I just wanted to let you know how disappointed I am with the
Tech Notes article in the February 2005 edition of RRW&MT.
With a tag line like,
“With Horsepower, Technique Matters,” and three images of
dynamometers, one could reasonably conclude that the
focus of the article would be on dynamometer operating
and testing procedures. Indeed, I was looking forward
to reading what appeared to be four pages of technical notes
on this topic. Unfortunately, the technical portion of
these notes never materialized.
Apparently, the author feels as though operating procedures
are significant in the context of dynamometer
accuracy. However, the magnitude of this influence is
never quantified in the text of this article. We, as
dynamometer service consumers, are encouraged to get
intrusively involved with the dyno run to make sure it’s
done right, but what’s the payoff? One percent
tolerance? Twenty percent tolerance?
Perhaps, if the Test Methodology section comprised
more than 20% of the total word count, this critical
detail would not have been overlooked.
If errors of omission were not enough, there were also glaring errors in
the descriptions of the various dynamometer types. According
to the author, the two most common dynamometers are
both of the inertial type, but one uses an
eddy-current brake, “to create a variable load which can be
precisely matched to the mass, inertia and horsepower
characteristics,” of a test vehicle, provided one is willing
to, “accept the risk of error from incorrect
calibration…” If, as one is led to believe by the next
paragraph, that the two dynos to which the author
refers are Dynojet and Factory Pro Tuning products, then
this description is completely wrong.
Dynojet’s test procedures are based on sweep tests, where
horsepower and oxygen levels in the exhaust gas are
evaluated during a steady-throttle run-up (sweep)
through the engine’s operating range. In this procedure,
horsepower is derived from the angular acceleration of a
~900lb. rotating drum that’s driven by the rear wheel.
This differs significantly from Factory Pro’s procedures
where horsepower and four exhaust gases are evaluated
steady-throttle, fixed-RPM step tests that increment through the
engine’s operating range. In this procedure, each fixed-RPM
run is a separate test and horsepower is derived from
the steady-state force that the rear wheel exerts on a
lightweight rotating drum.
Clearly, because the Factory Pro system calculates
horsepower under steady-state (zero acceleration)
conditions, it is not an inertial type system. Since
it is not an inertial device, it need not be calibrated to
the inertial characteristics of the test vehicle, thus the
risk of error introduction is non-existent.
Note that Dynojet also makes dynos that include an
eddy-current brake, such as the model 250, but that
addition brings with it both advantages and
disadvantages. The advantages are that the oxygen level
readings can be better synchronized with the power
readings by controlling the rate of acceleration, and
by actively controlling the rate of deceleration, wear
on the test vehicles' mechanical systems can be reduced. The
disadvantages include the fact that the horsepower readings
are obfuscated even further. With the addition of an eddy-
current brake to the system, the calibration software must
take into consideration both the inertial forces from
the drum and the power absorption of the eddy-current
What is probably the most important aspect of chassis dyno tuning was
completely missing from this article. That aspect is how the
various dynamometer systems are used to optimize power
output. Power output optimization is, after all, the
ultimate goal of dyno tuning.
With the Dynojet system, horsepower is treated like a
byproduct of proper air/fuel ratio selection. This is
evident when a Power Commander map is developed, as this
process is based on setting air/fuel ratios to a
pre-determined, arbitrary number across the RPM range. While
this procedure can be accomplished very quickly using sweep
tests and the integrated software suite that Dynojet has
developed, the optimization criteria are based exclusively
on circumstantial data. If the optimization criteria are
changed to include maximum power output and smoothness,
expect an exponential increase in testing time.
In contrast, the Factory Pro EC997 system uses horsepower
figures directly. Each RPM/throttle position point is tested
and optimized for peak power regardless of what the O2
numbers are. The exhaust gas chemistry is used only to guide
the operator in his iterative quest for maximum power.
Compared with Dynojet's normal approach, this can be a very
time-consuming process, but it always results in optimized
power output. But, because the operator is presented with
200% more empirical data (power and oxygen plus three other
exhaust gases and a "flux factor"), converging on optimal
power and smoothness takes
relatively less time.
Which system is better depends on whether you place greater
emphasis on quickly achieving the same O2 levels in your
exhaust gas as your competitors or getting the most power
and drivability out of your
Finally, I fail to see the certainty in spending a
“lottery-sized” sum of money on a water-brake test cell and
a staff of engineers to maintain and calibrate the system.
(Use of hyperbole of these dimensions is typically
discouraged within technical writings.) Clearly, if the
author is willing to accept a five percent error band, a
considerably less expensive and substantially more practical
solution can be found.
X Dot Racing
Mr. Chin is a mechanical engineer,
graduated USC, 1988.
He works for Boeing on their B-1 Program.
He owns Dot Racing and it campaigning a gsxr1000.