Found this
http://www.bristoldyno.com/tech/airfuel.htm
Specifically..
"...Another important factor is that most air/fuel ratios are measured via a tailpipe sniffer. This method has proven to be an excellent way of measuring the ratio, but it is not perfect at low rpm. At low rpm, an engine may not be producing enough gas to displace all of the atmospheric air in the tailpipe, and this will produce a false lean reading because of the extra oxygen - as one can see in this chart. This phenomenon is going to be more pronounced in small-bore engines with large diameter exhaust piping. Two important things must be considered when one is tuning with a tailpipe sniffer because of this phenomenon. One, a flat line across the entire rpm band will mean that the actual air/fuel ratio is too rich at low rpm. Two, a real-world driver is almost never at wide-open-throttle at such a low rpm, so the air/fuel curve at that point is something that the driver will never see. One can also see from the chart that the catalytic converter has no significant effect on the air/fuel ratio in this particular vehicle."....
My engine is a large bore engine, but I also had 3.5" tips on my Cougar at the time.
This may help some guys on the dyno.