legacycentral bbs

Just one, right? At the base of the "Y" on non-turbos and in the downpipe on turbos?

2 on both turbo and n/a as far as i know. turbo in the downpipe and in the midpipe. N/a in the Y and also in the midpipe. (this is the case on both the exhausts i have)

MK

Ah, so that second thing is also a cat and not a resonator?

Thanks.

heh i just asked this question in irc last night, didnt get a response though

I thought the second one was a resonator, but indeed it is a cat. There are some pics of it on my exhaust upgrade page.

The second one on the N/A is definitely a cat. Spent a fun time under it trying to remove that razzing heat shields with vise grips and metal cutters.

Steve

N/a's have one, turbo 2

Mark,

I know my 2.5GT had two.

there are definitely two cats.....again....subaru uses a two part catalyst system. This is on both non-turbo & turbo.

Two part? What does that mean?

Are they not both normal 3-way catalytic converters?

They're still a 3-way converter....just a multi-staged process.

This page gives a nice pic/explanation

http://www.uyseg.org/catalysis/catalytic/cat2new.htm

also....if you look here
http://auto.howstuffworks.com/catalytic-converter2.htm

they show a converter that has both stages in one converter. In our cars the Reduction & Oxidation stages are in separate converters.

I see. Thanks for the explanation, Josh.

So if I were to swap in a catless downpipe, I'd end up increasing my NOx output but HC and CO output would be more or less unaffected? Or would the missing reduction stage result in not enough free oxygen for the oxidation stage?

NOx would probably go up a little.....but by how much.....really couldn't tell you.

If I were going to do an entire exhaust system swap.....I'd just use a cat that has both stages in a single housing.

If you have a previous emissions report....look at it....see where your NOx readings are.....if they're extremely under the limit.....you'd probably be fine without the reduction part....

NOx is high heat. Our cars run richer then most....so IMO NOx isn't as big an issue as CO.

Last time my car was tested, the HC and CO outputs were below the machine's precision. Nitrogen oxides were the only pollutants that registered.

Of course, that was at low load... and I had the leaky intake manifold gasket at the time which probably caused a somewhat lean mixture.

I was just curious from an academic viewpoint; I'm fairly confident that if I had to I would pass testing even without catalytic converters.

I would also do what you describe if I were doing a complete exhaust. Get rid of the restriction in the downpipe and use a high-flow cat in the midpipe.

I unfortunately don't remember that much about my chemistry stuff.....so I really can't tell you how that will actually affect the oxidation stage in terms of enough free oxygen.