No, not only is it rather far away from the actual collector...there are cats getting in the way. A good tuner will be able to adjust for that and still get it close, but if you see 12.5 reading on the wideband I wouldn't believe that as the true output.
depends on the style of wideband, i have found the dynojet style sniffer widebands to be off at the tailpipe, but my Autotronic on my dyno is pretty damn accurate through a nice flow cat, the only time i get skewed numbers is through a restrictive cat system
really it depends on your exhaust setup...
single in and dual out? one pipe will read really lean, the other will be nuts on within .1~.2 AFR
you also have to take into account how old the sensor on the dyno is and how well it has been taken care of...
if you dont get the car hot and get all the moisture out of the exhaust, you get a sensor that gets damaged and out of calibration sooner..
if its been there for a while and the operator cant recall the last time they changed it down to within a particular month and year...they probably need a new one.
if you want to really guarantee accuracy.. have them put the wideband in your exhaust bung...
if its not included in the cost, then no big deal...offer them a few more dollars to put it there.. it shouldnt take more than 5 minutes to put it in and about the same to get it out, and thats with nothing more than a floor jack, and a 7/8 open ended wrench.
if they wont do that when you ask them to, go to a different place.
cats dont change the AFR readings....contamination to free air does...
I can prove it over and over on a local dyno...my NGK in a bung, the shops NGK in the tail...
dead nuts on within .1 AFR every time as long as I get it far enough in and away from any possible open air contamination.
most people have a pipe they stick in your exhaust tips...this pipe is typically not long enough on the exit end and does not get in far enough on the exhaust pipe end(just another reason to ask for it to go into your exhaust bungs further up stream.)
if you cant get an AFR reading at idle, then you are too far out or getting too much open air contamination which means your placement is incorrect.
I alway's put my reference wideband in the exhaust and put the dyno wideband in the tailpipe just for verification/failsafe. I have found that my dyno wideband reads about .005-.007 lambda leaner than my reference in the exhaust. This has been tested across a few different car's.
Also it's not a good idea to put an o2 in the exhaust right behind a cat either, not because of the cat's function of burning excess carbon emissions but mainely due to the heat genereated. It will make a wideband go nuts with varying temps across the sensor. This is the main reason to put your wideband before the cat's.
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