Factory OBD2 cats: A closer look

[m3f_Bimma360]

My current exhaust setup consists of Raceland euro headers, foler SS rep midsection w/ magnaflow metal core cats, and a euro spec SS performance exhaust. Unfortunately, the magnaflow cats just don't do a good enough job and the exhaust smell is still pretty significant. They are also 200cpsi, and definitely not as long as factory units, so the smell makes sense.



Since I saved my factory OBD2 cats, I decided that I was going to just replace the magnaflows with them. Today I started to prep them to integrate into the current system as cleanly as possible. I removed the baffles and trimmed them. Since I don't think there are too many pictures online of the factory cats without baffles, I figured I snap some pics to post. I also took the liberty to do my best to measure the cell count.



There are apparently 2 catalyst sections in each cat. The 1st has a smaller diameter (I will measure later) and is about 200cpsi, which then tapers up to larger catalyst that looks like 400cpsi. Notice how, at even 180k, the rear section of the catalyst is super clean. Really shows how long these can last with a good running motor. I'm sure it also helped that the car was an automatic the first 160k of its life haha.

[m3f_Braymond141]

Thanks for posting. Good to know they are semi normal shaped under the heat shields. Looks like you could make a rather large diameter exhaust system too. Similar to what I did on the front half of mine but I retained the tiny second half.

[m3f_T3AMFiSh]

So how many cells are the stock cats?

[m3f_Braymond141]

So how many cells are the stock cats?

He has that stated in the first post;

There are apparently 2 catalyst sections in each cat. The 1st has a smaller diameter (I will measure later) and is about 200cpsi, which then tapers up to larger catalyst that looks like 400cpsi.

[m3f_Bimma360]

Thanks for posting. Good to know they are semi normal shaped under the heat shields. Looks like you could make a rather large diameter exhaust system too. Similar to what I did on the front half of mine but I retained the tiny second half.

Yeah definitely not too bad. It's super interesting that they have two different sized catalysts as well, both in diameter and the amount of cells. I am not sure of the overall length and diameter of the US e46 M3 cats (I'd bet they are a tad bigger), but are also 400cpsi. I really don't see why the factory S52 cats wouldn't be more than ample for even a built NA S52.

[m3f_Braymond141]

Yeah definitely not too bad. It's super interesting that they have two different sized catalysts as well, both in diameter and the amount of cells. I am not sure of the overall length and diameter of the US e46 M3 cats (I'd bet they are a tad bigger), but are also 400cpsi. I really don't see why the factory S52 cats wouldn't be more than ample for even a built NA S52.

I wonder if the meshing on the two catalyst creates a denser overall caralyst. The 200cell cutting the 400cell in halve no longer gives you 400 cell. Probably reduces exhaust gas speed too unlike an easy straight shot honey comb to pass through.

[m3f_Bimma360]

I am not really sure. What we know is that 200 cells flows better than 400 cells. But in this case the 200 cell substrate has a smaller diameter than the 400 cell substrate. I am pretty certain that the area that tapers up to the bigger catalyst is dead space, so the exhaust gasses do have an opportunity to expand into that second substrate.


In the very brief time I cared to spend looking this up, it looks like some catalysts actually have a two substrate system. The first, which in this case would be the smaller diameter 200 cell one, performs the "oxidation" activity, and the second (400 cell one), performs the "reduction" activity. So it is making a bit more sense now. What this design does for overall flow, beats me. I imagine that at a certain diameter a 200 cell will flow the same as a 400 cell. So I suppose, the question is if thats what they did here.

[m3f_tsmith25]

Thanks for posting. Good to know they are semi normal shaped under the heat shields. Looks like you could make a rather large diameter exhaust system too. Similar to what I did on the front half of mine but I retained the tiny second half.

What diameter tubing did you use, and what manifold/header are you using?

It is similar to how mine currently is with raceland euro headers then it has flanges with 2.25 between the flanges and cats similar to that.