Comment by sbierwagen

>I don't know where you got the 14.7:1 number

The cat has to be hot to catalyze. The engine is run rich so unburnt fuel makes it to the cat and is combusted there, warming it up enough to also kill the undesirable gases. This is wasted heat... unless you mount a turbocharger after the cat, which has its own set of weird tradeoffs. (I've never heard of a factory car with a rear turbo)