coffeegoat
Adventurer
Alrighty - Update time
I got the exhaust manifolds back from the muffler shop here's the results:
I chatted with the guy that does the work and the basic method was pretty straightforward and they've had good success with it in the past.
The starting point is they can only weld cast steel parts and not cast iron (apparently most Japanese cars are cast steel). The difference is the amount of carbon in the metal, as soon as you exceed 2.0% carbon in the metal it is classified as cast iron rather than cast steel. A good overview of the differences can be found here. In order to weld the cast steel the guys v-grind out the crack and then heat the entire part up to cherry red hot (couple of torches) before they hit it with the mig. This is the shop equivalent of doing a few things metallurgically - first grinding out a crack is vitally important, if you don't you end up with a stress concentration right at the base of your weld - which cases it to re-crack basically immediately - grinding it out reduces this possibility. Secondly, when you weld as the metal solidifies and cools it contracts, this results in distortion in parts that can move - or tremendous stress if the part is rigid enough (like a cast manifold) that it can't distort it introduces a ton of stress into the part, which can cause it to fail later. The worst case is called solidification cracking where the part cracks before it even cools down to room temp - as the heat is pulled out of the weld by the rest of the part the solidification process is weak near the center and it will crack wide open during cooling (audibly in fact). By preheating the part you dramatically slow the cooling process, increase weld penetration, and decrease the stress on the weld, making it more likely to last. They welded the shorter crack successfully in one try, the second wasn't preheated enough and re-cracked on cooling, so they had to do it again and get it hotter. Second time around you also get a bit more weld metal alloying which helps avoid cracking later.
I think a better solution in the long term is to furnace or torch braze the part because it will automatically wick to the end of the crack to avoid stress concentrations, it's less molten metal so the stress is reduced, and it's ductile enough that it won't crack afterwards. But until I have an oxy-fuel torch and another cracked manifold I won't be able to try it.
On buying new units I came up without a ton of new info, the people I called in the US about the OEM manifolds didn't know if we are getting welded manifolds from the factory now and the imports from Australia were outrageously expensive. To handle the 3-cat systems you need a "Exhaust Manifold Replacement," part number A349MR (which is stainless) and it'll run you roughly $550 plus another $75 for shipping. It doesn't sound like a group buy will save anything either....
Anyways, I was pretty pleased with the work so far, we'll see how they mate up and how they last, if they crack again I'll update the thread.
Shop in question was AA Performance Muffler & Brokers in Englewood, CO, price ended up around $50. Seemed like a great shop, I'll be looking there when I go to replace my suitcase muffler.
I got the exhaust manifolds back from the muffler shop here's the results:
I chatted with the guy that does the work and the basic method was pretty straightforward and they've had good success with it in the past.
The starting point is they can only weld cast steel parts and not cast iron (apparently most Japanese cars are cast steel). The difference is the amount of carbon in the metal, as soon as you exceed 2.0% carbon in the metal it is classified as cast iron rather than cast steel. A good overview of the differences can be found here. In order to weld the cast steel the guys v-grind out the crack and then heat the entire part up to cherry red hot (couple of torches) before they hit it with the mig. This is the shop equivalent of doing a few things metallurgically - first grinding out a crack is vitally important, if you don't you end up with a stress concentration right at the base of your weld - which cases it to re-crack basically immediately - grinding it out reduces this possibility. Secondly, when you weld as the metal solidifies and cools it contracts, this results in distortion in parts that can move - or tremendous stress if the part is rigid enough (like a cast manifold) that it can't distort it introduces a ton of stress into the part, which can cause it to fail later. The worst case is called solidification cracking where the part cracks before it even cools down to room temp - as the heat is pulled out of the weld by the rest of the part the solidification process is weak near the center and it will crack wide open during cooling (audibly in fact). By preheating the part you dramatically slow the cooling process, increase weld penetration, and decrease the stress on the weld, making it more likely to last. They welded the shorter crack successfully in one try, the second wasn't preheated enough and re-cracked on cooling, so they had to do it again and get it hotter. Second time around you also get a bit more weld metal alloying which helps avoid cracking later.
I think a better solution in the long term is to furnace or torch braze the part because it will automatically wick to the end of the crack to avoid stress concentrations, it's less molten metal so the stress is reduced, and it's ductile enough that it won't crack afterwards. But until I have an oxy-fuel torch and another cracked manifold I won't be able to try it.
On buying new units I came up without a ton of new info, the people I called in the US about the OEM manifolds didn't know if we are getting welded manifolds from the factory now and the imports from Australia were outrageously expensive. To handle the 3-cat systems you need a "Exhaust Manifold Replacement," part number A349MR (which is stainless) and it'll run you roughly $550 plus another $75 for shipping. It doesn't sound like a group buy will save anything either....
Anyways, I was pretty pleased with the work so far, we'll see how they mate up and how they last, if they crack again I'll update the thread.
Shop in question was AA Performance Muffler & Brokers in Englewood, CO, price ended up around $50. Seemed like a great shop, I'll be looking there when I go to replace my suitcase muffler.
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