There are two distinct camps within the the Pontiac community, based upon the year of the car. The deciding factor seems to be the fuel delivery method-carburetor or EFI. The fuel-injection boys for the most part have embraced the theory of using a chassis dyno to extract the most power from their Pontiacs. But the technology of a modern chassis dyno never really caught on as much with traditional Pontiac owners.
This is unfortunate, since as far back as the '50s the hot-rod community welcomed the opportunity to tune engines on a Clayton-brand chassis dyno. This forerunner to the modern system was often the catalyst for record-setting performance, either at the local track or on the national level. And today's chassis dynos are much more sophisticated, provide better information, and are less abusive to the car than ever before. Even in the days of the old Clayton, a well-executed dyno session put less wear and tear on the engine than a pass down the dragstrip.
Over the years, many questions about chassis-dyno testing and tuning have come into the HPP editorial office. Common queries center on the accuracy of the tests, dyno computer input, varied driver techniques, engine temperature, and gear selection. Others wonder how the gear ratio and tire pressure impact the results.
The EFI SD-455 engine in our...
The EFI SD-455 engine in our subject Firebird was fully modified but appears stock. It ran flawlessly all day on the dyno.
To provide some insight about the dyno testing procedure and answer these questions, we created this primer. For the tests, we enlisted the help of Melvin Benzaquen, owner of Classic Restoration Enterprises. He has a portable chassis dyno from Dynocom Industries (DC-1800sz-I trailer [mobile] 2,000-plus-horsepower/ 225-plus-mph) for testing and tuning customers' cars.
To even the casual reader of HPP, Melvin is no stranger, and neither is his '74 SD-455 T/A-our subject car. What is unique about this T/A is that it straddles technological timelines. It is a traditional 455 Pontiac with the factory cylinder heads and EFI. Thus, the car is the blending of old and new technologies that both factions of the Pontiac community value-a rare bird to say the least. (No pun intended!) Its engine build and EFI swap were covered in HPP, as was the installation of its 4L85E SuperMatic overdrive transmission. To get more details on the engine and drivetrain combination, log on to www.highperformancepontiac.com.
Always make sure the Pontiac...
Always make sure the Pontiac is mechanically sound and the engine oil is full (but not overfull as it will reduce power) before you go to the dyno.
To make this a comprehensive guide to chassis dyno testing, basic information and the proper protocols will be covered. Since adjusting the air/fuel ratio and timing will be fully explained in the coming paragraphs and the results are combination-specific for each Pontiac, we have dedicated the dyno time for testing to answer the specific questions that have been posed over the years but are normally not discussed in a chassis dyno test. By presenting the information in this manner, you will not only be provided with the prescribed methods, but you will be exposed to minor changes that can have the ability to drastically impact the results.
Chassis Dyno Types
The two types of chassis dynos you are most likely to encounter are inertia and load-bearing (or brake).
The portable Dynocom dyno...
The portable Dynocom dyno we used is mounted on a trailer so it can be taken to shows and racing events, but the results would be the same if it was in the floor of a shop.
On an inertia dynamometer, power measurements are derived using the fixed mass of the roller or drum. During a pull, the dyno's computer measures rpm and the acceleration rate of that fixed mass-no other load applied-to calculate torque and horsepower. Some inertia dynos tend to produce higher power numbers than load-bearing dynos based on how they perform their calculations.
A load-bearing dyno employs a brake-most use an eddy current brake-to apply an adjustable load on the engine. The computer then notes the applied braking torque, and measures the engine's ability to maintain or raise the rpm against it via inputs from a speed sensor and strain gauge to calculate torque and horsepower.
The ability to provide steady-state load in order to tune or diagnose the engine at a certain rpm, and the ability to provide a test that accurately depicts the on-the-road or on-the-track experience of the car, are two characteristics of the load-bearing dyno that an inertia dyno does not share.