Pros and Cons of Flex/No Flex section on Downpipes

[edwinm3]

Originally Posted by 0-100 ninja real quick

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.

I like piece of mind

[CaryTheLabelGuy]

Originally Posted by Loboost

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.

I am not an engineer, but I was a dyno tuner for years and have tons of experience with many many aftermarket and OEM turbo charged setups. It's just common sense grounded in a lot of experience. I've seen enough things with my own two eyes and have a good understanding of how the parts interact with each other.

[sA x sKy]

If my 335i had any worth, I had AR downpipes installed from 10K miles to 60K miles for 4 years and the AR DP did not have flex... and guess what? No problems at all.

[marcvtec]

i,ve had my AR downpipes since 2007, and still have the car to this day, no flex sections, no issue.
Car still does 12's at the drag strip and i drive it every day.
only thing i did was replace turbos, since 6 years of 19 psi on them, i felt they were getting bad. glad i did cause now its reborn
160xxxkm on it

[r33_RGSport]

+1 to CaryTheLabelGuy . When I was employed for some coilover company, I have to help in building their drift car, Nissan S13, S14, S15, Z33 350Z, etc, all of the turbo car's downpipe are just plain straight. Imagine how much stress we put on those drift car during practice and events. With all of those, the non flex piped downpipe that we use are held very well.
Without a cat, all the heat are transferred all the way to the tail. Even ER's own M4 with the downpipe doesn't have issue with it. Other customers with other brand catless downpipe that doesn't have flex pipe, also don't have a problem with it while daily driving and tracking the car.
But, of course, if you don't feel comfortable, nothing will convince you but yourself.

[mshab356]

Originally Posted by CaryTheLabelGuy

I am not an engineer, but I was a dyno tuner for years and have tons of experience with many many aftermarket and OEM turbo charged setups. It's just common sense grounded in a lot of experience. I've seen enough things with my own two eyes and have a good understanding of how the parts interact with each other.

Now let me ask you something else if you don't mind: what about those DPs that offer heat shielding or ceramic coating? Do you think that is worth it or not necessary?

[Loboost]

Originally Posted by CaryTheLabelGuy

I am not an engineer, but I was a dyno tuner for years and have tons of experience with many many aftermarket and OEM turbo charged setups. It's just common sense grounded in a lot of experience. I've seen enough things with my own two eyes and have a good understanding of how the parts interact with each other.

Ok thanks for the response! It seems as though many including yourself have never had a issue. That being said it would be nice to see the engineering reason for the flex section.

I know when a factory engineer designs something there is usually a reason behind it. I agree many have not had any issues without a flex pipe but the S55 is also using 2 instead of 1 downpipe that are side by side and there may be more of a benefit but I certainly do not know as I am not a engineer.

I think time will tell how the solid pipes work out for those that have them. But I don't think anyone truly knows on this platform just because they have done solid downpipes on others. Just my 2 cents, again not arguing just talking out loud. Regardless any mods are considered do at your own risk.

[NorCal f80]

Originally Posted by Loboost

That being said it would be nice to see the engineering reason for the flex section.

I'm pretty sure the heat theory is accurate.

Originally Posted by Loboost

I know when a factory engineer designs something there is usually a reason behind it.

I'm pretty sure the heat theory is accurate.

[NorCal f80]

Originally Posted by WERKE

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

Some member on here sold his brand new Zima version 2 for $940 shipped. It was strange because it took about two weeks to sell and I would have guess a day to sell.

[r33_RGSport]

Originally Posted by Loboost

Ok thanks for the response! It seems as though many including yourself have never had a issue. That being said it would be nice to see the engineering reason for the flex section.

I know when a factory engineer designs something there is usually a reason behind it. I agree many have not had any issues without a flex pipe but the S55 is also using 2 instead of 1 downpipe that are side by side and there may be more of a benefit but I certainly do not know as I am not a engineer.

I think time will tell how the solid pipes work out for those that have them. But I don't think anyone truly knows on this platform just because they have done solid downpipes on others. Just my 2 cents, again not arguing just talking out loud. Regardless any mods are considered do at your own risk.

