Portable Air/Fuel Ratio meter
I worked for a Harley Shop for a while. The shop had a sniffer that we rammed up the pipe for dyno runs, but it had to almost go up to the cylinder heard, into a part of the pipe that we were 100% sure was sealed to both the head and full-length. Harley guys run pretty big cams, and atmospheric air is always trying to flow back up the tailpipe, until the rpm gets up.