Stainless vs. Mild

[TimmyET]

Are there any major advantages in purchasing a stainless steel exhaust over a mild steel, as I currently have a 2.5" mild steel installed and wondering if its worth 'upgrading' or not.
Thanks

[Callumgw]

on a like for like basis there is no performance improvement of stainless over mild steel. In theory the stainless will last longer, but the mild steel one will probably last longer than you will have the car.

If your getting a new improved system then its more about money and bling.

C

[TimmyET]

Hmmm I see. I thought at one stage that stainless retained more heat than mild resulting in faster moving exhaust molecules. But I dunno
thanks Callum anyways

[shanec86]

actually the hotter the exhaust, the more efficient it is. Look at the Turbo blankets, and people heat wrapping the manifolds/dump pipes

[SiniT]

afaik , stainless is slightly better for performance , but in saying that , i doubt that a 1.5 would even notice the difference ... i go with mild as it is cheaper ...

[Kimmo]

I can see a polished stainless manifold making a tiny bit more boost, and along with a stainless dump slightly lowering engine bay temps... but it's prolly a matter of poofteenths.

[SashET]

I dont think stainless or mild would make any difference to performance....

For performance, I'd be more worried about the welding of it all, like requesting purge welding (where they fill tubes being welded with gas whilst welding to make the inner weld smooth as without any slug in there) would be better for air flow...... It would minimise turbulance within the pipe....

But again, its probably only a poofteenth difference.......

Cheers
Sash

[Ash]

sweet F all really. stainless will sound raspier where the mild will give you more rumble. stainless looks better and lasts longer but is more expensive where mild will last long enough anyway not look as good but do the same thing for cheaper.

money?

[Callumgw]

I can see a polished stainless manifold making a tiny bit more boost, and along with a stainless dump slightly lowering engine bay temps... but it's prolly a matter of poofteenths.

Kimmo,
AFAIK, Stainless and Mild steel have very similar thermal properties, so I doubt that there would be any reduction of heat getting OUT of the pipe. The steel has very little - if any - insulating effect. So if stainless does happen to be a tiny bit better, it's tiny bit better than the none of mild steel. Which would make it a tiny bit of none.

Polishing a pipe would reduce heat getting through from the polished side. So the temp from the engine bay wouldn't getting into the pipe, which is a bit useless because the engine bay is cooler than the pipe. So polishing an exhaust is useless unless you polished the inside of the pipes......but thats not what you meant is it?

Polishing the outside is pure bling. High performance exhaust systems are focused more on weight reduction. Using very thin, light high strength alloys. Any thermal effect comes from the heat wraps, which reduces under bonnet temperatures and improves the efficiency of flow in the pipe by keeping the energy higher (higher temp-higher energy).

C

[etking]

only upgrade to stainless if your going for a 3" system but to go that big ud want to be serisouly modified, save your money and buy something else =)

[Kimmo]

Cal, as I understood it, polishing the pipe, whether on the inside or outside, will reduce the amount of heat it radiates.

This might even be true even if it's covered in heat wrap; not sure about that one, though

[TimmyET]

Thankyou all for your wonderful input. i have learned much ^^

[Callumgw]

Cal, as I understood it, polishing the pipe, whether on the inside or outside, will reduce the amount of heat it radiates.

Nah reckon thats a fallacy, polishing works on reflection so it only stops heat entering through the shiny side, not exiting. Reflective house insulation, for example, works best if you have to reflective panels facing each other across an air gap.

Ohh we haven't mentioned ceramic coating! now thats cool, but expensive.

C

[Damo]

On exhaust manifolds:

Stainless is a poorer conductor of heat compared to mild steel, so that in itself would allow a little more heat to remain in the ex manifold before the turbo giving quicker spool up. Quantifiable? Probably not.

The other benift of stainless over mild steel and especialy cast maifolds, is the fact that there is far less material used to make the manifold itself. Which means the heat sink capability of the manifold is relativly low. So it only takes a couple of seconds at 'full noise' to saturate the manifold ensuring all the heat energy makes it to the turbo and is not wasted heating a great lump of cast iron.


Damo

[Kimmo]

Cal, as I understood it, polishing the pipe, whether on the inside or outside, will reduce the amount of heat it radiates.

Nah reckon thats a fallacy, polishing works on reflection so it only stops heat entering through the shiny side, not exiting. Reflective house insulation, for example, works best if you have to reflective panels facing each other across an air gap.

Radiation and absorption is the same thing in reverse. To minimise it, a mirrored surface is the go, to maximise it, matt black.
http://en.wikipedia.org/wiki/Black_body

To go with the house insulation example for a bit, the air gap works for two reasons; the air in between has heat conducted and radiated into it, and convection keeps replacing the warmed air with cooler stuff. And it's shiny on both sides of the gap; where you want it to reflect, and where you don't want it to radiate.

Now if you fill the gap with something solid, it'll conduct more heat than air, but I'm pretty sure you'll still want the silver surfaces to minimise radiation. Since whatever you fill the gap (or wrap the pipe) with, will prolly be a poor conductor anyway, like fibreglass, the difference in radiation may be more than a piddling theoretical percentage of the amount transmitted.

To have the heat travelling through mutiple shiny surfaces would be a fine thing, I reckon... polished pipe with reflective heat wrap would radiate less heat than black pipe with reflective heat wrap, I bet.

Ceramic coatings prolly work by being a really poor conductor, forcing more of the heat to dissipate through the metal of the manifold into the head, turbo and exhaust, I guess