Dyno Numbers - Engine Professional

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Dyno
Numbers
This
screen from a
DTS dyno can seem
extremely intimidating
at first. Let’s learn how
to interpret the data
shown here.
What to do with
all the numbers?
BY BILL HANCOCK
The sheer amount and variety of
numbers involved in dynamometer
testing can be overwhelming for the
casual user and, in some cases, even for
the seasoned professional. We will
attempt to take some of the mystery out
of these numbers and see what they
mean, but more importantly see how we
can use them effectively. For this article
we will focus on an engine dynamometer
where we are measuring what is called
output at the flywheel, as opposed to a
chassis dynamometer which measures
output at the rear wheels.
There are two sets of numbers that
a dyno test produces. We will refer to the
two sets as basic and calculated. Basic
numbers are those numbers that are
measured by the dynamometer system.
Calculated numbers are those which use
the result of observed or measured basic
numbers applied in a formula.
A good example of a calculated
number might be everyone’s favorite
number; horsepower. Dynamometers do
not directly measure horsepower, in fact,
they measure torque and rpm and a
formula programmed into the
dynamometer control computer system
called an internal algorithm makes the
actual calculation and posts the
horsepower value in the appropriate data
field. Any seasoned dynamometer pro
will always refer to the engine output as
lb.-ft. of torque at a given RPM. Hot
Rodders and beginners like to talk about
horsepower and are usually enthralled
with only one number, the peak
horsepower produced. Professional
Engine builders will talk about the shape
of a torque curve, where it starts and
where it noses over. We will discuss these
terms and some more so hopefully the
readers will be able to use more of the
available data from their dyno testing.
20 OCT-DEC 2009 engine professional
Basic Number
Overviews
Revolutions per Minute or RPM
Let’s look at the basic numbers first.
These are the numbers that form the
foundation for all of the dynamometer
data. The first and most obvious is RPM
or revolutions per minute that the engine
is producing. This number serves as the
scale or reference point for all testing
data. No matter what data you have or
produce, you always need to know how
fast the engine was turning when the
data was taken. Whenever you graph
any of the dyno data, one axis of the
graph will usually always be RPM.
Torque or Lbs.-Ft. (pounds-feet)
Torque or the twisting action of the
engine is the next basic number and by
far the most important number for
measuring raw output of the engine. This
is the amount of torque produced at the
flywheel.
Pressures usually measured
in PSI (Pounds per square inch)
Oil, fuel, boost, vacuum, coolant,
exhaust — various pressures are
recorded during dyno testing so they can
be posted next to the various other data
entries. This enables the dyno operator
to quickly see trouble coming or explain
gains or losses in the torque. For
example if the fuel pressure drops from
one rpm point to the next and is
accompanied by a sudden change of
torque, a sharp dyno operator would
immediately stop the test and look at the
fuel delivery system to ensure it did not
have a problem.
Flow – usually measured in pounds
per hour or cubic feet per minute
Air, fuel, oil — proper flow is
critical to good engine performance,
whether it be engine oil flow or fuel flow
through the injectors. If the flow is not
there, regardless of the pressure, find the
problem and fix it.
Temperature
Oil, coolant, exhaust gas, wet bulb, dry
bulb, inlet air temperatures are very
important because we not only use them
to keep track of safety items, but also for
comparative analysis for correction
factor calculations.
CAT or Carb Air Temperature
This is very important because it serves
as a basis for the correction factor
calculations. It is also an indicator of
how good the dyno room is at
circulating cool fresh air. If the air in the
dyno room is too hot, there is also a
good chance that it is getting some
exhaust contamination, which will lead
to severe power loss.
Fuel Temperature or Fuel Temp
This number is also important because it
is used in the fuel density equation to
determine the fuel flow for fuel
consumption calculations.
Oil Temperature or Oil Temp
This value is very critical. To illustrate
this to yourself; warm up an engine to
the minimum acceptable starting oil
temperature and then make a number of
successive acceleration runs, one after
another, changing nothing. Watch how
the horsepower changes as the engine
gets hotter. This experiment will prove
better than any explanation I can give,
just how important it is to make each
run at the same starting oil temperature.
Also as a side note, make sure that the
oil temperature transducer is in the same
location for all of your test engines. If a
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DYNO NUMBERS
Pressure vs. Flow
BY BILL HANCOCK
Does good pressure indicate
good flow? Do not confuse the
This basic engine combination is
called a dyno mule engine. Its specific
purpose is to serve as a test engine.
You would do well to build an inexpensive moderately powered but
extremely durable engine just to learn
how to run your dyno before ruining
an expensive customer engine.
transducer is off in a corner of a
particular oil pan it affects the observed
temperature rise rate.
Water Temperature or Wat. Temp.
Much of the above goes for water
temperature. Additionally you can use
strategically located temperature
transducers to measure the temperature
difference often referred to by engineers
as the “Delta T” across oil coolers or
radiators. The higher the delta T, the
more efficient the cooler is. Of course,
having air blowing across the cooler or
submerging it in a tank of cold water is
necessary in the dyno room, since the
engine is stationary.
Calculated Numbers
Horsepower or HP
As we said in the beginning we use the
basic numbers to derive the calculated
numbers. The formula for Brake
Horsepower as we all know is:
HP = T x RPM / 5252
Where T = Torque
HP = Horsepower
RPM = Revolutions / minute
Corrected Brake Horsepower
= Corr. BHP
©Bill Hancock 2009
This number is important to
determine the optimum RPM range for
the engine. If you stay within the power
band, the engine will perform well.
