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Exhaust flex bellows & the 4 factors you need to consider before choosing yours

Posted by David Taylor on

Exhaust Flex Bellows

Exhaust Flex Bellow Stainless Steel

What do they do and why are they needed in an exhaust system?

We will cover that soon but first.


Let's start with the basics.

Exhaust flex bellows are a convoluted tube most often made by hydroforming tube inside a die, or cold forming (rolling) both of these methods have advantages and disadvantages that affect you. Hydroformed bellows have a thicker wall; this translates to less flexibility but better resistance to heat & higher pressures. 
Cold-formed bellows can have a thinner wall ( or multiple ultra-thin walls to make one) this allows for better flexibility but a much less durable product.
Thin Wall Exhaust Flex Bellow 
Generally, an exhaust flex joint is placed just after the extractors, downpipe or dump pipe. Sometimes they are even used further down the exhaust to isolate expensive components such as catalytic converters or electronically controlled mufflers.   
The primary use is to soak up excess vibrations & movement caused by your engine; this helps in preventing extra strain from being applied to your exhaust system.
They also help to soak up linear movement as the exhaust system heats up and expands during use.

Another convenient advantage of using a flex bellow becomes apparent when fitting the exhaust system to the car as they allow for a slight amount of movement that can help when aligning and fastening exhaust flanges & V-Band's.

Most common flex bellows 

Each flex bellow has its advantages and disadvantages, but you can whittle it down to what you need with the helpful list below.
Here are 4 factors to consider when it comes to deciding what flex bellow is most suited to your application.

The 4 factors you need to know before starting

  •  The budget of your build.

  • Exhaust size & flow requirements 

  • Usage conditions i.e drift, drag, streetcar. 

  • What material are you making it from. 

Your budget can some times be the most significant deciding factor when choosing components for your build. If you have the room in your budget, then you should select the highest quality you can to avoid broken parts or premature replacement.
Exhaust flow some builds will only require basic flow, like a naturally aspirated motor or low boost applications. For these, you could use a cheap, lower quality flex bellow, but it still isn't ideal.
If you are building a high-performance exhaust system or your build has a larger displacement motor, then you will need to concentrate on the volume and flow that is required. Some deciding factors are engine displacement, turbo size & target boost pressure.
What conditions does your exhaust need to survive? is it destined for high flow, high-temperature runs down the 1/4 mile, violent & bumpy drifting or maybe it's a chilled streetcar. These are all things that will make a difference in what parts you choose.
The Material you are building your exhaust from is another big factor, flex bellows are available for most common exhaust metals, although this doesn't mean you get the same options across the board so choose carefully.

Braided flex bellows

Braided flex bellows are the most commonly found bellow on the market, but they generally do not last due to the braid breaking down quickly from the heat and gasses.
Exhaust Flex Bellow Stainless Steel Braid
Once the braid has deteriorated, it causes restriction and unwanted turbulence with the exhaust flow.
Another issue is that many flex bellows on the market get advertised as being made from "stainless steel" however, they do not state the grade of stainless used.
The 409-grade stainless steel commonly used for lower quality bellows deteriorates much faster and is not as strong as 304 grade.

Lined flex bellows 

In contrast, if your requirements are high flow and strength, then a better quality flex bellow is your go-to.
3" 76mm Stainless Steel Exhaust Flex Bellow
Flexible lined bellows have advantages such as lighter weight, marginally more flexibility & generally no restriction inside.
Exhaust Flex Bellow Braided Outer Flexible Tube Liner
But the flexible tube liner does not withstand heat or pressure as well.
Alternatively, Solid lined flexible bellows deal with harsher conditions like withstanding excessive heat and higher pressures, such as high boost drag car's, competitive drifting applications, large horsepower race boats or even a show spec streetcar.
However, they are generally a little more restrictive than the flexible style mention above do to the solid liners needing to overlap in the centre leading to a reduced internal diameter.
You can overcome the restriction by using a slightly larger than required flex bellow to account for the reduction in internal diameter, allowing the extra flow of performance or race car applications.
A larger exhaust even has the possibility to help you achieve higher power levels with the same or reduced boost; however, this is a subject for another day.
These factors make a solid lined bellow ideal for high-performance applications such as pre-turbo crossover pipes, pro-mod style exhaust manifolds and race car turbo downpipes.
The length of a flex bellow will affect its life span, a longer bellow has more area to stretch and soak up the vibration whereas a shorter bellow will be stiffer meaning a slightly shorter service life. Don't go scrambling for a longer flexie though as you need to remember this will change other factors such as exhaust sag. 
Flex bellows are an essential part of any exhaust system regardless of its application. They increase longevity, can improve performance & allow for a higher quality end product meaning you must select the correct component for your needs.
This information is to help answer your questions and make it easier to select the correct components for your build if this has helped you, please leave a comment and tell us about your build and how this benefitted you.


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    wendy on

  • Do you have 6 inch diameter flex . It’s for a Cummins KTA19M bellows.


  • Hi id like to ask a question as to is it bad to wrap the flexy mesh with a flexible aluminium sheet.? Thanks

    Gerry Ballesta on

  • What is the best flex pipe for a jeep Cherokee limited ,2018 2.4L engine.

    Binnie Warren on

  • Thank you so much for this post. I love the part where you talk about” Exhaust flex bellows “. I have found some worthy guides on this website:

    Mark Williams on

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