i put my wideband lambda up the arse of my 205 dturbo as i was curious just like you! At ticover it was soo lean it never registered, and only when you accelerated hard did it show an AFR of 17:1, and my car was quite black at this! not sure about the science. Basicaly i think the leaner the more efficeint (obviously!) and if you are too rich you just make smoke! i too would like to know the science!