Dyno Tuning Basics - Jets, Timing, Camera, Action

Dyno Preparation
Preparing to dyno-tune your Pontiac requires forethought. Ensure that your car is in top operational condition, gather your carburetor tuning kit and timing light, and call up your favorite shop to schedule an appointment.

Market rates dictate how much it will cost but in general, expect to pay between $100-$150 per hour for dyno tuning. Tuning time depends on myriad factors including the current tune of the combination, as well as the tuning ability of the dyno operator and enthusiasts involved. For most, a single hour session is often sufficient to optimize power, assuming that quick changes, such as carburetor jetting and timing adjustments are all that are required.

According to Keith Lohse of RPM, "Enthusiasts who have never put their car on a chassis dyno shouldn't be put off by the Internet stories of people damaging their engines. An engine and drivetrain that are in mechanically sound condition will experience no additional risks over what would be experienced on the street or track. At least in a dyno environment, we carefully watch the air/fuel ratio and immediately shut off the car if there is even a hint of a problem. In addition, topping out the car on a dyno is far safer than doing so on the street."

After the initial pulls, since the car was running so rich, the tuning team decided to first work on the secondaries of the carburetor and make a big change. Marty Parker jumped in and efficiently removed the rear bowl and replaced the No. 92 jets with No. 89s (three sizes smaller). After pulling the car again, the air/fuel ratio improved to 10.5:1, and with it both horsepower (375.2) and torque (430.7 lb-ft) improved by 20.3 and 2.7, respectively.	While slowly accelerating through the gears, Keith noted that, even at light throttle, the Tempest was still in excess of 11.5:1 air/fuel ratio, so it was time to work on the primary side of the carb. After quickly removing the front bowl, the No. 79 jets were replaced by No. 76s. On the following pulls, the horsepower increased marginally but the torque started to increase substantially. As fuel was pulled out of the primary circuits, the car was crisper going through the gears.The results were 376.9 hp and 439.3 lb-ft of torque; a gain of 1.7 hp and 8.6 lb-ft of torque.	With both horsepower and torque on the rise, rather than changing jet sizes Keith recommended that the fuel pressure regulator be adjusted down to 7 psi from 7.5. Horsepower and torque both posted nice gains with 382 horsepower and 455.4 lb-ft of torque (increase of 5.1 hp and 16.1 lb-ft tq.). The A/F ratio at wide open throttle got progressively leaner and was now right at 11.3:1.
Evidenced by the fact that the engine was responding favorably to leaner mixtures, a final front jet swap to No. 72 jets from No. 76s was performed. Horsepower fell off to 375.7, but torque increased slightly to 457.5. The air/fuel ratio continued to rise to 11.5:1.	After determining that the timing was at 28 degrees total, it was adjusted up to 30 degrees and the car was pulled a final time. Horsepower increased to 383.2, but torque was down slightly to 455.6.	A great way to learn what a chassis dyno session is all about is by attending a "Dyno-Day" sponsored by a local car club. Here, Tom Garner and his '69 Limelight Green GTO convertible spin the rollers at a recent DAPA Dyno-Day. Tom is a member of GTOANT (GTO Association of North Texas), one of the participating clubs. For $35, owners received three pulls on the dyno, a dyno chart and tons of entertainment watching Ponchos spanning four decades spin the rollers.

Dyno Results
Tests were conducted on a Dynojet 248 chassis dynamometer equipped with a wide-band O2 sensor. All horsepower and torque readings were converted back to SAE. Average horsepower and torque were taken from 3,900-5,900 rpm. In both the baseline- and dyno-tuned configurations, the peak horsepower and torque numbers were recorded at the same rpm levels. Peak horsepower occurred at 5,850 rpm, while peak torque registered at 3,950 rpm. Since the Tempest was equipped with a Turbo 400 automatic transmission and a 3,200 stall tight 10-inch Continental torque converter (Jim Hand Special), the car was pulled in Third gear. Rather than simply romping on it, which caused the car to downshift into Second (and invalidate the dyno pull), the throttle was eased down until just over 3,200 rpm, then mashed to the gunwales.

Since we traded horsepower for torque and the air/fuel ratio was up to 11.8:1 with nice part-throttle ratios of 13.8-14.3, it was time to call it a day. After all, in a period of just over one hour we managed to find 28 hp and an equivalent amount of torque at the peaks. Further testing might have found a few more ponies or managed to get the air/fuel ratio in the low 12s, but time was running short and the Tempest was well prepared to test out its new-found power on the street and strip. In our book, that's $100 well spent!

Conclusion
After establishing a baseline for horsepower, torque and air/fuel ratio on the Tempest, a series of changes were made to the carburetion and timing in order to enhance driveability along with overall power and torque. When complete, the Pontiac picked up peak horsepower and torque to the tune of 28.3 and 27.7, respectively. More important than the peak numbers were the gains in average horsepower and torque, as those are the changes that can be felt on the street and track as the engine accelerates through the power band.

Keith Lohse of RPM states, "As the generation that grew up tuning and racing carbureted powerplants such as the Pontiac dwindles, we are seeing a greater influx of cars that are either overly rich or dangerously lean coming into the shop. Since a carburetor and distributor are 'wear items' and are not capable of self-adjusting like the late-model EFI-equipped cars, a car can go out of tune with minimal use. Even with highly-skilled tuners-such as Floyd Hand and Marty Parker-due to the demands of everyday life, there just isn't enough time available to take repeated trips to the dragstrip in order to get the car tuned again. In a normal 1-hour tuning session, an enthusiast can generally get their car tuned well enough so that when they return to the track, they are very close from an optimal performance standpoint."

After reflecting on the dyno session, it strikes you that the chassis dyno is simply a tool for tuning, and the real expertise is what both the dyno operator and enthusiast bring to the table in the way of experience and tuning ability. Rather than simply trying to optimize overall power or torque, RPM carefully watched the car's air/fuel ratio under various throttle conditions in order to improve the overall driveability of the car and consulted with the tuners on a recommendation in order to optimize performance.

Although the snow may be swirling, it may be worth considering dyno tuning your Pontiac. It doesn't matter if you run the same combination for 10 years or have recently installed a new set of cylinder heads and camshaft, isn't it worth knowing that when you roll your prized possession out onto the streets or drop the hammer for a pass down the strip that you will be doing so with the confidence that you have objective data to rely on?

Knock the toboggan out of the way, check the bearings on your trailer and spend an hour with your friends or club members engulfing yourselves in the melodious rhythms of a Pontiac engine singing a song that's music to your ears and uplifting to your spirits.