Hi Andy
It requires the exhaust section containing the Secondary Cats to be removed, you then need remove the material which is contained within the Cat by one means or the other, I used a giant screw driver and a hammer. Didn't damage the Cat at all, but removed everything within a few minutes.
It depends how easy it is to get into the cat to ensure you remove every little bit of the decat material, which resembles a honeycomb.
Here is a pic of when I gutted a pair of Audi V8 cats, and the material you need to remove, it then leaves the cat totally empty and the exhaust gasses simply flow through the original cat, which now has the restriction removed.
You need to ensure your wearing gloves and something to prevent you inhaling the dust, as this is hazardous and why many people just removed the cat in it's entirety.
The Audi Cat was easy to gut, as it was located at the end of a downpipe section, so was easy to see what you were doing and ensure that every little bit was removed
You should end up with a bin liner full like this. (Note - There are normally some spring looking rings that assist in retaining the honeycomb content in the event that the material starts to break up when in normal use, I'm not sure if Mercedes cats have these, but that's why it's important to be able to see what your doing.
I found it easiest just to get MSL to simply remove the entire cat and replace the section with a straight through section. They do a great job and unless an MOT Tester was very familiar with C63's they wouldn't even spot that the secondary cat had been removed, the quality of MSL's work leaves a perfect finish.
Mine passed it's MOT at Mercedes and they didn't realise it had both secondary CATs removed and it passed the emissions test with no problem.
