I pulled the pipes loose on my '91 300SE, and thought I'd look
down into them to see if I could check the condition of what I thought were the little pre-cats. These are bulges in the pipes about the size of a short loaf of French bread, about 1 foot from the manifold. Corvettes have similar pre-cats, so I expected to see the typical converter brick, the honeycomb ceramic.

Well, to my surprise there was no converter brick in them at all!
All I saw was a funnel-shaped metal piece with several holes in the sides about 10mm in diameter.
I could not see all the way through due to the bend in the pipe, but I took a piece of half-inch hose, and stuck it right through the pipe. i was concerned at first that I had lost all the "brick", but a closer look convinced me these were just not shaped right to hold a cat brick.

So, these are not cats at all. They must be a type of resonance chamber or muffler, they kinda looked like the inner pipe of an old "smitty" glasspack, with all the holes.

now I know how they managed to put the O2 sensor downstream of these "cats" and still get proper readings.

Well, at least I feel good knowing I have only one cat instead of three.

Anyone else ever seen inside one of these?