The Flex section is needed due to the heat caused by the catalytic converter that will gets red hot and expand/weaken the pipes, while with engine movement, the flex pipe will help reducing the stress on that red hot expanded/weakened pipes.
While, on the catless, the pipe will not get as red hot as the catted one since no hot gas or excess fuel got trapped.

As why it uses 2 downpipe instead of 1, that is because there is 2 turbo's in the car. Downpipe is a pipe connecting the turbo to the exhaust pipes. True you can design it with 1 downpipe, but it will have 2 inlet pipes that need to be connected to the 2 turbo's. Also, by designing them in 1 pipe, it will put more stress and difficulty on aligning the pipes during installation, and more restrictive for putting the collective pipes to close to the turbo.

[Loboost]

As why it uses 2 downpipe instead of 1, that is because there is 2 turbo's in the car. Downpipe is a pipe connecting the turbo to the exhaust pipes. True you can design it with 1 downpipe, but it will have 2 inlet pipes that need to be connected to the 2 turbo's. Also, by designing them in 1 pipe, it will put more stress and difficulty on aligning the pipes during installation, and more restrictive for putting the collective pipes to close to the turbo.[/QUOTE]

Ok I know it has two downpipes because it has two turbos, since there are two pipes instead of one and they are different lengths they will not expand at the same rate. I am not a engineer but two pipes with different bends are not likely two expand at the same rate and in the same direction.

The one thing that concerns me more than the expansion is that without some flex the as the rest of the exhaust moves around it will put a lot of stress on the front of the exhaust system.

Again it would be nice to understand all of the engineering behind the flex. I am not telling anyone to not buy a solid or flexed pipe. I just would like to understand all the dynamics involved, I do value people's opinion that have a lot of past experience but this platform is fairly new still so long term durability has not been proven in either case.

[Mit_Boost]

Originally Posted by 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.

UMMM..... Both the N54 & S55 OEM DP's have flex sections. Maybe not a mesh style, but they do have flex.

[apexit]

Originally Posted by Mit_Boost

UMMM..... Both the N54 & S55 OEM DP's have flex sections. Maybe not a mesh style, but they do have flex.

Guess I'm wrong then. My brother told me, he has an N54, whoops

[ryank382]

exhaust moves around quite a bit. i feel like that exhaust play could be tugging down on the turbo potentially causing issues down the line. Flex section could minimize the effect of exhaust movement. just an idea

[r33_RGSport]

Just FYI, I just dropped by at ER and talked to them regarding this issue.
He said that he can add that flex join if needed. Got to be special ordered.

[Bowser330]

Originally Posted by r33_RGSport

Just FYI, I just dropped by at ER and talked to them regarding this issue.
He said that he can add that flex join if needed. Got to be special ordered.

What's the added cost?

[r33_RGSport]

Originally Posted by Bowser330

What's the added cost?

No added cost.
Just need some patience to wait.

[Mike@N54Tuning.com]

Originally Posted by r33_RGSport

No added cost.
Just need some patience to wait.

Are you sure.. I was quoted 1-150.00 to add a flex section for a couple custom jobs.

Mike

[r33_RGSport]

Originally Posted by Mike@N54Tuning.com

Are you sure.. I was quoted 1-150.00 to add a flex section for a couple custom jobs.

Mike

Mike, Correct. I visit them yesterday and he said same price.
I will ask him again today as I am going to go there again for some test fitting.

[r33_RGSport]

Update. Mike is correct. It is $150 additional for flexpipe.
Just confirmed with ER on that when I visited them earlier.

[Henry655]

Originally Posted by ryank382

exhaust moves around quite a bit. i feel like that exhaust play could be tugging down on the turbo potentially causing issues down the line. Flex section could minimize the effect of exhaust movement. just an idea

Yes, there could be reaons other than heat that BMW and Akrapovic have taken into account when designing downpipes for the M4.

Quite often there is more than one reason/benefit to a design feature such as having a flexible section on the downpipes.

I would much prefer mine to have it than not.

[Henry655]

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 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?