[Christ]

Originally Posted by cfg83

delslo -



Yeah, I've heard of some people at higher altitudes (i.e. Colorado) getting great MPG, because they are "running lean" as a function of their altitude. They are not actually "lean" per se, but they are using less fuel than they would be at sea level.



For OBDII controlled cars (1996 to present), the AFR is controlled by the 02 sensor, which shoots for the magic 14.7 stoichiometric ratio. If the cold air is denser, there is more "air" being drawn into the car. The 02 sensor will report this to the ECU/PCM, which will increase the amount of fuel injected.

I lost the "perfect URL" for this, but here is a good explanation :

Tips on Reading Gauges; Air-Fuel Ratio Monitor




Other people have posted that this was done on some full size trucks in the 1970's. It used the WAI when the engine was cold, and switched to the CAI (with a flap, just like you said) once the engine was all warmed up. All mechanical stuff in those days, no carputer. I think that some Hondas use WAI by placing the intake deliberately close to "hot stuff", but I don't know the details.

CarloSW2

Any vehicle with a "heat riser" on the intake piping switches from HAI to CAI based on engine temps. The warmer the engine gets, the more the valve closes to allow colder air from the end of the piping to enter the intake. When the engine is cold, the thermal valve is completely open, which only accepts air from near the engine, so that the thermal cycling warms up the engine faster. (Each degree the engine warms up also warms the intake temperature, creating an effect of non-linear heating, or a "heat curve", since intake temperature effects engine warm-up times, until the thermal valve begins to close, which allows cooler air intake. In the event that the thermal valve is stuck open, allowing only hot air to enter the intake system, the net effect of engine cooling would become the determining factor in overall intake temperature, all other things equal.)