to the OP. you should read up on lean burn engines. quite interesting in my mind. lean burn enginer run hotter. and hotter = more efficient from a thermodynamic point of view. however normal fuel injected engines run into detonation problems if its too hot. one way they get around this is fuel stratification. ie. the area around the spark is closer to stoichiometric and it gets leaner the further out and the area where the fuel is is cooler because of that fuel. the areas with little fuel can be hotter because there is nothing to ignite. this can be done by direct injection and theyr'e are other proprietary means the differ by manufacturer.
but, even these engines tend towards stoichometric when under heavy load, and lean out at cruising
for auto-x: '04 Liquid Grey ZX3
for street: '02 Infrared SVT