[brucepick]

OK. I know a bit about LB from driving my HX, with ScanGauge and MPGuino. SG tells me when it's in and out of LB, among other good stuff. Guino of course tells me the mpg.

There are two factors that enable lean burn to actually get you more mpg.

First and foremost, the air/fuel ratio that's actually maintained in a conventional modern gasoline engine ensures that a small amount of unburned fuel passes THROUGH the engine. This is intentional, so that it's available to burn inside the catalytic converter, keeping it at the desired very hot operating temperature. Whatever that amount is, it's "wasted" as far as getting motion from that fuel.

So - by leaning out the mixture - you don't waste that small percentage of fuel. It burns in the cylinder where it can help move the car forward.

The HX keeps the cat HOT by putting it inside the exhaust manifold. To get it any hotter/sooner, you'd have to put a mini-cat into each exhaust port on the head. Now there's an idea...

Now - when my Civic enters lean burn - I can feel the power drop just a bit, if my foot is dead steady on the accelerator and the road grade stays the same, etc. Of course there's a power drop at that instant because the air volume to the cylinder is unchanged, and the fuel qty dropped. Some of that air isn't "getting fuel". Less fuel is being burned in the cylinder - where it counts. But that's OK. I step into the gas a bit and keep the car moving at the speed I want. Net result is that mpg goes up, and I can see it on the MPGuino. I've seen 100-120 mpg on the highway, on a gentle downgrade where some power is needed to maintain speed. On a moderate upgrade, if I can maintain LB it will get 30-35 mpg. But if I take it out of LB, it gets about 25-30 mpg for the same climb. Dont' ask me about level roads, we don't have them here!! Up and down all the way, usually not too steep.

The second benefit - which was already mentioned somewhere in this thread - is the reduced vacuum. The throttle is more open in lean burn, for any given power/rpm combination, so there's less mechanical effort expended to create + maintain that vacuum.