[bigdonnyd40]

Hi folks. I come from the 540i forum, where modding is much less active.

Like many of you, I run a modded car and want to go as close to catless in performance without the annoying toxic smell, so I'm considering some catted options. Obviously I could pickup a catted DP, though I thought this was an interesting alternative. I read the following on the F30 forum:

From the BMW Group University Technical Training document for the B58 Engine ST1505 p.53 it states that it is a 3-way catalytic converter with 2 monoliths of different sizes and a total volume of 2.8 liters:

• 1st monolith: 600 cells per square inch, 125 x 98 mm
• 2nd monolith: 400 cells per square inch, 125 x 130 mm

I also note on RealOEM that there are several different part numbers for 340i and a different part number for the M140i all with the B58.

I'm interested in understanding the free area calculations (flow rate) of the monoliths in the stock downpipe and comparing to some of the aftermarket options that are smaller diameter but have lower cell density.

Has anyone considered removing the first 600 CEL monolith in the OEM downpipe? This would effectively result in a 400 CEL cat and could be stage 2 friendly. The only issue is that you'd have to tune out the CEL like the catless guys do as the 400 CEL monolith is behind the second O2 sensor.

Would this pose any issue? Would this be any worse than the Active Autowerks downpipe (which uses a 400 CEL Gesi cat)? I imagine BMW sources a high quality metallic substrate cat for its cars.

[pdxtiger]

If you could somehow do this perfectly it would still be worse because the OEM downpipe is around 3" at the joint with the rest of the exhaust.

[mike@x-ph.com]

You will destroy your stock downpipe, which costs thousands of dollars to get this modification; it's a lot of work, which might not even work by the time you are done. And when its time to sell the car, you must buy a stock downpipe.

It makes more sense to buy a catted downpipe instead; they cost a few hundred dollars. Investing in the active autowerke downpipe would still make more sense if you do not wish to mess with the CEL.

Once it's time to sell the car, put the stock downpipe back on, and sell the aftermarket downpipe to get some of your money back.

[jpowers36]

Just use AA and be done with it.

[mike@x-ph.com]

Originally Posted by jpowers36

Just use AA and be done with it.

+1

[Stag77]

Originally Posted by jpowers36

Just use AA and be done with it.

This. If you're going to deal with a CEL you might as well just go catless.

[Info@mad-us.com]

Go with our 200 cell cat it will outflow all the other cats mentioned.