I heard the WB oxygen sensor will show a leaner air fuel ratio if it's in the tailpipe on a car with cats. Does it have to be upstream of the cat to get a correct reading?

 

Not necessarily true.

You can tune a car with cats just fine with the WB in the tailpipe...its done all the time that way all over the world.

In my experience tuning my car with and w/o cats, the car actually showed leaner after removing the cats, so I had to add more fuel with the Off road pipes in. Somehow the cats had actually made it show richer.

I believe that is because the cats use excess oxygen to burn off unused hydrocarbons, so less air present is going to show rich on the WB and you'll tune accordingly.

 

If it's the "sniffer" type that you are going to see on the dyno then it'll probably work just fine.
If you've bought an aftermarket wideband, such as the innovate motorsports piece, I was told that it needs to be located in front of any catalytic converters.

 

I've heard that the same Bosch sensor that comes with the LM-1 is used at many dynos. I'm actually trying to figure out where to mount mine. Before the cats would only monitor one side of the engine at a time. I was thinking of mounting it in the Y pipe.