legacycentral bbs

I think I know the answer, but wanted to get some additional feedback. Taking readings from a wideband post catalytic converter is not going to give you an accurate reading of the AFR's, correct?

I ask because the shop I plan to use for tuning has indicated in the past they have an O2 setup on their dyno they use for tuning. My concern is that using this reading to tune with will not give accurate readings of the actual AFR. However I may just be over thinking things.

Thoughts?

Thought: watching both pre- and post-cat O2 signals on a scantool will show that, in closed-loop, the post-cat sensor cycles rich to lean, only much more slowly.

From what I read, cars with the ability to monitor the cat, the rear O2 sensor should really stay pretty steady if the cat is working, at least that's what I read.....it could be wrong.

If the cat is not working up to snuff, it would mirror the front O2 sensor change.

If the cats not working, the rear O2 should stay lean. I think? I'm pretty sure it doesn't cycle, though.

A wideband O2 sensor will typically read about 1/2 point leaner than actual if it's installed after a catalytic convertor.

When I worked at the local dyno shop, we always installed a bung on the customer's car in front of the cat, and then used our wideband sensor for the dyno runs. Hopefully your shop will be doing something like that, otherwise I'd be looking for another dyno shop.

JMHO

I had asked Miles @ Fineline the same question. Here is his response:

"The answer is yes, the AFR reading before and after the cat are different. However, not by much, maybe 1-2%. They most likely will use a wideband O2 sensor in the tail pipe unless they are tuning for idle, then they will either use your wideband or install one (if there is a bung for it) as close to the turbo as they can. One more thing to think about, every wide band O2 sensor will read a little bit differently, some by .25% and others up to 1%, which is not a big deal."

SLODRIVE wrote:A wideband O2 sensor will typically read about 1/2 point leaner than actual if it's installed after a catalytic convertor.

When I worked at the local dyno shop, we always installed a bung on the customer's car in front of the cat, and then used our wideband sensor for the dyno runs. Hopefully your shop will be doing something like that, otherwise I'd be looking for another dyno shop.

JMHO

I haven't talked with the shop yet about specifics, so that will be something I will be inquiring about.....and definitely something I'm a little cautious about. I unfortunately don't have too many options here in Houston.

You really want the readings Pre-cat. No telling how used up your cat or cats are used up. Just remember that the cats are used to remove as much unburned fuel. Oh, I get to use my example
Baking soda = Catalytic Converter and Vinegar = Fuel. A drop of Vinegar on Baking Soda will bubble, but eventually there will be no more Baking Soda to bubble. Same thing happens with the cat, just no bubbling effect.

Erm, no.

Read up on the definition of catalyst.

This is the important part: Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself.

Cats don't get used up, they get clogged or burned. But if your motor is kept healthy, a cat will last longer than the iron its housed in.

May I suggest that you get an LC-1 and install it into the first 02 sensor position with its second output configured to use narrowband voltages? The car will run better and your tuner can connect right to it.

I did this recently on a JDM WRX and the tuner was very happy to plug right into the LC1 wires I had left in the glove box (they are two headphone plugs)

C'mon Josh, pony up the dough and get a wideband installed!

ALL the shops here in Portland (not 100% sure with Surgeline, but would be willing to bet) stick the O2 sensor in the tailpipe. However, these O2 sensors are gererally NOT just a cheapy wideband, they are more "sophisticated" and so forth.

My wideband readings, when compared to the tailpipe readings are always different. HOWEVER, its consistantly different, like its always .5 off, and so forth.

So IF you feel you are lean, even if their wideband reads 11.5, just add some fuel in...

Also, invest in some knock ears.

The PLX wideband I have has a headphone jack so I just plug in headphones when tuning. CAUSE YOU CAN STILL KNOCK WHEN 10-1 AFR!!!!!

Ummm....not sure if you guys are talking to me....


I've got a wideband installed, have had one installed for a while. The narrowband output is of no use to me for a stand alone. I can do WB closed loop.

As for the variance, my Zeitronix is about 0.5 AFR high at full throttle compared to the calibrated/fancy WB's, (ie reads 11.5 AFR when it's actually 11.0). Around idle low rpm it's pretty much dead on, or within 0.1 AFR.