Dyno tune AFR from tailpipe on catted car?

[trinity_gt]

At a recent dyno session I went to I noticed the exhaust sniffer for AFR monitoring was placed into the tailpipe of a car with catalytic converters. This got me thinking...

How valid a reading can one get from this? Modern three-way catalysts convert HCs into water and CO2...what is the sniffer actually measuring? HCs? When you go WOT, the cats would be "overwhelmed" but will still convert at least a portion of the HCs, right? So if the tuner plays with the fuel deliver to get 13:1 at the tailpipe, doesn't that imply the engine is richer still since the cats have scrubbed the exhaust some?

Is the best option a bung welded into the downpipe before the cat with a wideband sensor?

 

[04DarkShadowGT]

Its definatly better to get the readings in front of the cats. I got my car dynoed at the tail pipe as well before I got my wideband.

The dyno guy said it was usually a half a point off at the tail pipe if you have cats.

 

[Don 95Vert]

There are two problems with that. First, cats hold and release 02 and usually release it at WOT, so the readings will be false lean. It is a bit safer, but not accurate. Second, there is a pretty decent delay in the readings and what's happeneing in the engine. It could take like 1/2 second for the gases to get to the tailpipe and into the wideband, which is fairly significant. A welded bung before the cats is the best choice followed by using a rear 02 sensor bung - doing the latter eliminates the delay but still is after the cats.
Don

 

[2002BLGT]

get a bung welded in before the cats if at all possible

 

[the98stang]

Steve at Powertrain Dynamics tunes that way. He said the difference is minimal, and it's easier to check the readings that way. Worse comes to worse, the car is tuned to run a little more rich because the readings are lean. I'd rather sacrafice some HP than blow my engine from a lean condition. Just my .02

 

[01Steeda]

I'd rather sacrafice some HP than blow my engine from a lean condition. Just my .02

I agree, but where would the lean condition be coming from... if he welds a bung in before the cats and the tune is done properly there is no lean condition.