[turbodan]

At some point overly rich AFR's can make an engine more susceptible to detonation but thats only because an extremely rich mixture burns slowly enough that it allows more time for detonation to take place. The cooling effect of relatively rich AFR's during cylinder filling and the cooler burn during combustion makes motors much less susceptible to detonation than they are running leaner AFR's. Its not like you have saturated rich pockets in the combuston chamber. Generally the amount of turbulence that occurs during the cylinder filling process makes for a pretty well mixed fuel and air charge.

I can say for sure that if I were to lean out my turbo car past 14:1 the results would be quite expensive.

If you want to utilize direct injection and very fancy combustion chamber designs, you can get away with running some traditionally ridiculous AFR's. Even then, you're still going to need a throttle to make the engine suitable for use in an automotive application.