Chassis Dynamometer Differences & Testing Goals:
Much debate revolves around measuring the power and torque levels of your vehicle and engine on a
chassis dynamometer (dyno). One machine may give you measurements drastically higher than the
next. My first objective is to explain some important differences amongst the most widely used dynos.
However, my main objective is to discuss the importance of setting dyno testing goals.
Inertia vs. Eddy Current Dynos
There are a variety of different types of chassis dynos; however, the two most common machines are
either based on inertia or eddy current magnetic retarders.
Inertia dynos calculate power by measuring the power required to accelerate a drive roller. The drive
roller has a known mass. There is no load that is moved, in turn there is no measurement of load. The
most common inertia dynos in use are manufactured by DynoJet and Mustang Dyno.
Eddy current dynamometers have an electrically conductive core, shaft, or disc. It moves a across a
magnetic field to measure resistance to movement. The eddy current magnetic absorber allows for quick
load change rates. The best models enable the operator to conduct true steady-state and controlled rate
operation. The most common eddy current dynos in use are manufactured by DynoDynamics and
DynaPack.
Roller vs. Hub Style Dynos
There are also two main types of design used in chassis dynos.
Most dynos consists of rollers mounted in a frame or in a pit in the floor. The vehicle is strapped to the
frame or floor and driven on the rollers. The most popular manufacturers of roller style dynos are:
DynoJet, Mustang Dyno, and DynoDynamics.
Another type of chassis dyno is one that is mounted to the hubs of the vehicle after the wheels have
been removed. A popular brand is DynaPack.
It is obvious that a roller type dyno will deliver lower measurements of power and torque, since the drive
train loss is carried further. This loss is increased the further we move from the engine’s flywheel, to the
transmission, to the axles, to the wheels, and finally to the tires. A dyno that measures at the hubs does
not suffer from drive train losses at the wheels and tires. Therefore, it will measure higher power and
torque numbers that are closer to those measured at the flywheel.
Correction Factors and Smoothing Factors
Dynamometer manufacturers also incorporate different features with regard to atmospheric correction
and smoothing. DynoJet uses a smoothing factor to average out the bumps and spikes to compensate
for the fact that the raw data from the DynoJet’s acceleration sensors is very jagged and noisy. The scale
runs from 0, least smoothing, to 5, which eliminates nearly all spikes and dips. You can imagine that the
spikes and dips can significantly distort the measured power and torque numbers. Atmospheric
corrections are handled by incorporating dyno room temperature, humidity, and barometric pressure in to
the dyno’s software. With these variables, the dyno operator ensures that the measurements are
equivalent during widely varying seasonal conditions.
Testing Goals
All types of chassis dynos share one common characteristic: they are tools to measure output levels and
to prepare appropriate tunes for the intended purpose of the vehicle. Therefore, it is of lesser importance
what type or kind of chassis dyno is used, than what the dyno professional does with the machine.
A good tuner will prepare your street vehicle with a consistent combination of air/fuel, ignition timing, and
cam timing. The tuner’s focus will be on safety and reliability, not power numbers. On the other hand, a
good tuner will prepare a track-oriented vehicle for the highest power and torque results that are
necessary for the type of racing intended. A much more aggressive tune is justified in race vehicles, with
a greater focus on power numbers. Regardless of what the intended purpose is of the vehicle, the tune
should reflect a repeatable, lasting combination of air/fuel, ignition timing, and cam timing.
In conclusion, considering the large amount of differences from one dyno to another, it may seem difficult
to determine the best approach when you want to put your vehicle on a chassis dyno. However, your
decision will most likely depend on the proximity of the dyno shop, their rates, their customer service, and
their availability.
Here are some useful tips for successive dyno testing sessions:
1. Use the same dyno for all your testing
2. Ask the dyno operator to use the same smoothing factor (DynoJet)
3. Ask the dyno operator to use the same correction factor or method (all)
4. Establish a baseline first, which can be a stock vehicle, a stock engine, or just the start of a new
project on your already modified engine
5. Compare your baseline results to the results of the next trip to the dyno and every subsequent one
6. Ask the dyno operator to attempt to conduct successive dyno pulls within the same range of engine
and intake temperatures
7. Be consistent with gear selection, dyno pulls in different gears will produce different power and torque
measurements
8. Always check with the dyno operator how to properly prepare your vehicle for your visit
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