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Originally Posted by ROLLMODEL
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Originally Posted by Smokin
Why did you choose to use mild steel instead of stainless?
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Stainless is not the be-all, end-all of performance materials. Stainless expands and contracts at 5 times the rate of mild steel. Keep cycling a material like that and it is prone to crack over time. Many people use mild steel in turbo applications because of this. Durability. It doesnt look pretty, but it gets the job done, and done well. |
Roll Model, hey my man! What's happening?
The reason I like mild steel over stainless to work with is two-fold:
1. Generally, it's easier to cut/drill/etc...
2. Do you see those 1/4" wide stripes? I love to use those to doublecheck the degree of squareness of a proposed cut. When I was first starting out, it was fairly easy to determine the angle on the radius to cut. I got used to that and was a little worried when I got the first piece of stainless and there was no stripe.
Mild steel is a better material that stainless for flanges only. (They can be nickel plated to prevent corrosion.) ==> The reason mild is better for flanges is because it does not have memory to the degree stainless does. So once they are clamped into the final position and heat cycled, the mild steel will just conform to that shape. A stainless flange could possibly warp at the sealing surface if the memory is such to want to pull it that way.
With that being said, stainless may expand and contract more than mild steel, but it's metalurgy allows it to do so without a problem.
There are a lot of opinions out there, but I think we need to separate fact from fiction.
I have heard people say something like," I have welded this stainless Supra manifold 4 times this year so far..... all the mild steel ones we have made this year have not cracked yet!"
The main reason any exhaust system will crack from expanding and contracting is really a threefold proposition:
1. First and foremost; the design of the system==> If an exhaust manifold(the headers and collector) supports the weight of the turbo and exhaust, it WILL CRACK NO MATTER WHAT unless it is made out thick enough material for the distance away from the collector and to accomodate the particular design.
==> In a nutshell, the weight bearing aspect is the biggest reason even the best materials and welding techniques will crack.
2. The welding technique and mode comes into play as well.
Technique==> When something is TIG welded with low amperage(low heat), with the proper filler rod and proper penetration(the women will understand this part quite well,{JGK's wife less than most others-Jay, just kidding, I couldn't resist!LOL!}), the weld has been done very well.
What I have just discussed is for the outside of the weld. Now the other thing we can usually see is the inside of the weld which is at least as important as the outside.
I will define TIG for the people new to welding:==> The TIG, Tungsten Inert Gas is basically a tungsten tip on the torch with inert shielding gas flowing over the tungsten tip and the weld puddle while the amperage is applied. The inert gas keeps the weld from being contaminated by oxygen. The integrity of the weld is definitely compromised if contaminated by oxygen. Now, the outside of the tubes being welded together is certainly shielded by the gas, but the inside of the tube (where oxygen is free to flow into) allows for oxygen to contaminate the weld making it weaker PERIOD.
There is a technique called back purging which essentially caps off both ends of the tube being welded and pumps a secondary line(split off from the shielding gas going to the torch)through one of the caps filling the space with shielding gas thereby displacing the oxygen that will contaminate the weld.
Nutshell: Back-purging is essential to maximizing the strength of each and every weld in the system.
Mode: Some people MIG weld instead. Although TIG is generally regarded as the premier mode for exhaust headers and such, some people believe MIG(Metal Inert Gas){Thanks WV98GTS for the correction} is better for welding frames and exhaust parts as the bead is thicker and some people believe therefore beefier. MIG is much easier as you just set the amperage(heat) and wire speed and off you go. Pull the trigger on the torch and the wire just comes out. With TIG, you have the torch in one hand, the apropriate filler rod in the other hand modulating the amperage with the footpedal. TIG for me is easier when welding different thicknesses of metals as you can add more heat on the thicker mass and either move the puddle to the thinner metal or back off the heat when you get to the thinner metal. with MIG, I just seem to blow holes in the thinner material. I have to take a welding class to get more familiar with MIG welding. I like TIG better.
3. Flex pipes
The other main reason exhaust systems crack is due to not having the proper flex joints at the right place in the exhaust system.
==> Any exhaust system WILL expand and contract as it heats and cools.
Seeing and designing the system as a whole allowing everything to move, expand and contract WITHOUT stressing the welds is the key to the longevity of any system period.
I would also like to add that stainless steel is much better than mild steel in allowing heat to cross the barrier from outside to inside or vice versa.
So, all other things being equal(both either coated or noncoated), the stainless will keep the heat in the exhaust{ie more kinetic energy to spool the turbos} much better than mild steel.
Also from a radiant heat perspective, again, all other things being equal as far as coatings and heat shields are concerned, the stainless system will result in a cooler engine compartment==> More power, cooler running car, less chance for all your wiring harnesses and brake lines to get burned up.....How many people here would be interested in having that???? That's what I thought! [img]/images/graemlins/laughing.gif[/img]
I would like apologize to the people who already know all this as this post was primarily to educate the people who do not know much about welding and exhaust metalurgy.
I hope what I have learned and incorporated in my twin turbo system and will be implementing in future single applications gives future customers for all Viper turbo tuners a base knowledge of what is technically correct practically functional in the real world application for their Viper. :thumb:
