After further reviewing the stock tuning, it appeared to our tuner that advancing the timing was what the engine needed most. He noted the inlet air temperature adjustment was too excessive. When it measures 85 degrees ambient with road temperatures at 113 degrees, it pulled nine degrees of timing. He changed it so it doesn't pull timing out until there's 120 degrees intake air temperature. He also advanced the timing several degrees in many areas of the part-throttle, light-load, low-to-medium rpm range of the low-octane map. Once he was done, we repeated the test loop and recorded the data.
During his analysis, our tuner looked primarily at the engine-speed trace, throttle position, manifold pressure fuel-injector trim, and knock retard trace. The goal is to run as much advance as possible without forcing knock retard. The primary mechanism is dialing in the most timing advance (without causing knock) to turn more of the heat in the fuel into work. Retarded spark timing starts produce a shorter power stroke, and thus you lose some of the potential work of the fuel. In addition to advancing the timing, we could tune the catalytic converter overheating fuel enrichment. If the converter begins to overheat, the computer will provide up to 25 percent more fuel. Our tuning specialist turns the circuit off in his GTO, but on this truck he limits the enrichment to 10 percent.
After the final fine-tuning, we were satisfied with the results. In top gear around 2200 rpm at 40 kilopascals (approximately 40-percent throttle), the truck was set to a 38-degree timing advance. That's good for fuel mileage, and there wasn't any knock retard where there had been in previously troublesome areas. It was time to put miles on the rig and check the mileage.
The owner of the truck reports that with his normal 80/20 city/highway driving route, he was originally getting 11 to 12 mpg. He installed an Airaid intake and Borla exhaust, which improved his mileage to 13 to 14 mpg. After burning two tanks of gas driving his normal routine since the computer tuning, he reports he's now getting 15.7 mpg. His truck also has new perks like improved throttle response and an additional 30 horsepower, and driveability and performance will get even better as the tuning is refined. This is the best of both worlds: increased performance and improved mileage. What's better than that?
Is Premium Fuel Worth the Price?
Being able to advance the spark at full load certainly provides better economy, since the engine is making more efficient use of the energy in the fuel. It also potentially allows application of a higher gear in certain situations (due to the increased torque), which is more economical. Finally, many engines run rich at full load to cool down the exhaust gas, and if the ignition is retarded due to knock, even more fuel is needed. So under high-load conditions, higher octane is definitely good for fuel economy.-From Tony's Guide to Fuel-Saving Gadgets