Measuring and
Estimating Engine
Power
by
Joe Abbin
Roadrunner
Engineering
Corrected vs.
Uncorrected Engine
Dyno Results
The
above topic would
require an entire
book to thoroughly
explain the
differences in
quoted horsepower
readings.
Dyno Testing and
Tuning
by Bettes and
Hancock is
recommended. The
short version
follows.
On an engine
dynamometer,
measuring the
torque, TQ in
ft-lbs, and engine
rotational speed,
RPM, allows us to
calculate the
“observed”
horsepower using the
equation:
HP= (TQ x
RPM)/5250
That may be all we
want to know.
However if we want
to compare our
results with
measurements made on
a different day
under different
atmospheric
conditions (assuming
the dyno and engine
are identical) then
we have to “correct”
our “observed”
numbers to some
“standard”
conditions using
accepted standard
techniques. The
Society of
Automotive Engineers
(SAE) has provided
several
specifications to do
this. One of the
oldest and most
commonly used SAE
specifications is
J607. J607 provides
formulas to correct
our observed
readings to what we
would have observed
if out tests were
conducted at a
temperature of 60F,
a pressure of 29.92
in.Hg and dry air.
As an example,
suppose we observed
100 HP on our dyno
when the ambient
temperature was 75F,
the pressure was
29.4 in.Hg and the
relative humidity
was 40%. Using the
formulas in J607, we
would find that the
atmospheric
correction factor is
about 1.05 . The
corrected HP is then
the observed number
times the correction
factor or 100 x 1.05
= 105 HP. This is a
reasonable estimate
of what we might
have observed if we
had conducted our
test at the standard
conditions.
To compare dyno
results we must know
the test conditions
and correct the
readings to the same
standard conditions.
Also, because
different dyno
manufacturers have
different ways of
correcting for dyno
internal losses and
inertia, really
comparable tests
should be run on the
same dyno.
Engine Dyno
Results vs. Chassis
Dyno Results
If we install our
engine above in a
vehicle and test on
a chassis dyno we
find that not all
the power measured
at the engine
flywheel or the
flexplate gets to
the chassis dyno
roller. This is due
to driveline and
traction losses.
These losses are
often substantial
with numbers of
15-20% being common
for street cars.
Thus our engine that
produced 100 HP on
our engine dyno at
75F, etc above may
only deliver 80-85
HP to the chassis
dyno roller. Nature
is a bitch.
Estimating flywheel
horsepower from
chassis dyno or drag
strip results
requires us to know
or estimate dyno,
driveline and
traction losses.
Drag Strip
Performance
Correlations
If we install our
engine above in a
vehicle and test on
the drag strip we
again find that not
all the power
measured at the
engine flywheel gets
to the track. This
is again due to
driveline and
traction losses.
These losses are
often similar to the
numbers experienced
on the chassis dyno
(15-20%). Thus our
engine that produced
100 HP on our engine
dyno at 75F, etc
above may only
deliver 80-85 HP to
the drag strip.
Estimating flywheel
horsepower from drag
strip results
requires us to know
or estimate traction
losses, weather
conditions, vehicle
characteristics,
engine and driveline
characteristics, and
driver inputs.
Examples of some of
these variables are
horsepower-torque
characteristics of
the engine, starting
line track
preparation,
temperature,
pressure, humidity,
wind speed and
direction, vehicle
weight, vehicle drag
characteristics,
transmission type
and efficiency,
converter or clutch
type and efficiency,
tire
characteristics,
launch and shift
RPM, shift speed,
etc. There are
several software
packages that can
account for all of
these variables and
more with reasonable
accuracy. One is
Drag Racing Analyzer
Pro
or the standard
version. This
software can also
simulate Bonneville
type racing.
There are also
common “pocket dyno”
tables, calculators
and slide rules for
estimating
horsepower from drag
strip results. The
only variable these
calculators include
is vehicle weight.
They are really only
useful for comparing
run to run
horsepower
differences
on the same vehicle.
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