409 will corrode to an extent while 304 won't. A magnet will stick to 409 and not to 304 if you need help determining which stainless steel an exhaust is made of. Please understand this does not mean 409 is bad. It's still much better than aluminized steel. It's just that it is cheaper than 304 so the price should reflect that. My pypes system is 409.
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Trey
Current ride: 2001 BMW 540iT soon to be manual swapped.
Former rides: 1979 Trans Am WS6: LT1/T56, Kore 3 C5/6 brakes, BMW 18in rims
00 BMW 540i/6: Suspension, wheels, and ACS bits.
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