[Shop-talk] How do they do that?

[Randall]

> So listers, how does the car's computer system immediately 
> recognize that I
> have just dumped in a load of higher octane gas?

You're basically driving a mobile fuel laboratory!  With a knock sensor, the
ECU advances the spark until it gets knock (and backs off just a bit); which
gives it a pretty good estimate of octane.   And since it also has to adjust
the mixture until the O2 sensor reports that it's perfect, it's got a good
estimate of how much energy per gallon is in the fuel. 

Randall