Im curious since I have seen a few people saw that it was inaccurate. Is the probe in the end of the exhaust not accurate with and without a cat (is there a difference)?

Also, can you just unscrew the rear O2 sensor and plug in the probe there? Or will that cause issues with the sensor being out of the stream.

 

If you have a cat then yes, it'll be different. If you don't it should be ok.

If you pull out the 02 sensor you'll get a CEL. I'm not sure if the second one is only for emissions, if it is, then your cars performance will be unaffected.

They say a wideband should be *I think* around 36" downstream of the turbo.

 

Hopefully someone with a good WB has had a tailpipe sniff and can comment. But jumping in with a SWAG, I don't think a cat will make that much difference. You're only interested in high-load conditions so you're running very rich and there's not much a cat can do at that point. What might throw you off more though is that an O2 sensor needs atmospheric O2 as a reference. It doesn't need much so a quick in and out may be OK, but on a long pull, like steady-state measurements, it may not work.

Dave

 

I think the cat has very little effect on the AFR. My guess would be that most people say it's not as "accurate" because the readings will be rather delayed, considering its further from the engine.

 

Moreboost,

I'd recommend you read this http://auto.howstuffworks.com/question66.htm

 

Yeah, the whole idea of a cat is to catalyze the exhaust coming out of the engine and clean it up. Good link bolsen.

So does anyone know how much leaner a post-cat wideband reading will be compared to pre-cat? I remember seeing Shiv write 0.3 on a thread before.

 

Yes, but that catalysis is the oxidation of CO, HC, and H2 by O2 and NOx to spit out CO2, H2O, and N2. A rich exhaust has very little O2 and NOx so there's not much catalysis going on. That's why in closed loop the engine oscillates between slightly rich and slightly lean to make sure there's enough of all the above to let the cat do its thing.

Dave

 

Originally I thought the cat might effect the readings, however I was reading the PLX message board and one of the owners themselves said the cat would not effect the AFR readings...

 

Ok, then let me ask this, if the cat has no effect on a/f why is the first o2 before the cat?

 

OK, you asked for it, you have no idea how detailed and boring I can be. When operating around stoichiometric or bouncing back and forth between slightly rich and slightly lean there's always some unburned bits of hydrocarbon, a little hydrogen, some left-over oxygen, and unwanted nitrogen oxides in the exhaust. When this goes through the cat the good stuff happens and out comes pretty clean carbon dioxide, water, and nitrogen. The O2 sensor before the cat lets the ECU bounce the AFR between slightly rich and lean. If all goes well the one after the cat should always read right around stoichiometric. Just like what you'd expect.

But if you run very rich, there's no leftover oxygen, and very little nitrogen oxides so there's nothing for the cat to do. The exhaust coming out of the cat is practically unchanged and both O2 sensors, and a WB, will read rich. Similarly if you run very lean, there's no unburned hydrocarbon left and the exhaust still comes out lean.

It's only in the region around stoich that the cat can do anything to clean up the exhaust. Especially back in the late carburetor days some cars pumped air into the exhaust before the cat so that even when running rich the cat could clean up. Modern 3-way cats need that rich-lean cycling to remove the nitrogen oxides, they can't do that when there's always excess oxygen. In daily driving you spend most of your time in closed-loop mode and the periods of very rich operation are pretty much a loss from an emissions point of view, though NOx traps in some cats can cover short bursts.

I'm still more concerned about a tail-pipe sniffer tending to read lean the longer it's in place. See there's this thing called a Nerst cell...
Yeah, I'll stop now.

Dave

 

Don't get me wrong, I agree that it has little effect of the extremes. But, the entire point of running a wideband is to get an accurate reading across the board....

I guess what I'm trying to say... is if you have skewed #'s what's the point of running a wideband to begin with? That's like saying... I have a boost gauge installed, but it's got a crack in the hose so it only works at certain times. Yea you have a boost guage, but if it's not accurate or reliable, what are you doing with it????

So back to my original comment, based off my explanation above, if you run a cat, put the o2 sensor in the DP ahead of the cat... not in the tip. If you run without a cat, then you won't have a problem.

 

Oh, yeah? Well I agree more! For the price that you can get a good WB these days there's no reason not to, and to put it before the cat, then you know you're set for anything. But for max power, at least on pump gas, you'll be 11.5:1 or richer and that's in the extreme range where the cat doesn't matter. I'd go so far as to wager a 4pk of Guinness in those funky cans with the nitrogen capsule! Mmmm, Guinness...

Man, the guy who started this thread is probably long gone, but if not, if you're getting a sniff just to make sure you're rich enough you're ok; if you're trying to tune to the edge put a WB where it's supposed to go.

Dave

 

well from my personal experience. i had a wideband welded into my downpipe about 1" before it bolted to my cat. my car was tuned at 11.4 afr going by my wideband in the car. when i put a sniffer on it at the dyno my readings were wayyyyy different. i can't remember exactly what they were and i don't have the dyno graph laying around here, but i want to say they were about 2 points different. more recently i have added a test pipe and been back to the same dyno. my final numbers the second time around had my in-car final afr at 7000rpm at 11.3 and the sniffer recorded a 11.4.