A primer on chassis output testing
In assembling our Engine Power Special Section this month, it seemed appropriate to take a look at dynamometer testing—the process of measuring how much power an engine produces. For automotive use, dynamometers— or “dynos” as they’re more commonly known—typically come in two forms: the engine dyno and the chassis dyno. The engine dyno mounts just the engine itself and takes its reading at the crankshaft output, typically referred to as “flywheel horsepower”; the chassis dyno mounts the entire vehicle to a platform and measures output at the drive wheels, resulting in the term “wheel horsepower.” A general rule regarding the difference in measured output between the two is that there is roughly a 20-percent loss from the flywheel to the drive wheels. Note that there are many variables involved, the most significant being the type of transmission— traditional automatics tend to sap more power than manuals.
Among engine and chassis dynos are further variations, mostly based on the method in which power is measured. A dyno must place a load on the test subject in order to quantify its ability to do work; engine dynos often accomplish this by using either a water brake or some form of electrically generated resistance. Chassis dynos may also use an electrically sourced load, but many are considered inertial dynos. This type uses a weighted roller or drum of specific mass, and calculates the vehicle’s power output based on its ability to accelerate the drum.
The Dynojet chassis dyno uses this principle, and it’s the type of unit we’d be using during our trip to Quintin Brothers Performance in Williston, Vermont. The Quintin brothers, Tim and Pete, have been working on cars since their teens in the early ’80s, starting at their father’s service station and later moving to new car dealership service bays. During that same time, they both got involved with drag racing, leading to performance engine building and chassis work. Today, their shop is dedicated primarily to performance and restoration work, with an on-site body shop, engine dyno cell, and the chassis dyno we’d be using.
To explore a basic day on the dyno, we took the 1971 Plymouth Road Runner of Hemmings advertising staffer Rowland George. Rowland is a Mopar guy with a particular penchant for Plymouth’s fuselage cars, and he returned this Road Runner to proper form and function several years back. But, as is the case with many of us, his car is still the subject of ongoing improvements.
In fact, Rowland is embarking on an engine build this winter, to yield a fresh 383 from his car’s numbers-matching block, which he received with the Plymouth when purchased. The engine currently powering the car is a 1970 383 that runs a Mopar Performance “Purple Shaft” hydraulic cam (though we’re not sure which one) and an older Edelbrock TMG single-plane aluminum intake manifold, mounting an Edelbrock 625-cfm carb. There are also Hooker headers and a Magnaflow exhaust system.
The Road Runner runs very well, but there are still some mysteries surrounding its engine, including the question of whether or not it is a Magnum-spec version or just a standard 383 (it appears to have been turquoise originally, where a 383 Magnum or “Road Runner Engine” would have been orange). To us, it seemed like a very typical muscle car engine—not quite stock, and with a past that is not entirely known. A perfect subject for some basic dyno testing and tuning.
We loaded up the car and took it to Quintin Brothers, about three hours north of Hemmings HQ, and the staff mounted it to the dyno. The Dynojet unit at Quintin Brothers is an above-ground type, utilizing a four-post lift to raise the test vehicle into place; some shops have the dyno unit mounted in a pit in the floor so no lift is required. Still others use a portable unit incorporated into a trailer. Once the car is in place with the rear wheels up on the roller, it is tied down with ratchet straps at both ends, similar to what you’d do when tying a car to a trailer for transport. Large wheel chocks are also used on the non-driving wheels.
After some basic checks, the Road Runner was put through its paces and the results analyzed. As is typical today, this dyno provides output numbers and charted graphs for horsepower and torque, and also gives air/fuel readings. The details of our session are covered in the photo captions, but the results were enlightening. We didn’t find much in the way of hidden horsepower, but we did get the Road Runner running more efficiently. Take a look at the steps involved and the findings, and consider locating a similar facility in your area.
Quintin Brothers Auto & Performance
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