Chassis Dynamometers: Is Your Dyno Operator Cheating You?

Chris Holley

The 2006 Dodge Charger R/T equipped with a 5.7L HEMI is ready for action. The Charger is strapped to a Mustang Chassis Dyno (MD-150).

Just uttering the words “chassis dyno” will send many enthusiasts into a deep and often never-ending bench-racing session that devolves into trash talking that expands well beyond the rational or even realistic. For “proof,” the trash talk is often accompanied by page after page of dyno graphs passing hands to compare and ultimately determine who has the meanest ride in town. Without a real basis for comparison other than the dyno graphs, the peak horsepower (hp) or peak torque numbers are king for the dyno crowd. Forget about elapsed times from “lining ‘em up” at the drag strip; it is all about those little dyno graphs providing all the information necessary to satisfy the bench racers. While this scenario seems far-fetched, it plays out on daily occurrence even though there is no way to have a level playing field for an actual comparison between different types dynamometers or the real usage of those dynamometers. In an attempt to dispel some misconceptions as well as enlightening enthusiasts on the mistakes and downright lies perpetuated by dynamometer operators, several tests were conducted on the Pennsylvania College of Technology’s Mustang Chassis Dynamometer and the Penn College 2006 Dodge Charger R/T.

During my years of teaching chassis dyno classes, several of my students have stated that they do not like the Penn College Mustang dyno because “it makes less power than the one back home.” This is one of the biggest misconceptions about chassis dynos; many enthusiasts hold a genuine belief that one dyno “makes” more power when compared to another. This belief is blatantly incorrect. Regardless of the load-type of the dyno, the dyno absorbs torque and software monitors the acceleration rate and speed of the rollers. The drive wheels of the vehicle spin the rollers against a preset load or a load applied to the rollers. The preset load is an inertia wheel style dyno, which uses its own roller weight as the resistance to provide a load on the vehicle’s drivetrain. A water-type brake or an eddy-current (electric) brake provide a more accurate means of variable loading and delivers a more easily repeatable outcome from run to run. Water-brakes and electric-brakes (such as the double-roller Mustang dyno used for this story) are in most cases used when performance considerations are being tested.

Another misconception perpetuated throughout the industry is the belief that each automotive manufacturer’s drivetrain layout has a certain amount of loss (parasitic loss) due to the rotating components (tires, wheels, drive axles, driveshaft, transmission, torque converter, ect.). For instance, it is often stated that all front wheel drive (FWD) vehicles lose 15% of the engine’s torque because of the drivetrain. If a FWD engine produced 200hp, based upon the belief of the 15% fixed loss, there would a 30 hp loss due to the need for the engine to drive the torque through the transmission and the rest of the drivetrain. Moreover, it is generally and incorrectly agreed upon belief that rear wheel drive (RWD) vehicles lose up to 25% of the engine’s output and that 4-wheel drive/all-wheel drive (4WD/AWD) vehicles can lose up to 35% output or more. Without having the engine dynamometer pulls and data graphs prior to running the vehicle on the chassis dyno, an accurate determination of actual drivetrain losses cannot be ascertained. In an attempt to mimic parasitic loses of the drivetrain, dyno manufacturers provide a fixed loss built into the dyno software or the dyno operator must accelerate the vehicle to a specific miles per hour (MPH), and then push in the clutch or nudge the automatic selector into neutral. As the vehicle’s drivetrain coasts down on the rollers, the dyno software monitors the deceleration rate and determines the drivetrain losses based upon the deceleration. In both cases, the parasitic losses are a “pretty close” measurement for the dyno testing but not the ultimate indication of the drivetrain losses.