Chassis-Dyno Tuning - Dyno-Mite!

The carburetor and distributor found on a typical Pontiac V-8 are responsible for feeding the engine a combustible cocktail of fuel and air, and igniting the compressed mixture. Too much or too little fuel and/or inopportune spark timing can result in significant performance degradation. While small changes followed by a thorough road test can sometimes get one close, oftentimes the minute changes required to attain maximum performance must be measured on a dragstrip or dynamometer.

The Vehicle Our '72 Trans Am is powered by its original code-YE 455 H.O. engine. Several years ago it was rebuilt to stock specifications by a previous owner. Since then we've made some subtle modifications to improve performance without grossly affecting originality or appearance, including an aftermarket hydraulic flat-tappet camshaft from Nunzi's Automotive in Brooklyn, New York, that features slightly more duration and lift than the original No. 068, reproduction round-port exhaust manifolds with oversized outlets from Ram Air Restoration Enterprises (R.A.R.E.), and a 2.5-inch exhaust system with a reproduction crossflow muffler from Waldron Exhaust.

With continual carburetor and distributor tuning, we arrived at what we felt offered the best part-throttle street manners and full-throttle performance on the street. The Firebird ran and drove extremely well, and while we were confident that we were fairly close to the optimal tune, to date we hadn't ever measured its performance in a controlled testing session, which would tell us just how close we really were.

The Chassis Dyno A chassis dynamometer is used to measure the amount of rear-wheel horsepower (RWHP) and torque (RWTQ) that a vehicle transfers to the road surface. Output is measured by placing the vehicle's drive wheels on a roller drum, and it's calculated using several factors, such as vehicle speed and the drum's acceleration rate.

Tom VanVugt of C&S Dyno Shop in Omaha, Nebraska, has been using his company's Mustang Dynamometer MD-1750 chassis dynamometer to tune various cars in and around the Omaha area for the past four years. In his opinion, his inertia-based Mustang Dyno unit is among the most accurate on the market. While the roller drum of some is a simple fixed weight, the Mustang Dyno unit incorporates a 2,500-pound roller drum that uses a power-absorption feature (eddy current brake) to apply a variable amount of resistance on the roller drum that equals the vehicle weight.

Dyno Day
Confident in VanVugt's ability to tune our Firebird for the best practical results, we scheduled our appointment at C&S Dyno Shop and began preparing our Firebird for the session using many of the chassis-dyno tips found in "Let The Good Times Roll" (Dec. '10). When our day arrived, VanVugt positioned the Trans Am's rear tires over the MD-1750's roller drum, securely strapped it into place, and clamped an air/fuel (A/F) probe onto an exhaust pipe to record mixture consistency.

Reviewing the pull, we learned that the Quadrajet's 0.040-inch AX secondary metering rods were providing more fuel than our 455 H.O. needed, causing a slightly rich condition. The next several pulls were spent swapping metering jets and rods, which brought the A/F ratio closer to 13:1, which VanVugt says should yield best performance. The 455 H.O responded with a slight power increase. With the A/F ratio in the ideal range, he made timing adjustments on the next few pulls. Once peak power was achieved, the chassis dyno session was complete.

Analyzing The Results
In less than two hours and after a dozen full-throttle pulls, our Trans Am was running as well as it possibly could. From our baseline pull, we were able to increase engine output by 14 rwhp and 15 rwtq to a respective total of 264 rwhp and 330 lb-ft, with minor adjustments. Though others could see much larger gains depending upon the initial state of tune, these results told us that our extensive tuning efforts proved fruitful as the Firebird's fuel and spark curves were relatively close to the optimal settings for maximum performance-and now they are ideal.

Conclusion
On the road, our Trans Am feels more responsive at part-throttle while full-throttle performance feels even stronger than before. With the engine running at its peak potential, we expect long-range fuel economy testing to show a detectible increase as well.

Even if your particular Pontiac makes more or less rear-wheel horsepower and torque than our example, there's a good chance performance can improve some. So if having your Pontiac running its best is important to you, a chassis-dyno session is wise investment toward its future!