Makes sense... If you want to get 25% better mileage by using 25% less fuel, you would have to run the engine at an AFR of about 18:1 assuming the same amount of throttle was used as would otherwise be done with a standard AFR. Of course, a similar effect can be had by just using less gas pedal in the first place...Originally Posted by oly884
Problem is, if you're running 25% less gas (and assuming you're running a water injection system to keep your EGT down), you'll make 25% less horsepower. Odds are people would tend to use more gas pedal to make up for this fact since the engine would probably feel sluggish as well as rough-idling. Because they are using more throttle, they would either use about the same amount of gas as before the modification (except now the engine runs like crap and uses distilled water too), or they might even use more than 80% throttle while accelerating, pushing the engine into open-loop where the ECU ignores oxygen sensor feedback and dumps in about 12.0:1 AFR.
Basically, everyone wants a magic bullet where they can drive the same way they always have, but get 100% better mileage (I would love to bolt a magic gadget to to my engine and get the same horsepower plus 50 mpg). This is of course impossible if you have the same weight, same aerodynamics, same engine efficiency, and same driving style.