Tjabo

I know it's an odd question, but is there anything that can be learned about the AFRs if you have your sensor placed like this? I assume not. . .

srmofo

you can learn if the cat conv is operating effienctly....but who cares

Tjabo

What kind of reading would you see?

kyden

lean, which would mean the cat is working.

blkb18

A wide band sensor after the cat is a gross waste of money, unless your a superhippie treehugger. Even new model cars with air fuel sensors still use oxygen sensors after the converter.

Tjabo

I'm not really looking for any judgments, I'm just curious what the readings would be like, and what if anything you could learn if you had it set up like this.

blkb18

Im just kidding, Im not judging you. It would not show you any valuable information other than how well your convertor is working. Any valuable info on how well tuned your motor is gonna come from a wideband properly mounted in the manifold after the collector and BERORE the convertor. A convertor does exactly what it says, it converts gases into other gases and water. That being said, you would be doing yourself a disservice by mounting it after the cat. Why monitor an altered exhaust stream? You could run one before and one after, but that gets us back to that whole wasteing money thing (unless your a supperhippie treehugger) Just kidding again.

Tjabo

Oh, I'm good with you calling me names like "superhippie treehugger", or whatever, goofing around is always good!

My point was just that I wanted to really find out if anyone had actually tried this to see what happened, and I wasn't interested in judgments like "wasted money" or something.

Just to be clear, this is not something I've ever thought was a good idea, for the OBVIOUS reasons. However, recently it seems that I may have run across a couple of threads here and there where it looks like dyno shops had "measured the AFR" of cars by simply sticking a sniffer up the tailpipe of a car that had catalytic converter(s) on it. One way I noticed that things looked goofy was that it was too lean at WOT, but still followed kind of a normal (albeit too lean) AFR curve.

I've always thought that this would render useless information/no information, but then I started wondering. . . It doesn't seem like the measurement of the exhaust gasses after the converter would be consistent if AFR varies considerably, so I was wondering what the measurements would actually look like.

Now, for the confession part. . . . . My 540i has a V-8, and a converter on each side, prior to a Y-pipe where the two sides merge. I was thinking of a couple of ways to do some tuning, and I was weighing out my options for checking my AFRs. I could take out the converters for long enough to do my tests (heaven forbid I would leave them out of there for longer than necessary with me being a supertreehugger and all! LOL.), and that way I could use a single wideband after the merge.

OR, I could mount the wideband separately in the two banks ahead of the converters and take two sets of measurements (maybe both banks will turn out to be essentially the same, in which case it won't be an issue). Orrrrrrr, the expensive and therefore unacceptable option is to setup with dual widebands, one in each bank at the same time.

The other problem with having the converters out is that the car is already rather loud with them in, and would undoubtedly be louder with them out. Here is video to show current loudness:

http://www.youtube.com/watch?v=2pBRFg2BThQ

I'm thinking of using an NGK AFX this time, and I don't think they make one with dual sensors, so I'm not aware of any middle ground like that. . . .

Mainly though, I know there are a lot of guys on this F/I forum who spend a LOT of time dynoing, so I was wondering if any of them may have run across some data like this by accident or whatever?

Tjabo

I ran across another thread where it became apparent that the dyno operator had simply stuck the AFR sniffer up the tailpipe of the car after the cats. . . I am just really curious whether there is any usable information to be gained this way? About a third of the way down this page there is a dyno chart from a 540i, that turns out to have an AFR trace from a sniffer after cat. Can anything be learned from that reading?:

http://forums.bimmerforums.com/forum...1221704&page=3

Here's an interesting page that seems like it might have some information indicating maybe the after cat readings would appear leaner as kyden said above, but I'm not sure I'm interpreting it correctly:

http://www.troublecodes.net/articles/catfailure/

Anyone have any first hand information/experience, or any well thought out comments about this?

Thanks in advance as always!

kyden

i doubt it. if the cat is in good working order, depending on the wideband, it might just read AIR and not even give a value.

it WILL read leaner. end of story. its what a cat is designed to do.

Tjabo

I just happened to be talking with a tuner about an hour west of me, and he was very sure that the downstream location can give some useful information, albeit that he said his Dynojet tailpipe sniffer ordinarily reads about .2 AFR leaner than an upstream wideband. . . I suppose this is because, as kyden has alluded to above, the converter is doing what it's designed to do, which should ultimately result in additional O2 in the exhaust after the oxidation stage I guess.

Interesting stuff.

Tjabo

There's some more interesting stuff in this thread:

http://efi101.com/forum/viewtopic.ph...af93cd5b14742c

Looks like a sniffer after the cat for a dyno run might actually not be so useless after all. . .

HamiltonRex

It should read the same.. Oxygen sensors measure the amount of oxygen.. the amount of oxygen flowing though your exhaust system at any given place is the same, unless there are leaks.

One at the tailpipe vs at the downpipe will be less responsive when it comes to realtime data.

blkb18

http://auto.howstuffworks.com/catalytic-converter2.htm

Tjabo

Exactly:

The reduction catalyst is the first stage of the catalytic converter. It uses platinum and rhodium to help reduce the NOx emissions. When an NO or NO2 molecule contacts the catalyst, the catalyst rips the nitrogen atom out of the molecule and holds on to it, freeing the oxygen in the form of O2. The nitrogen atoms bond with other nitrogen atoms that are also stuck to the catalyst, forming N2. For example:

2NO => N2 + O2 or 2NO2 => N2 + 2O2


The oxidation catalyst is the second stage of the catalytic converter. It reduces the unburned hydrocarbons and carbon monoxide by burning (oxidizing) them over a platinum and palladium catalyst. This catalyst aids the reaction of the CO and hydrocarbons with the remaining oxygen in the exhaust gas. For example:

2CO + O2 => 2CO2

But if the excess fuel and flow overwhelm the process as described in that last thread I linked to, it could have very little chance to function (only effective at stoich).