I just ordered an A/F gauge partly because my last dyno numbers showed that I was running very lean, partly because eventually I am probably going to go FI or nitrous an I think that it will come in handy and mainly because I think that it will enhance my understanding of the engine.

Now for the stupid question, does the sensor go before or after the cat? Seems to me that it would be after since that's where the stock sensor is located, but I seem to remember someone saying that it is more accurate to take readings prior to the cats.

there are by default 4 sensors. (i am talking from an '05)
There is an A/F sensor on each header (prior to the cats)
and there is a Heated o2 sensor on each cat after the elements

If you want to know A/F it would absolutely go prior to the cats

You'll add a bung to the top of the cat BUT good luck welding one in. I went with the Crawford cats so I would have room to work with. The stockers offer no extra pipe on top.

I've got the Crawfords. I think that I'll probably use the passenger side cat since it seems to be easier to get in and out.

Yep thats what I used

Before the cats. The cats will mess up the A/F readings.

Some of the turbo F/I kits will have an A/F probe bung prefabricated at a location before the cats.

I agree with SickOne, befoe the cats.

i'm curious, but where would you get the bung welding done... would any shop that works on exhausts be able to do it?

you could also tap the existing the A/F sensor and get a reading

Barr's Mufflers, just down the road from my house can do it.

Oh, except you are in NYC, and I'm in Hawaii. That might be a bit long to get here to get something like that done...

Any shop that does exhaust work should be able to do it, but many might not, as it involves work on the exhaust before the cats, which some shops are leery of due to federal emissions laws. Best bet is to call around and ask at shops close to you and ask them if they will do it.

Dave

You can do that?

How then would you compare a A/F result from a Dyno that could be used for a Techosquare ECU adjustment, to the reading from the wide band sensor?

To me, it would make more sense, to mount the AEM wide band O2 sensor after the cats in the y-pipe, so that I could have the average O2 from both banks of the engine, rather than one side and be able to compare the live readings to the Dyno results chart.

Please free to correct and educate me, if neccesary

G35_6MT...

for that purpose, it might be better to have a live 02 reading...
But if you want to know actual air/fuel you'd want it prior to the cats.
The readings from the cats are gonna vary (I would think) they certainly changed based on how hot the cats are.

roneski: I stumbled accross it in the service manual. Don't have time to go dig it up right now. But the stock A/F sensors feed back to the ECM(ECU) creating a closed loop control, the 02 sensors after tha cats are a fail safe incase things get out of wack

if memory serves, the A/F sensor puts out a reading up to 1.5 volts (which was 15:1 I think - but it is not linear) they gave the scale in the manual.
So it would seem fairly easy to tap the sensor output at the ECM, (with a high impedance meter) and get those readings. It also likely serves as a way to slightly alter the A/F depending on how the reading / sensor is done.