I am still using my stock sequential turbos and will be keeping them for a long time since they can meet my power goals. I started doing some research on downpipes and mid pipe (test pipe) and pretty much the options are stainless steel and mild steel; stainless steel being half the cost of mild steel. The brand names such as HKS use mild steel, while not so famous brands such as Megan or OBX use stainless steel to construct theirs.

My question is, what are the advantages and disadvantages of each? Is the additional cost of mild steel (i.e. HKS, FEED) worth it over the stainless steel? I am under the assumption that mild steel will do better in terms of thermal resistance, keeping the energy within the pipe and therefore preventing heat loss (heat escaping the pipe) and possibly maintaining a higher exhaust gas velocity. But, what are the experiences on this?

Thanks in advance.

 

So the added price you are paying for mild steel is only because of the companies selling those pipes in this case. The material cost for stainless is typically double that of mild, not the other way around. For exhaust applications, stainless is a superior metal, owing to its higher chromium and nickel content. This translates into better corrosion resistance, especially at temperature, better strength at temperature, and better weld properties at high temperatures as well. Why do HKS and FEED use mild over stainless for this application? Well as a mechanical engineer, I'm a little bit baffled about this, as they have to have much thicker wall construction with mild vs stainless (304 or 321). And that means a heavier pipe, on a not very heavy car. Further, the thermal conductivity of carbon steel (mild steel) is almost triple that of 304 stainless, so stainless will lose alot less heat for each unit thickness.. On top of that, there are thermal coatings available that can be applied to either material to further reduce their thermal conductivity.

 

First of all thanks for your input. It is interesting to know stainless steel has better thermal management properties than mild steel. I was thinking the other way around.

Would love to hear if anyone has had any experiences with them to see how well they hold or how they perform.

 

When I was looking for a DP (14 years ago), the quality units were all stainless steel. The cheap stuff was mild steel. For durability and corrosion resistance, stainless has always been the way to go.

 

Have what I believe is a Bonez or an old M2 mild steel. At least 15 yrs old. Never had it commercially coated but painted with DEI sealant and header wrapped. Still going strong.

 

Yeah...'back in the day' there were more stainless options for DP and other exhaust pipes.
I've got a Pettit DP and high flow cat, all 3" diameter and all stainless, plus a stainless Racing Beat exhaust.
ONE thing that is indisputable...stainless resists corrosion better than non-stainless steel.

 

+1 for stainless; it is the material of choice for exhaust, which is a hot and corrosive atmosphere. Why would anyone use anything else, really? Take to heart everything that F1Eng wrote above, which is, in fact, correct.

Worth mentioning, I once made a downpipe from a FD precat assy using some heavy mild steel tubing I happened to have on hand and while it worked ok, it didn't last for ****--it was almost all gone internally after about 5 years of continuous use (it was wrapped). I also should note, however, that aluminized mild steel tube may produce acceptable results and is widely used in oem exhaust systems.

Finally, I have seen quite a few FD downpipes that are S/S tube but have mild steel flanges--I don't see much downside to this practice and expect these units should be fine.

 

Hey...the car came with it and it just won't wear out.

Just don't forget to look for good thick flanges and the downstream ear for the support bracket.

 

Im no engineer, but here's my take;
Mild pro's;
* better sound absorbtion - less tinny
* much less thermal expansion - it doesnt move and warp anywhere near like SS does.
* less resistant to cracking due to better heat dissipation and being more maleable.
* Very durable in the face of repeated heat cycles
* cheaper to buy, easier to work with
Cons - it rusts,
- Much more conductive of heat - theoretical energy loss (negligible effect on performance IMO)
but both points can be remedied with paint and wrap.

Stainless pros;
* doesnt rust.
* looks pretty when new and shiny
* much better heat retention

Cons;
* prone to cracking if bad design/welding/wrong grade due to thermal expansion and increased brittleness over heat cycles
* can be a bit louder but wrap will fix that.

Its a bit of either or IMO they are both good.

 

Thanks all, so SS is the way to go

 

Meh...OK. Unless it's cheaply made. And you mentioned a midpipe in your title. I wouldn't get one of those at all.

 

Have any of you check out the downpipe mid pipe combo from RX7store? I think they want $200 for everything, what do you guys think of that?