Air/Fuel Meter/gauge

[leon bee]

This has been mentioned lately and I'm interested, but completely ignorant about it. When we get past about the mid 80s, cars got a lot of stuff I don't know about.

I googled this, mostly what I found was 2 or 3 hundred dollar gauges to install. I'm guessing there must be a version for shop testing? Looks like you weld a bung in your exhaust, is this always required?

Anyway, I'd just like a little practical information about this with our old cars. Thanks!