While the subjects of spark timing and air/fuel ratio (AFR) are sufficiently complex to fill an entire article, the bottom line is that finding the right combination of these two parameters is critical to the task of extracting maximum safe output from any engine combination. Tuning on a chassis dyno is the ideal way to achieve this balance, since it allows the tuner to make minute changes to spark advance and fuel-injector timing over the engine's entire rpm range.
The terms rich and lean are central to any discussion of AFR. A rich combustion mixture-that is, one with an overabundance of fuel-is typically safe under a wide range of operating conditions but less than optimal for generating optimum horsepower. A lean mixture-one with an overabundance of air-makes more power but can become dangerous beyond a certain point. Again, balance is key.
Our C4 started out with an overly rich AFR, which Gonyon tweaked via laptop over the course of several dyno runs until he reached near-ideal readings on the final pull. After settling on a complementary spark curve, he fine-tuned the car's shift points and firmness, rev-limiter settings, cooling-fan operation thresholds, and various other parameters to better suit the new cylinder heads and camshaft. The entire process took a little more than two hours, mostly due to the protracted upload times of the LT1's outdated ECM. (Indeed, we've seen similar tuning sessions take half as long on C5s and C6s.)
The final dyno run of the evening brought readings of 323.39 hp and 302.89 lb-ft of torque at the rear wheels, representing improvements of nearly 52 horses and 7 lb-ft over the session's first pull. While it's perhaps unreasonable to expect similar results from a stock or lightly upgraded car, our test provides ample demonstration of the value of dyno-based tuning on a heavily modified engine combo. Even better, our C4 now operates with a stocker's civility in everyday driving, a faint syncopated patter from the exhaust serving as a subtle reminder of the 400 (crank)-horse performer under the hood. Roll into the throttle, though, and the big rear Nittos claw for purchase, the trans snaps off perfectly timed redline shifts, and the C4's Cyrano schnoz takes on a distinctly skyward cant.
Will these most recent tweaks enable us to hit our previously outlined bogey of mid-12-second e.t.'s in the quarter-mile? We'll know soon enough, when we take the car to nearby Gainesville Raceway for a full dragstrip test session. In the meantime, it's nice to know that the car is performing at the full limit of its capabilities-thanks to the magic of dyno-based ECM tuning.
To see a video of our final dyno pull, click here.