Doing a bit of research, it looks like lean mixes alone do no harm, but at low rpms and high load, lean mixes can lead to
detonation and its the detonation that can damage pistons, burn valves etc.
This is why the Civic VX lean burn does not damage the engine and why the Honda insight can go up to 40:1 under cruising conditions and do no damage.
Here are a few good reads:
Aircraft engines and leaning/richening mixtures (pilots do this all the time when they change altitude)
A discussion amongst engineers on the subject "Running lean and melting pistons"
Combustion temps actually decrease as you get leaner and leaner
When I first heard about the EFIE devices and clogged fuel injectors, I wondered why ppl weren't killing their engines since these would cause one or all cylinders to go lean. The above links explain the lack of damage. If the above info is correct- as long as you have a functional knock sensor (so the ECU can retard timing as needed), leaning an engine a moderate amount should be a safe way to gain MPG as long as you do not ignore the sounds of detonation.
These are my thoughts assuming that the above webpages are accurate.
Threaded Mode
