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Originally Posted by DHK
Paolo, no question 321 is "the good stuff" and can be made bullet proof. All I know is from what I ran and had built -- i.e. from direct experience. The afformentioned 321 manifold was from an A&P airframe certified outfit out at Troutdale airport in Oregon who does that kind of stuff -- but it still wouldn't hold up. EGT's were running from 800 to 1100 degrees (used water injection). The manifold was TIG welded from Burns tube sections, and as overkill it was welded up in a sulpher hexaflouride tank and packed with that expensive white paste to boot (whatever it is). It was a thing of beauty, but after 6 months it cracked anyway... The guy who did it had fantastic welds -- it fractured along a straight section of tubing, not a weld. I was told that in practice stainless would expand much more than mild steel as the mild steel would sink the heat to the heads and therefore wouldn't reach as high a temperature. But, I chose 321 as it seemed the best at the time (I thought it would be much better than 304 -- which failed). In retrospect, other guys who were cutting up and altering cast iron manifolds didn't have these cracking problems. But, their manifolds looked like a mangled mess worthy of a kitchen garbage disposal -- visually very ugly.
There seems to be two schools of thought when it comes to exhaust manifolds... #1 Use stainless. #2 Use mild steel. Both sides have very good points and both types are currently made. Turbo manifolds are different in that the temps will be higher. This is why I chose stainless alloys to begin with! However, depending on the geometry of the manifold and whether or not slip-joints or bellows can be used, mild steel may end up being more durable. I found out the hard way, not once but twice, that stainless wasn't the best material for the manifold shape I needed. If I had to do all this over again, I'd get myself an infrared camera and check the manifold for hot spots. No doubt, it was a high turbulence hot spot just after a junction which failed. Had I spotted that, I probably would have ceramic coated the inside of the manifold and just run the risk of a few particles of that stuff going into the turbine... But, that was before IR cameras and thermometers were readily available. Heck, now-a-days you can get an IR thermometer with a built in laser pointer for something like $70. As a digression, I went through 2 turbo CHRA's because of particulates mangling the turbine. The first manifold, made out of 304 stainless, had rough spots inside where a dremel just couldn't reach. Some of that stuff burnt off, hit the turbine and threw the rotating assembly out of balance. Talk about weirds sounds! That and smoke from oil going out the exhaust...
Paolo -- I'm sure you'll have trouble-free results... The Viper block is way longer and there's much more elbow room under the hood. Slip joints and bellows won't be a problem. The tubing is much larger in diameter to boot, so the insides can be smoothed out. The gotcha's for me were high thermal stresses -- everything was packed into 3/4 of a square foot including turbo. When heat cycled as a daily driver (3 to 4 start/stop cycles a day) both 304 and 321 alloys failed within 6 months. I used the very best stuff I could get my hands on and spared no expense -- yet I was humbled by the backyard mechanics and their mangled cast iron turbo manifolds. Well -- I was humbled until I got a Viper that is ;-)
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DHK, Your real world experience is very informative and interesting. That is too bad that the physical displacement of what you had to work with led to limited ways to designing your setup that failed. The high turbulence hot spot is definitely going to cause problems. I remember initially thinking about doing a log style manifold for the Viper and was advised against it after several people advised me that having such a short primary would cause such a restriction that the heat generated by not having at least 6-8 inches before the log(Butthead," UUHH, he said..... 'LOG'") could possibly burn an exhaust valve. It was after this that I really got into designing a set of equal length exhaust manifolds with a double slip fit 5 into one burns collector whose primary tubes have beautiful sweeping radii for minimal restriction. Well, enough of my system on another's thread.
Sorry your high tech setup did not work, but there's hope for you to pick from the many growing possibilities of turbo systems that have been and are continuing to be developed for the Viper. You are obviously a man who knows his stuff and I am sure you will be making the best decision for yourself the your desired application. I am glad I cut and pasted the article off the Burns site and started the discussions about the qualities, properties and differences between 321,304, and mild steel on my thread. Take care and good luck! :thumb: