I posted this in FI, so sorry to post it again, but this is where it should be. Looking for feedback from people who actually have experience with both. If possible, please post more than just opinion, ie quantify, share experiences of durability, results of testing both, applicable metallurgical and thermal properties, etc.

I'm trying to decide between mild steel vs stainless steel turbo manifold flanges, and also 3/8" vs 1/2" thick.

In several places I seem to read mild steel for some reason is stronger or more resistant to thermal expansion, so is 1/2" thick. Just spoke with Sam at bmcRace and he said the same thing, and referenced the tuner Jason (aka st00pid), as that is his opinion as well.

Casey at burns stainless however, said he recommends 3/8" and stainless.

So I'm getting mixed opinions, what are the facts? What are the pros and cons of mild vs stainless, and 1/2" vs 3/8" for turbo manifold flanges?

Can 3/8" mild steel be reliable long term for a street use turbo manifold? Does anyone here make cuts to separate the flanges for each exhaust port to help prevent problems from expansion?

 

Did Casey recommend 3/8 ss for a boosted application or for NA?

I know he loves his stainless but in this case 1/2 mild steel is more cost effective and has less risk of creating a problem down the road. Unless you are very very against surface rust of any kind just get the mild steel.

While both sides can argue back and forth about which is better the simple truth of it is this.

Both work if done correctly...

...but mild will work even if it's not done exactly correctly so you can't really F it up...and you know it's cheaper.

 

i've done manifolds with 3/8" and 1/2" mild steel flanges... i don't really see much of a difference. it also depends on what kind of manifold you are building.... personally i don't think anything thicker than 3/8" is necassary but everyone will have a different opinion on it. matching base metals is nice if you go all stainless but if you use 309L filler then you can mix ms and ss without an issue... also be sure to weld the inside of the runners where they meet the head flange and turbo flange just for added strength. if you do get stainless flanges try to get 304L as they will warp less than other grades of stainless but will not be as strong structurally but i think the difference is negligible. regardless of how you build your manifold it is going to warp when you weld it together if you use ms or ss, when you heat cycle it the first couple times you drive your car just tighten down the flange and it will flatten out. i put on a 1/2" flanged manifold that was warped about 1/8" and took it off at the end of a summer of driving and it was dead flat.

 

I'd say 1/2", in case it ever warps and you need to have it milled. My mani was 1/2", and was warped terribly. The machine shop took off 1/8", and its straight again. The PO apparently never checked the stud torque, and it got twisted really badly. If the flange had been 3/8", I'd have to toss the mani.

1/2" Steel.

 

Depends on what type of stainless I guess. But the high Nickel variety does grow more under temp, than mild steel. Stainless is great for high temps if it is taking a lot of load, like weight support under high temp, bolts for example.