[Sevn86]

Originally Posted by AK Mike

My wife called from work and wanted some stuff she forgot this morning, so I had an excuse to drive the Camaro (still too many rocks on the road!) Man, this car is FUN!! Probably put another 30 miles or so on...

Sevn86:
Just to add to what Brad already said, an internal combustion engine has an ideal fuel/air ratio (14.7:1) called a stochiometric ratio. Fuel doesn't burn without being mixed with air and with carburetors, the ratio would be all over the board sometimes....racers would 'tune' their carbs for WOT (wide open throttle) and most of those cars wouldn't run very well at low/medium RPM's (plus they would use big cams that didn't help the idle quality either.) Anyway, with fuel injection and computers, engineers could control the fuel/air ratio much more precisely and keep the engine running at optimal...providing they still met all the emissions/mileage requirements! Usually, that meant the car would run toward the 'lean' side (more air/less fuel) that helped with mileage...but at the cost of burned valves/pistons and hotter engine temps. Generally speaking, the fuel/air ratio should be pretty consistent, depending on the engine load, etc. Higher altitudes require leaner mixtures (more air/less fuel) because the air density is less; a cold engine requires more fuel/less air to prevent backfires/misfires. Colder air is denser than warmer air so my Camaro will make more horsepower (sea level and cool/damp air) than somebody in the high desert on a warm day....everything else being equal, of course!

So, the engineers would typically run a green (new) engine fatter/richer (higher fuel to air ratio) to keep temps lower and allow the engine some break-in time before leaning out the mixture for best performance. I'm sure there's a 'trigger' so the computer 'knows' when to lean out the mixture; time/mileage or whatever....could be the oil change reset? Only your engineer knows for sure. Anyway, for the average owner, it really doesn't make a huge difference one way or the other, your car will likely run better 'overnight' some day because it's designed to do that! (Most people never notice the difference!)

thanks for that informative explanation...