Chassis dynos exposed

[Daniel Ciobota]

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.
>

That's an engine dyno setup, I've never seen a car dyno of this type.
The dynojet type systems have been around for at least three years (my
earliest chassis dyno runs)

[...]

> 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.
>

Since the rollers are 1000lb a piece, a 2000lb weight is enough to
stress the motor to the detonation point, if any.  Detonation has less
to do with load than combustion chamber temps, timing advance, octane,
etc.

> 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.

Dunno about your case, but I know several race mustang owners that will
disagree (one's a former NMCA champ. Lee Bender).  I have seen motors
set up on the ragged edge on those dynos (read, lean and advanced),
without detonating on the street.  No dyno can give you absolute,
incontestable hp numbers, but as a tuning tool, it's a big bang for the
buck.

> 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.

Sounds to me like you're marketing these units.  I'm not saying they're
bad, but don't knock down the other dynos.  They're very useful tuning
tools, at least for me and some other fellow racers.

 All dynos are good, whether chassis, engine, or, as in the case of my
former vertible stang, "car mounted".

Daniel