Dyno tune: AFR reading from tailpipe?
 

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.

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?

If the exhaust stream content is being chemically altered by the cats, which way does it err? If a tailpipe AFR is 13:1, what would a wide-band sensor placed in the downpipe before the cat read? Richer or leaner?

leaner would be my guess... but by how much???

By 12 pounds per hour.

If your bringing your car to a place to get it tuned and they use a tail pipe sniffer............run, and i mean run they have no clue. They should be using ther own wibeband system to get proper Air/Fuel Readings.

Well, I assume the sniffer is a wideband since it doesn't simply show rich or lean. I'm just concerned about what the thing is actually reading since it's in exhaust that's been modified by the catalysts.

But okay, if the only other option is a wideband sensor pre-cat, is every dyno shop equipped to lift the car in the air, punch a hole in the midpipe, weld in a bung and install their WBO2 sensor? What's the drill?

The tailpipe sniffer used by "tooners" is junk and these people have no place tuning a car....period

If a tailpipe is all that can be used, at least buy a quality one that limits the amount of air onto the sensor not the dynojet copper tube. Not to mention, the sensors used by 90% of these shops never get calibrated, and drift rich/lean because they are standard off the shelf crap wideband sensors that have had all types of fuel on them.

A reliable tuning shop should be using HAND BUILT lab quality sensors

Thanks Paul. Okay, so, again: When a car comes into your shop for a dyno tune, do you put it up on the hoist first, cut a hole in the midpipe, weld in a bung, screw in your WBO2S and, when you're done tuning, put the car back on the hoist, remove the WBO2S and screw in a plug and let the guy go on his way?

What does your shop do?

Yes. I will not tune a car unless our wideband is installed before the cats. We use a lab quality setup with hand built sensors

Thanks. My 02 GT has a stainless MRT catted midpipe: Does your shop offer the service of welding in the needed bung (and, obviously, a plug for it once tuning is done)? What does this process cost over and above the tune?

Yes. We have all the tools necessary for this job. You are looking at $75.00 with the bung and plug including welding