From what little I know it is basically the same for all internal combustion engines except maybe the stratified charge style that Honda was playing with.

I believe it is referred to as the stoiometric ratio and it is variable depending upon load, engine speed, temperature etc.

Generally a cold engine on initial start up will run rich say as low as 12-13 to 1 air to fuel. An engine at operating temperature and cruise (light throttle opening) is around 14 - 14.5 to 1 and under full load and acceleration like on a dyno run for instance it is best to be a little but not too rich.

I have a recent dyno run for my Beeza Cupra 2.0 16V and it is 13.1 at 2190 rpm and stays betwwen 12.9 and 13.3 all the way to 6278 rpm where it starts to lean out a bit before it hits the limiter. Between 6350 and 7000 rpm (rev limiter) it is 13.4 - 13.6.

The guy who did the dyno run is big into turbos and big horsepower applications. He told me that for a totally factory setup he felt that VW couldn't have gotten it any better.

I hope this helps.