quote:There are two kinds of stainless that are in general usage for exhaust systems. One is the 409 which is what is now used by Borla and most of the new car manufacturers.
400 series are martensitic steels whereas 300 series are austenitic. “Stainless” is a generalism in reference to resistance to oxidation but generally speaking 400 series alloys are not true stainless steels. They most frequently are used because they are more workable and able to be precipitation hardened after forming to increase mechanical properties. These gains can not typically be preserved at exhaust gas temperatures. They can also be more economical than 300 series alloys. 300 series contain more chromium and nickel which provide better corrosion resistance but cost more.
quote:What I don't like about 304 stainless is it is going to blue from the heat of operation of the engine just like chromed systems do.
304 still oxidizes. It just doesn’t look like rust. Tan is the more usual discoloration for 304.
quote:It will also eventually split or crack through the mandrel bends from the normal use of the engine and the heat cycles the engine puts it through. The term is work harden. Regular mild steel exhaust tubing does not. That makes it better than 304 stainless and in my opinion makes 304 stainless obsolete, with the exception of use for the mufflers, which do not get as hot as the headers do in operation.
I don’t think I can agree with any of that. If we’re talking a normally aspirated street car, a properly designed and fabricated 304 system would be a lifelong system. The only cracking on such an install would come from improper welding by either no shield/purge gas or use of improper filler rod. The only other mode of failure would be from lack of strain relief due to thermally induced growth and stresses. The exhaust temperature cycling tends relieve stresses induced in forming, even in the austenitic 300 series.
Mild/carbon steel exhaust systems, coated or not, are not a superior material to 304 and it’s mode of failure in an exhaust system is usually due to the operating temperature of the system causing carbon to precipitate making it more susceptible to accelerated corrosion. This is especially true in proximity to welds. Exhaust temperatures seriously degrade the mechanical properties of mild/carbon steels and they wouldn’t be used even in moderately stressed applications at these temperatures, but they are very economical, lightly stressed in an exhaust system, practical, and good enough for most folks given the application.
For what it’s worth, a street car isn’t much of a challenge to either material because the exhaust gas temperatures and duration of exposure are nothing compared to the racing environment. The far more difficult exhaust test is turbo installations which need to preserve the heat/velocity of the exhaust gas to be efficient and effective. These systems will operate at much higher temperatures on the street and especially at the track. Great care is spent in the design of high performance turbo systems for strain relief and supporting components such as the turbines because these systems operate at temperatures much closer to those that seriously reduce the mechanical properties of even stainless steels, and they’re made from thin walled material to be as light as possible. FWIW, the choice of materials for pros is typically 321 stainless, other high nickel alloys, or even Titanium.
If you are coating carbon steel because you expect big temperature reductions, you’ll likely be disappointed. Most of that is marketing BS and is the thermal equivalent of bench racing. The popular coatings just aren’t thick enough to have the claimed affects. There are thicker ceramic coatings that are much more effective but they aren’t pretty like Jet Hot and the other popular cosmetic coatings and they are not easily applied. The primary benefit of commonly available ceramic coatings is cosmetic and durability/life due to it limiting the ability of the carbon steel to be exposed to the reducing environment when it is most vulnerable at temperature and these coatings are actually very good at both. In a street car environment you really do get a big increase in service life and they look good to boot. They still won’t outlast a 300 series stainless system.
A 300 series stainless system will typically run cooler than a mild steel system because it is a poorer conductor than mild steel and all heat must conduct through the header walls before it can be transferred to the engine compartment. Initially, heat is transferred primarily by convection from the high velocity exhaust gas, then conducted through the header wall. Once conducted, heat is imparted to the engine compartment by convection and radiation. The best hope for the commercial coatings reducing temperature is reducing radiative heat transfer. Ironically, people like to have the ceramic coatings polished which make them a better emitter of radiative heat transfer.
And BTW, engine compartment temperatures are a strong function of exhaust system surface area and 180s have way more surface area than the typical 4:1 system.
-My 2 cents.
K