quote:Originally posted by PanteraDoug: Beautiful work Kelly.
Thanks Doug, much appreciated.
quote:Considering the recent improvements in the ceramic coatings available, I seriously doubt the need and desirability of building headers in stainless?
Does anyone really need a stainless exhaust system? Just depends on what you want out of them. Certainly more expensive but if offered the choice between a mild and stainless steel exhaust system I can’t imagine why anyone would choose mild steel.
quote:I had mine coated inside and outside and the coater has flow bench numbers that show the headers flowing a lot better coated inside.
Other than cosmetics, there’s really no need to cosmetically coat mild steel headers either. As discussed earlier in this thread, they’re great cosmetic coatings, but after Mark (ehpantera) posted earlier, I revisited a little basic heat transfer and quickly concluded the heat reduction claims of the paint based cosmetic ceramic coatings are greatly exaggerated. As far as flowing better, don’t want to burst your bubble but I seriously doubt there are any material gains to be had. IMO, that’s mostly marketing BS that goes along with the claims of reduced heat transfer equating to higher velocity hot gas flow. Sure, the theoretical argument is sound but in practice is not material. It’s not like a cast iron exhaust manifold. The inside of a mandrel bent tube is already pretty nice surface and even extrude honing won’t do much to the welds as far as flow losses go.
quote:I have seen several stainless sets recently and they turn all sorts of colors in usage.
Virtually all bare metals (at least the ones you would use for an exhaust system) will discolor to varying degrees depending upon the metal and usage temperature.
quote:Cosmetic stainless, 303, 304 always had the reputation of splitting from the heat. I already know it is very difficult to bend without kinks and ripples and it isn't that simple to weld.
Not sure what you mean by 304 being cosmetic stainless. You see it used in a lot of cosmetic applications because in annealed state 304 is fairly easy to cold work as far a stainless steels go. It’s certainly not Inconel or even 321 but it is a true austenitic stainless steel. It’s 18% Chromium and 8% Nickel. Some folks still refer to it as 18/8. I used 304L which is typical for applications that will be welded because of lower carbon content the post weld heat treatment recommended on 304 is not required. As far as weldability, the guy who welded it says it’s very easy to weld compared to say aluminum which tends to puddle and flow all over the place.
The biggest reason for using austenitic stainless (other than avoiding rust) on a performance automotive exhaust system is that it retains its mechanical properties at elevated temperatures and only conducts heat at about 1/3 the rate of mild steel with thermal conductivity 16 versus 43 W/m.K. By way of comparison, Inconel is 14W/m.K in similar temperature range. Tell-ya-what, I’ll put that up against the heat transfer reduction afforded by paint based ceramic coatings on mild steel. I could really tell when I was cutting and using abrasive belt to fit the stainless pieces. You can get a localized area at the tip of the tube red on the belt and you can still touch near the point for quite a while before the heat is conducted up the tube.
Normally aspirated cars are not as hard on exhausts as turbo cars. If this was a turbo header, I would likely have used 321. Mild steel headers are typically short lived in on turbo cars (at least the header to turbine part). Strength at temperature is why lighter gauge material can typically used on stainless exhaust systems. I opted for 16 gauge because the heavier gauge bends more easily in tight radii and the price of bends reflected this. It was only an additional 6 lbs using 16ga vs 18 ga on the primaries.
Bottom line for me on this was given the amount of effort to plan and fabricate, with any reasonable valuation of time, it made sense to use the most durable material. It will handle the heat mechanically and be a very durable life-time exhaust system. It will also significantly reduce heat transfer and yes, will oxidize, but can be repeatedly re-polished if so desired. It’s a matter of taste but I like the look of stainless. Some guys on the GT40 forum wipe their stainless down with WD40 because it produces a more golden oxidation layer similar to Inconel. I haven’t decided how or if I’ll finish the surface.