Ive had 4 cracks in my Exhaust and am wondering if its metal Fatigue. And which exhaust to buy? i had a crack right after the header, one right before the cat, right after the cat, and where the pipe hits the muffler can. any help is appereciated

 

Which year & model? How many miles?

I think I've read here before that the easly turbo's had some problems w. manifild cracking; the na exhausts seem to be pretty durable, at least to over 100K miles, except for the muffler exit tip.

Jim, more info needed... and more coffee...

 

We are very lucky with the quality of the stainless steel used by the Porsche factory in our exhaust systems. I have a hard time believing "metal fatigue" is a factor - unless something is out of alignment. Have you had your motor mounts inspected? A collapsed mount would encourage torque on your exhaust system, and problems might occur in your drivetrain. Just a thought....
it might save you rewelding your exhaust (which is best removed from the car to avoid damaging your very expensive DME !)

 

I think RC is right on about a bad motor mount allowing the engine to shift or torque up.However stainless steel in very prone to work hardening and cracking at welds.

 

Exhausts doe not normally get that hot & stressed that stainless AISI 304 will suffer Fatique problems, specially near the muffler were everything is reasonably cold.
Vibration must be the cause of your problem.
You can have the cracks welded, just find a shop with a TIG welder and make them use backing gas (fill up the pipe with inert gas to prevent corrosion of the weld from the inside).
Drill a small hole at the end of the crack (size 1/8", center at the end if the crack)and clean out the pipe in the welding area on both in- and outside.
Have the welding shop pickle the weld with acid when done.
Good luck