[psalt]

Perhaps the fact that the dithering also keeps the A/F average ratio where it is wanted for fuel efficiency is a happy convergence of two requirements? Your explanation of the pollution effect is really clear (even if borrowed) and I may even be able to remember it.

Walt,

Best fuel efficiency is around 16:1 AFR. Stoich 14.7:1 is all about converter life and the best compromise of reducing the legally ordained "evil" pollutants (and is mandated by law). GM certainly knows this, it used to put cruise tables above 14.7:1 in the ECU maps and was sued by the Federal government. Just like Cadillac used a separate (leaner warm up) ECU program to meet the IM240 test with the climate control system OFF and instructed the customers to never turn it off when operating the car. On a non cat aftermarket EFI conversion, you can get much better mpg by tuning AFR above 14.7 under low load cruise with ignition advance out near 50 BTDC. Many aftermarket ECUs allow you to shift the narrow band "dither" target higher than 14.7:1. I have done EFI conversions on 50's and 60's land cruisers and mpg can be improved +50% over 4 BBL or SU carburators. In other words, we have chosen to burn more fuel in everyday cruising because of the political decisions made in the name of the environment. Just like we are choosing to ship massive amounts of oil by rail, which burns even more fuel, to satisfy a few eco nazis that failed math class. Or the ethanol mandate. Politics and taxes drive these decisions, not engineering.