Originally Posted by JamesM3M5
304 SS is fine to use, provided the welder uses proper welding techniques. 409 has a higher iron content which makes it tougher, but it corrodes more than 304. However, 304 can crack over time, especially at the welds.
But otherwise, the primary cats are more restrictive than the secondaries. I'd rather remove the stock DPs than take the time/money to remove secondary cats as far as power upgrades go.
Thanks a lot for clearing that up.
Finding it hard to get 409 actually in the UK, maybe 304 will be easier