[T3] Lean Cruise

[Max Welton]

Now that someone else in the group is going the Megasquirt route, this might be of interest on the list.

Some while back I was trying to explain my lean tuning strategy as a component of getting better than stock mpg. The point was made that my high MPG was due to the thin air here in Colorado Springs and that if I tried that at lower altitudes I would fry my engine. So I went off and thought about that and believe I have an answer. 

Last spring I reconfigured my MS tables to give a wide flat "lean" plateau below a certain MAP value (65 kPa) and above about 2400 rpm. On that plateau I set the target AFR at 16.1:1. Above the plateau I drop to 13.5. And since I want to minimize the chance of spending much time around Stoichio, I arranged for a very abrupt transition. 

It was a nice day today, so I brought the laptop to work and took this screen-shot on the way home. The stretch of road in this sample was undulating a little so to maintain a constant speed required a little throttle (and thus MAP) movement. This shows the relationship rather nicely.

http://www.maxwelton2k.net/squareback68/MST3/AFRStepLeanTune.JPG

So, why do I think I won't fry my engine?

When load goes up for any reason and I increase power, the MAP increase will cause the transition from lean (low 16s) to power (mid 13s). If I choose not to apply more power, the engine will remain lean, but the car will slow down. Eventually the falling RPMs will take it below the lean plateau.

Max Welton
http://www.maxwelton2k.net/