What's the deal with our catalytic converters? 2way, 3way, air?

[AllanS]

I'm piecing together the components for a new 3" exhaust, and I need to buy a new catalytic converter. I'm looking at a Dynomax one, but they list them as 3 way, 2 way and 3 way, and with or without an air tube... what should I choose exactly? Does it matter if I do one over the other? I don't remember seeing an air tube connected to the cat last time I looked, so is this just something to do with other emissions crap like EGR?

Thanks,
Allan

[AllanS]

I guess no one knows then... oh well.

[Unregistered]

Are you installing a single 3" exhaust pipe, or essentially the stock setup?

[AllanS]

I'm replacing the whole thing, muffler/cat/etc. and going with a single 3" stainless steel system.

[Bill R.]

3 way preferably.... and you don't have an air pump so the air tube won't do you any good unless you plan on installing an air pump later... very unlikely.. n2 is nox by the way so if you live in an area that tests for nox it would be best to get a three way...

I'm piecing together the components for a new 3" exhaust, and I need to buy a new catalytic converter. I'm looking at a Dynomax one, but they list them as 3 way, 2 way and 3 way, and with or without an air tube... what should I choose exactly? Does it matter if I do one over the other? I don't remember seeing an air tube connected to the cat last time I looked, so is this just something to do with other emissions crap like EGR?

Thanks,
Allan

[Unregistered]

What advantages do you expect with a 3" single pipe system?

Disadvantages?

Noisier or quieter than stock?

How does its cost compare with conventional stock, Eberspaecher, etc.?

Where are you gonna get your new exhaust components?

I'm asking because I've been thinking along those lines, and don't want to re-invent the wheel or unwisely repeat mistakes of others.

[AllanS]

Thanks Bill, that was exactly what I was looking for!

Allan

[AllanS]

Well, there's this 3" hole at the rear end of my turbo, so I figured, why bother making a funky Ypipe for an already shot exhaust system, when I could spend $1k and build a new one from scratch?

The components come from JCWhitney (steel tubing and cat) and from Summit (Borla muffler, tips, exhaust bung, etc.) and comes to right about $1k. And you've got to figure in welding costs, which don't apply to me since I'm doing that myself. This is all stainless, so it's going to cost a bit more than aluminized steel tubing. The muffler was a little under 300$ including tips, the cat is 130-150, and the steel makes up the rest.

[Unregistered]

OK, how will your 3" system compare with the stock exhaust sytem in terms of weight and noise reduction? Performance?

Seems to me, it should be much lighter, could be as quiet, and could perform as well or better. I know that lots of folks feel that BMW already has the ultimate exhaust machine on our (stock) cars, but consider BMW motorcycles: Both K-Bikes (4 cylinders, water cooled) and R-Bikes (2 cylinder Boxer engines, oil/air cooled) feed into one pipe, and the system is quiet and efficient, as well as the cleanest of motorcycles from an environmental standpoint. So, dual pipes are not the only accepted theology at BMW.

[AllanS]

Well, it all travels into one pipe at the cat for the o2 sensor, and then back into two, and you have a silencer and a muffler, so i guess it may leave some room for improvement.

In terms of weight, I'm sure that a single 3" exhaust with only one muffler at the rear and the cat would be a bit lighter. Now, If you were to use a couple of2" or 2.5" pipes, and maybe a pair of cats and single inlet/outlet mufflers, you would end up with more flow, but then you'd have a weight penalty, of course. The only reason that I'm doing a 3" system is because it is convenient for the situation. If I was going to replace my exhaust, if everything was stock, and I was doing it myself, I might be persuaded to make my own headers, and do a larger 2 pipe system, but it would be hugely expensive I might imagine.

I'm not especially concerned about noise or performance in the conventional sense, since there's a turbocharger in the mix, but I would think that a single 3" system with a single quality muffler and cat would cause less of a restriction, weigh a bit less, and sound pretty damn loud and annoying.

Allan