My theory is that Toyota was meeting the EPA specs of the time. BAck then only CO and HC were a concern, the EPA didn't mandate anything about NOx. HC and CO increase when you're running rich, but taper off when you're lean (an oversimplification but bear with me here). In order to stay well clear of CO & HC, Toyota designed the smogged US engines to run lean - they bailed out on the lean side for safety, and because there weren't any repercussions for that. To their benefit the cooling system on the 2F could handle the additional heat running lean. There may be localized hotspots, particularly around the valves, but again the cast iron head is going to absorb and dissipate a lot of that. So Toyota didn't really have much in the way of drawbacks bailing out to the lean side. Fast forward a few decades and now we do test for NOx, so Toyota's engineering bites us in the ass - those of us who have to pass a smog test anyway. I will say, the super lean highway cruise condition is great for fuel economy. I've gotten as high as 19 over long stretches with the wind at my back, but more regularly 15-17 with a light load. Now that I have a bunch of armor, a roof rack, and the VTV (or is it VSV?), I get maybe 14 on the highway.Yes, I've thought about a VSV in some of those locations, but ultimately, I'm still smogged, and the HAC is working so I'd rather just let the system be automatic.
Target AFR for your elevation and light load should be around 15-ish. I'm at 800' above sea level and mine does the same.I always just attributed it to all the smog sheet, particularly Air Injection
That's my best guess as to the lean cruise thing. The HAC is the best way I've found for bringing it down.
