Wideband in tailpipe on dyno?

I've probable seen 100+ videos of people tuning cars on a chassis dyno.

They always put a wideband in the tailpipe to log the AFR separately.

What is the point of this on a car that already has wideband sensors from the factory?

Wouldn't the reading from the tailpipe have a much higher chance for error?

It helps to confirm what the built-in O2 sensors are reading (i.e. if you have a bad O2, then you can fix it). Also, in may cases it's because the dyno software will only log it's own O2 sensor, so if you want to provide a graph showing torque/power/afr, you have to use the dyno wideband.

You learn when to trust it (i.e. works fine at high flow rates), and when to ignore it (idle conditions). Now some dyno O2 will have a sample pipe that goes much further up the exhaust and allows more useful data, or even a pump to "pull" exhaust gases to an externally mounted sensor.

I've seen some of those venturi style "pumps" in action David, there was one that hooked into the shop air compressor and drew enough air through it that at idle there was a reverse flow into the tailpipe as the exhaust mass wasn't enough to supply the flow demands.

The other thing that wideband in tail pipe helps with is correct AFR reading with regards to ignition timing change affect. When wideband sensor is located very close to exhaust port changing the ignition timing can affect on lambda reading as much as 0.5 AFR depending on advancing or retarding... Placing WBO2 sensor as far as possible eliminates this issue...

There is also the option of using a 4/5 gas exhaust analyser in the tail-pipe, rather than just the basic O₂ lambda sensor.

TBH, I'm surprised they seem to be so uncommon, because they give so much more information.

Hi Gord, 4/5 gas analysers are good in steady state operation, but due to their latency in sampling (I've seen some automotive test units that have a 15+ second sampling response time) are not usable for transient and accelerative testing. Lambda does have it's drawbacks as well, but those are less than those of the 4/5 gas analysers.

Good point, especially as most use inertia dyno's, or acc'n ramping.