Every automobile performance enthusiast at some point wants to know how much horsepower his or her hotrod has after investing countless hours and dollars in order to make it unique. The easiest and most direct method of acquiring that information is by using a chassis dyno. There are engine dyno’s that measure horsepower at the flywheel. This “flywheel” number is
representative of how the O.E.M. manufacturers rate and advertise the
performance numbers that you see in magazines, on TV, and in the
showroom of your favorite brand of vehicle. Unfortunately, for the
vehicle owner, it’s not practical to remove the engine from an already
assembled unit, install it on an engine dynamometer, test it , and then
re-install it in the vehicle chassis. Money and time makes it cost
prohibitive. It’s just not practical.
The manufacturer however, has the advantage of being able to test the engine assembly before it is
Installed and becomes a finished automobile, ready to drive. The
alternative method of acquiring the horsepower number is by installing
the vehicle on a chassis dynamometer. It’s a much simpler method because
it only requires that the rear wheels to be driven onto the rollers,
the vehicle anchored with tie down straps at four corners, attach an
electrical pick up lead, and an exhaust gas sniffer hose in the exhaust
pipe, and a reference listing of performance upgrades. Once the vehicle
has been aligned properly, and the retention straps are tightened, the
vehicle is ready for testing. The driving procedure for the chassis dyno
is similar to driving on the road except for the fact that you are
sitting stationary and only the rear wheels are rotating. Something to
keep in mind is that the horsepower value obtained on the chassis dyno
will be less than the resulting number obtained on an engine dyno. This
lower number is due to the energy absorption requirements to transfer
motion from the crankshaft to the rear wheels. This lower horsepower
number can easily be adjusted to the flywheel equivalent using a simple
formula. Divide the rear wheel HP valve obtained at a drive ratio of 1:1
by .85. This will result in the approximate flywheel HP equivalent.
The industry offers several choices of chassis dynamometers, each of
them utilizing their own unique design. The two main engineering types
are the inertia dyno and the load dyno. In addition, there are
competitive name brands of each type. In this discussion we will focus
on the type or design, not the brand. The two types most commonly used
are the inertia dyno and the load dyno. Both designs will measure
horsepower at the rear wheels. Each design will provide accurate and
repeatable values, but the values are calculated using different engineering designs and therefore the actual HP “number” recorded by each type will be different , but does that really matter. I will explain…..
Inertia dynos extrapolate horsepower output by analyzing the dyno drum's
acceleration rate using a sophisticated accelerometer and computer
software. An inertia dyno works only when the car is accelerating. It
uses heavy roller drums of known mass mounted on bearings that allow
them to freely rotate. A vehicle is placed in position on the dyno with
the drive wheels sitting on the rollers. The car is placed in gear and
accelerated at wide-open throttle. It takes a certain amount of time and
force for the tires to accelerate the heavy rollers. The laws of
physics state that acceleration rate is directly proportional to how
much power the tires place on the heavy roller to get it to rotate.
The dyno software monitors roller velocity and the time it takes to
arrive at a rate of acceleration and estimates power at the rear wheels.
Using data from an engine-mounted inductive probe, the software then
graphs the power and gear-compensated engine torque against engine rpm.
Some inertia dynos also attempt to estimate flywheel power and torque
numbers based on mathematical models and data from additional sensors.
A pure inertia-only dyno can only calculate power by measuring the rate
of change in acceleration (that's why it's called an inertia dyno), so
it can't do loaded tests or step-tests. The inability to perform load
tests makes it difficult to accurately establish optimum timing and fuel
curves for use under varying driving conditions. On today's inertial
dynos, the static roller weight or resistance isn't adjustable to match
vehicle weight; depending on the dyno software and whether the inertial
test function can be combined with a strain gauge to effectively change
the rollers' inertia trim-as is possible on higher-end multifunction
dynos-extremely light or heavy vehicles might fall off the curve, and
turbocharged engines won't build boost as they do in the real world. On
the other hand, with mainstream vehicles weighing around 3,500 pounds,
the results are repeatable with minimal setup time. Dynojet is the most
common pure-inertia dyno in use today; however, some of its newer models
also have an eddy-current option. Load dyno’s, also known as an
eddy-current dyno. This type of dyno controls the brake/absorber using
electric current instead of fluid, measuring torque output and
calculating horsepower based on a strain-gauge. Electric current
provides much more
precise control and minimal spool-up lag-time, but you need a gonzo
electric supply and the dyno itself is more expensive than other types.
Top-flight eddy-current dynos are used for sophisticated R&D and
emissions testing where dead-nuts accuracy is extremely important. They
are especially useful in calibrating electronic engine-management
systems at varying vehicle loads.
On the other hand, utilizing the capabilities of this dyno to its full
potential requires a high degree of skill and setup time. Mustang and
SuperFlow are among those companies marketing high-end eddy-current
dynos. Some eddy-current dynos can also be run in "pure inertia mode,"
but then they have the same drawbacks as any other pure inertia dyno.
Many factors that influence test accuracy are common to all dynos,
including engine dynos; these include temperature, airflow, barometric
pressure, and torque calibration. But on chassis dynos, many additional
factors can affect the results,
factors much harder to control than those typically encountered on an
engine dyno. Drivetrain losses vary according to gear selection (testing
should usually be performed in the transmission's 1:1 gear to minimize
this factor), fluid temperatures, acceleration/ load factors, drivetrain
inertia, brake drag, the vehicle tie-down method, the weight over the
axle, and tire selection, growth, and slippage.
The bottom line, and the most important point of fact, is that no matter
what type of dyno you choose to use, it is the before and after NET
result that matters. What you gain in performance from the money you’ve
spent and the changes that you have made. The actual HP number is just
for bragging, bench racing, and impressing your competitors.
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