Chassis dynos exposed

[Jason_Leone at amat.com]

Since all this talk about chassis dyno figures and methods has erupted today, I
submit this. They're are a few distinct differences in chassis dynamometer
types. Each type has pros and cons.

Up until a year or so ago, all the load type dynos out there had primitive
operating
systems, if at all. They worked by running the car on the dyno, loading up the
rollers (usually via a water brake system), and reading the horse power number
when the load prevented the engine from accelerating past a predetermined rpm.

This gave you the max hp the engine could produce at that particular rpm, since
it
couldn't overcome the force being thrown at it. Take readings at several rpm
points
(usually manually recorded), and a power graph can be made, extrapolating torque
numbers mathematically. Thus, the load type dyno graphs are not "smooth"
because not every point along the curve is actually plotted, but just the
predetermined (by the operator) rpm points. The more points tested, the more
complete and smooth the curve. A primitive graph can be made with Excel, or any
spread sheet software.

However, that is quite time consuming, and hard on the car's engine and tires.
Plus,
operator skill is essential in preventing tire slip from corrupting the data
gathered.
Loading up an engine is how a manufacturer determines and engine's power output,
by the way.

This is all in contrast to the Dynojet inertia type testing, which simply
requires the
car to run on the rollers from standstill or low rpms, to redline or anywhere in
between (in one or several gears). The rate of acceleration is measured, and
power
is mathematically calculated using the constant of roller weight. The time it
takes to
accelerate a given mass to a particular speed can be used to calculate hp. It is
relatively painless to the car, and quickly measured. Plus, the Dynojet software
allows automatic graphing, which produces an attractive consumer product.

However, the downside to this measuring form is that, in the real world, the
vehicle
is working harder than what is required in accelerating the rollers on a
Dynojet.

Between the weight of the car itself, elevation changes, and surface friction,
the
actual environment the engine works in during use is quite different from its
experience on an inertia type dyno. This is an important factor to consider when
interpreting the hp gain of a particular mod/product. For example, lots of
ignition
timing can be thrown at an engine to produce big hp numbers on an inertia dyno,
but the same ignition setting can result in *less* hp when the engine is
actually
required to work on the street. Preignition or detonation can occur, tripping
the
knock sensors and causing the ECU to roll back the timing to less than stock.
Not
good for power. Same issue with fueling. Leaner fuel maps may work well when
subjected to the relatively gentle inertia rollers, but can be disastrous when
under
load on the street.

The Dynojet is a great marketing tool, but is not an accurate, real world
measuring
device. Dynojet now has a load type device that can be added to their existing
models, but it currently is fixed, and cannot be altered. So, it suffers from
the same
drawback as the older type load dynos.

The best of both worlds is the load type dyno with a sophisticated operating
system. For example, a Mustang MD-250 chassis dyno is rated at a maximum hp load
of 750.
However, we can load it from 0hp (simply spinning the attached flywheel and
rollers like the Dynojet) up to 750hp and anywhere in between. We can specify a
time period over which we want a load to be applied. We can simulate a series of
hills, as the car goes up and down in elevation, which obviously varies the load
on
the engine. We can set a specific rpm at which the dyno can apply a specific
load.

In other words, the load application on a MD-250 dyno is practically infinitely
variable,
and can be as punishing or gentle on the engine as we want. All controlled by
the
Windows based software on the controller PC, with hand held operator controls.
For development use, the load ability is crucial, to produce a product that will
perform correctly in the real world. For promotional use, such for dyno days,
the
inertia mode is useful.

Hope that clears us some confusion regarding the types of chassis dynamometers
out there.

Jason
'93 SLC