[elhigh]

I find the whole idea fascinating but I think your best money would be spent making yours the best SBC 350 it can be, not trying to retcon it into an Atkinson-ish model.

The TBI isn't really sophisticated enough to handle the tune you want for Atkinson function. Like the other guy said, the sensors are wrong.

Your truck is an aero disaster. I think there are likely a lot of things you can do, things that aren't too gross to look at, that will clean it up a lot. You're already doing nearly 15mpg and for a vehicle that size with that engine, that's pretty good. Tune your mill to its very best, and leave it alone.

I remember reading about Hot VWs "Mileage Motor" project - virtually none of which will make a hill of beans' difference to your project - but they found a lot of mileage after giving their test vehicle a chassis tune: setting up bearings correctly, alignment, fixing brake drag, etc.

You don't have to give your truck a heart and brain transplant in order to make it perform well, I think. I suspect you're just going for the biggest, sexiest project that has the greatest possible potential - really I think the Atkinson project does have enormous potential, but starting from a TBI SBC is probably the second-hardest way to do it (first hardest being carbureted) - but skipping past the more mundane but proven techniques that might get you closer to your goal anyway.