I found a thread about this ages ago when i was experimenting and it appears alot of mappers especially americans prefer using lambda rather then afr though your probably better off sticking with what makes it easier for you lambda always confuses me because the measurement is the opposite to what im used to on a 0.1 to 1.0v narrowband sensor,though that in mind we dont use them to tune and ive only ever used narrowband readings on a standard car.Think i went off topic a bit oh well.
lambda and afr are functionally related lambda = afr / afr at stoich - so if you tune for one or the other the results would be the same - with one possible exception - and that\'s differing fuel types. Since most people don\'t change the fuel type during running of the engine (I say most, because there are meth heads and people who have tanks of other stuff that get injected on boost), afr is fine - provided you tune for the fuel type.
Wiki says this : http://en.wikipedia.org/wiki/Air-fuel_ratio
Petrol / Gasoline is 14.7 to 1 afr for stoich burn, but for a turbo\'d car you want to be a lot richer in the boost part of the map - high 11\'s seems to be the favoured numbers - but it can be dependant on whether you run WI or meth on boost, as this can allow leaner afr with no knock.
I\'ve been running close on 13.5 on warm idle, can\'t get it higher without the idle becoming unstable, and 14.7-15 in cruise - which is great for fuel economy
I run about 11-11.5 on full boost which is too rich and the engine drops power, but it\'s staying that way till more mapping is done. I\'d rather bog than have to rebuild. :cheers