Stainless v Mild Steel Exhausts

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Yanker
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Mon Feb 23, 2015 11:31 pm

HxPaul wrote:I've heard that stainless exhaust's hold more of the heat than those made of mild steel,hence more chance of a heat seize.I have no proof of this,its just something that I have been told and I'm sure someone with more knowledge of the two metals will be able to say wether this is true or not.
That is complete hogwash.

Had a JL Vespa T5 and a Lam Scoot RS pipe both split on a stainless downpipe: both used hard for many years = no real complaints, but opt for normal steel pipes as the better exhausts made by the better exhaust designers are made in that material......

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dapper
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Tue Feb 24, 2015 11:26 am

I've had two SS ScootRS U bends crack at the exhaust flange. The SS was very very thin at this point.
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chris2470
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Wed Feb 25, 2015 7:47 am

Thank you for the feedback its much appreciated.
it seems the decision is most likely to be a reliability or an aesthetic one rather then down to outright performance, thank you.
Best Regards

Chris

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DigDug
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Wed Feb 25, 2015 9:03 am

chris2470 wrote:Thank you for the feedback its much appreciated.
it seems the decision is most likely to be a reliability or an aesthetic one rather then down to outright performance, thank you.
The fact that stainless resists rust is normally the primary reason that it is used for silencers.

It's the dimensions and shape of a 2T expansion pipe that gives it its performance - bugger all to do with what it's made of.
Did you have to do that?

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Knowledge
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Wed Feb 25, 2015 4:43 pm

Long ago, there was an interview with Charlie Edmunds in Scootering, and he stated that he made his pipes from mild steel and always gas welded them.

There could be many good reasons why Charlie said this (and I would say the same as mild streel is easy to work, and I prefer to gas-weld).

Persoanally, I bronze-weld brackets onto pipes as there is a little more flexibility in such welding (hence racing sidecars were made from steel tube and bronze welded together)
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Wed Feb 25, 2015 5:59 pm

the JL pipe on mine (stainless) is very thin, and all the welds are pretty flush with the plate its made from, making a good looking pipe, but it isn't very strong, most of the times its been the weld what has cracked out. I always tell the welder (Flavells engineering of Thornaby) to not dress the welds, as I think it makes it stronger, and being on a vespa, it isn't really on show.in fairness to JL, they do guarantee to repair the welding FOC on their pipes for original owners , but its a bit of a clart on sending it away, and not knowing when its coming back, when I can get it welded locally. (im not the original buyer, but he lives in the next street , so I could have cheated if I was that way inclined)

B-Race Tuning
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Wed Feb 25, 2015 7:37 pm

We set up my own TS1/225 scooter with exhaust a) mild steel, 1.5mm, and set out ten thermocouples along the length of the exhaust and run for an hour on the dyno under measured slight load. Repeat with exhaust b) Stainless steel 0.8/1mm. Both exhausts running the same AFR readings (at the start of set-up).
The temperature at various distances from exhaust flange differed by over 30% in places. Higher on the stainless exhaust. I have found similar but less difference on chrome exhausts. And at the end of an hour the AFR readings had changed enough to warrant going richer on the stainless pipe, but not the mild steel.
Bearing in mind what an expansion chamber should/does actually do and our own findings, I'll stick with mild steel. Additional to temps, stainless steel by its very chrome alloy make-up is more brittle, and especially after localised heat treatments (welding). This was in a controlled test on the dyno, whilst we were developing our exhausts to determine the material our own brand pipes were made of. Our decision was Cr1 Mild @ 1.5mm. Simon.

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Wed Feb 25, 2015 8:53 pm

B-Race Tuning wrote:stainless steel by its very chrome alloy make-up is more brittle, and especially after localised heat treatments (welding).
I've noticed the same thing, SS is very prone to cracking at the welds and is also very heavy compared to mild steel. It's okay for hardware and things that should be shiny but that's as far as I'd go with it.

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Wed Feb 25, 2015 10:28 pm

Shiny stuff, whether stainless or not will retain heat more than matt black. This is called its emissivity factor. What is known as an ideal black body radiates heat at the highest possible rate; factor of 1. Polished stainless is about as bad as it gets. And has an emissivity factor around 0.1.

Of course heat is also lost through conduction and convection and I would guess that when riding conduction is the dominant effect whereas radiation would dominate in still air.

If you took a sheet of sandpaper to your shiny exhaust you could raise the factor to something more like 0.4.

My point is that to get really meaningful results you'd need to do the thermocouple tests when riding.

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Wed Feb 25, 2015 11:02 pm

hendy wrote: My point is that to get really meaningful results you'd need to do the thermocouple tests when riding.

The difference to in the dyno cell to on the road running the thermocouples would have been negligible. Coupled to my computer/ data logger/ dyno, the ability to sup coffee and smoke tabs, and have the cooling fans running air flow we had at the last unit it was so much more comfortable to do indoors. I would also suggest from what we observed that on the road after a longer period of time / higher loads it would have been higher % differences. The Negligible bit relating to proving our point, rather than a defined "actual %" difference. I didnt need to know actual %, just that heat retention/ increase temps/ potential vapourisation of fresh charge occurring. Simon.

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