Chassis dynos exposed

[Shannen Durphey]

Helps me a bunch.
Shannen

Jason_Leone at amat.com wrote:
> 
> 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