when the catalytic converters are hot?

[CaryTheLabelGuy]

Originally Posted by Henry655

Also, you never answered, are you standing by the earlier comment you made about the reason for the flexible sections being to allow the downpipes to expand when the catalytic converters are hot?

Yes, I stand by my comments as I always do. Because you know... years of tuning experience?!?

Again, show me an instance where I'm wrong?

[Henry655]

Originally Posted by CaryTheLabelGuy

Yes, I stand by my comments as I always do. Because you know... years of tuning experience?!?

Again, show me an instance where I'm wrong?

As you will see in one of my earlier posts, I confirmed that I don't have an example to show you, where the lack of a flexible section on a downpipe has caused an issue, but that probably isn't the best confirmation that there would not be an issue at some point.

My point, is that if I am faced with the option of having a flexible section on the downpipes versus not having a flexible section, as far as I'm concerned the safer option would undoubtedly be to have downpipes with flexible sections which allow movement during driving.

You appear to be utterly convinced that the reason for the flexible sections on the downpipes is to allow for expansion of the catalytic converters under heat.

It genuinely appears that you are mistaken here, perhaps that would explain your highly defensive response earlier (which has since been edited).

You mention "years of tuning experience" with "?!?" after it. I might be pointing out the obvious here, but that hardly puts you in a commanding position in relation to motor engineering issues.

On the other hand, I do not claim to have great knowledge, but I have spoken with those who do.

What I find worrying, is that unless I'm mistaken, you are not a qualified motor engineer, and yet you appear to express your views in relation to motor engineering issues as fact on an open forum where young and/or inexperienced enthusiasts may read your posts and accept them as fact - which could lead to costly mistakes.

I wonder if a suitably qualified person is able to put us out of our misery here...

[CaryTheLabelGuy]

Originally Posted by Henry655

As you will see in one of my earlier posts, I confirmed that I don't have an example to show you, where the lack of a flexible section on a downpipe has caused an issue, but that probably isn't the best confirmation that there would not be an issue at some point.

My point, is that if I am faced with the option of having a flexible section on the downpipes versus not having a flexible section, as far as I'm concerned the safer option would undoubtedly be to have downpipes with flexible sections which allow movement during driving.

You appear to be utterly convinced that the reason for the flexible sections on the downpipes is to allow for expansion of the catalytic converters under heat.

It genuinely appears that you are mistaken here, perhaps that would explain your highly defensive response earlier (which has since been edited).

You mention "years of tuning experience" with "?!?" after it. I might be pointing out the obvious here, but that hardly puts you in a commanding position in relation to motor engineering issues.

On the other hand, I do not claim to have great knowledge, but I have spoken with those who do.

What I find worrying, is that unless I'm mistaken, you are not a qualified motor engineer, and yet you appear to express your views in relation to motor engineering issues as fact on an open forum where young and/or inexperienced enthusiasts may read your posts and accept them as fact - which could lead to costly mistakes.

I wonder if a suitably qualified person is able to put us out of our misery here...

If I'm just another forum idiot, why bother engaging me in any "discussion"? Think what you want to think and do what you wish with your car and money. I really don't care what you do, personally. This conversation, thus far, has been a huge waste of my time.

And yes, you are very mistaken.

P.s. A mod must have deleted the earlier comments as I haven't edited anything - I certainly don't blame them.

[Henry655]

Originally Posted by CaryTheLabelGuy

If I'm just another forum idiot, why bother engaging me in any "discussion"? Think what you want to think and do what you wish with your car and money. I really don't care what you do, personally. This conversation, thus far, has been a huge waste of my time.

And yes, you are very mistaken.

P.s. A mod must have deleted the earlier comments as I haven't edited anything - I certainly don't blame them.

At no point did I call you a "forum idiot", you certainly called me a few childish names in your deleted post however

What is it that makes me "very mistaken"? I can't see anything I have said which would make that the case.

I've never asked you to care about what I do or what I spend my money on, strange you would say that.

I'm just not satisfied that you are correct about the reasons for the flexible section in the downpipes.

I'll see if I can cite a reliable reference to put this one to bed.

[E92TiAg335i]

Originally Posted by 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.

Sorry to bring this back from the dead...

I recently got VRSF downpipes installed. Perfect fitment, finish, etc.

Installed 100% perfect.

They sound great blah blah blah.


I noticed at idle, i have VERY VERY VERY slight added vibration.

It annoyed me. Did a bunch of research... ended up buying AA catless downpipes with the flex joint and added hanger (VRSF has neither).

It is going on this Saturday. I really hope the flex joints and hanger eliminate this vibration i'm feeling.

Could it be because it's catless? Maybe? But i had a catless downpipe on my N55 and felt no difference in vibration at idle.

On my M4, however, i do.

Anybody?

[Brasko]

I'm flexin'

[E92TiAg335i]

Anybody else switch from non-flex to flex?

Or have a very subtle vibration in cabin from non-flex joint downpipes?

Anybody run AA catless? Any vibrations?

[jacobe92]

I went straight to an Eisenman DP with flex. I push the crap out of her and I have zero issues from the Get Go. Yeah, the other DP w/ out it are the ones w/ fitment and rattle issues. GL

[edwinm3]

flex is definitely not for heat expansion but for movement, makes more sense that way and that's why i'm flexing with some akra dp's. Some of the best brand exhaust makers (akra, eisenmann, etc) have them, probably for a reason.

[E92TiAg335i]

Originally Posted by jacobe92

I went straight to an Eisenman DP with flex. I push the crap out of her and I have zero issues from the Get Go. Yeah, the other DP w/ out it are the ones w/ fitment and rattle issues. GL

It's not the fitment with the VRSF. It's the vibrations in the cabin! I hate it!

[E92TiAg335i]

Originally Posted by edwinm3

flex is definitely not for heat expansion but for movement, makes more sense that way and that's why i'm flexing with some akra dp's. Some of the best brand exhaust makers (akra, eisenmann, etc) have them, probably for a reason.

This makes sense.

I just bought Active Auto with the flex joints and the bracket. Do you think this will be the difference in vibrations? That's the ONLY thing i hate with the VRSF.

[E92TiAg335i]

Bump

[chetrickerman]

For the record, the flex section is there for movement of the exhaust and to prevent cracking of the exhaust.

[E92TiAg335i]

Originally Posted by chetrickerman

For the record, the flex section is there for movement of the exhaust and to prevent cracking of the exhaust.

Yes, understood. But wouldn't it stop some "cabin vibration" as well?