Step 6: Back to the Dyno
Once again, Dyno Shop owner Mark MacNeil strapped our rig to the rollers to see what our efforts had accomplished. Unlike our Wrangler, which went severely rich in the mid-rpm levels, our XJ went over on the lean side right in the sweet spot of the power curve. Leanness builds horsepower to a point, but it can also kill engines. Our air/fuel ratio was dancing close to dangerous levels, with a high of 14.3 recorded from about 4,000 to 4,600 rpm. Ideally those numbers would be closer to 12:1 at wide-open throttle. We could have tried an adjustable fuel-pressure regulator to get a little more fuel in the engine, but our later '99 engine features a returnless fuel injection system that would require dropping the fuel tank. Too much hassle.
Again, MacNeil suggested the Unichip his company installs as a way to optimize the air/fuel ratio and timing events for an increased 10-percent more power and torque over what we generated. In the long run, that's probably what we'll do, but for now we'll just be careful if we hear detonation and make sure to always run premium fuel.