There are several computer programs
that will let you optimize shift points
and decide between 4, 5, and 6 speed
transmissions. Especially important is
where the power curve noses over and
increasing RPM just means dramatically
less power. Just because the racer in the
next pit is turning 8900 RPM doesn’t
mean that you should. Your engine may
be all done at 7800 or it may be just
22 OCT-DEC 2009 engine professional
leveling out at 8900! A good test session
will let you find out. In the end you may
make more power at 7800 than he
makes at 8900! KNOWLEDGE IS
POWER.
Air/Fuel Ratio or A/F
Air fuel ratio is the weight of air being
used per weight of fuel in the
combustion process. In a gasoline engine
with perfect combustion there are
approximately 12.7 parts of air
consumed for every part of fuel. This
results in a perfect burn where there is
nothing left. When there is perfect
combustion the ratio is referred to as
stoichiometric. In a case of 14:1 air fuel,
there is excess air left and the mixture is
called lean. The inverse or rich condition
would be an A/F ratio of 11.8:1 and
excess fuel is left. As tuners we typically
settle on an A/F ratio that is slightly rich
to give us some safety factor.
Brake Specific Fuel Consumption
or BSFC
This is a measure of the pounds of fuel
required to make one horsepower for
one hour. New dyno users often try to
use this number to reflect fuel mixture,
since it sometimes follows fuel flow. This
number is really a measure of engine
efficiency. If, for example, you had a
bearing tightening up, the torque and
hence the horsepower would drop but
the fuel flow would remain the same.
This would be a signal that the engine
efficiency was down since it was
producing less output for the same
amount of fuel.
Brake Specific Air Consumption
or BSAC
Like BSFC, BSAC is a measure of air
consumption efficiency. If you saw a
sudden drop in air consumption from
run to run, you might look for a worn
cam lobe or a plugged air filter. Going
relationship between pressure
and flow. Let’s take the engine
oiling system as an example. If
you had a plugged oil filter with
no bypass; depending on
where you had mounted your
oil pressure transducer, you
might have wonderful oil
pressure but virtually no flow. In
a different situation you might
have excellent oil flow through
an engine, but if it was not at
sufficient pressure, the oil
would never reach the rod
bearings at higher engine
speeds. Oil enters the
crankshaft from a main bearing,
and from there it must fight
centrifugal force as it heads
toward the center of the main
to meet the oil hole leading to
the rod journal where it gets a
free ride out to the rod bearing
courtesy of the same
centrifugal force. If there is not
sufficient oil pressure, the oil
can never reach the center of
the crank. The minimum oil
pressure required to reach the
center of the crankshaft varies
directly with RPM. This
minimum pressure is
often referred to as
the “drop dead
pressure”.
What is
Horsepower?
People often get horsepower
confused with torque. Try thinking of Torque as how much a
weight lifter can lift and horsepower as how many times in
an hour he can lift it. It
becomes apparent that the big
torque number may not be
everything. If the lifter can only
lift a 300 Lb. weight twenty
times in an hour versus a lifter
who can lift 5 lbs less but do it
seventy times in an hour which
one makes more horsepower?
Obviously the second lifter
would be the winner!
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This electric
dynamometer
made by Schenk
is capable of
motoring an
engine to measure friction. Being
able to measure
internal engine
friction can be a
very valuable
capability.
from one set of headers to another will
often change BSAC due to different wave
tuning.
Volumetric Efficiency or VE
This is a measure of how effective your
engine is at filling the cylinder at a given
RPM. It is not uncommon to see values
over 100% on some of the higher
powered well developed engines. This
number gives you a good benchmark on
how well your induction system is
coordinated. High flowing heads with
the wrong camshaft will show up here.
Mechanical Efficiency or ME
This is a measure of how much power is
lost to friction. A value of 100% would
indicate that there was no friction, so
obviously you want to get as close to
100% as possible.
Oil Pressure or Oil PSI
This value is critical because this is used
a key indicator for engine safety. On a
typical V8 try to have 10 psi for every
1000RPM, so at 6000 RPM you would
want to see 60 psi of pressure. A set of
safety limits should be set and engaged
before each run to force the engine to
stop if the oil pressure goes below a
predetermined value.
Manifold Air Pressure or MAP
or Vacuum
This is the absolute pressure in the inlet
manifold. It is important because it is a
good indicator if the manifold is sized
correctly for the engine. Because
manifold pressure is load sensitive this
pressure is frequently used as a reference
for spark timing, so it is very important
to keep your eye on this.
Frictional Horsepower or FHP
Imagine spinning an engine over with no
combustion or compression. The bearing
loads and valve train load plus the oil
This box is where the
various transducers and
thermocouples send
their signals so the dyno
computer can apply the
formulas for calculations
arrange the results for
presentation in a logical
order for the operator.
pump, piston rings, piston skirts and oil
seal loads are all contribute friction. The
engine has to produce power just to
overcome those loads which typically
increase with RPM. Think of frictional
horsepower as the power that is required
to stay in the engine just to get the
flywheel horsepower out. If you think of
it another way, if you were to reduce the
frictional horsepower by making the
short block turning torque lower, the
torque would end up as usable
horsepower at the flywheel. Frictional
horsepower is measured on some electric
motoring dynos, but for water brake
Barometric
pressure and
barometers
One of the most critical measurements in dyno testing is the
barometric pressure, since it
forms the core for the correction number applied to each
run. For that reason, all serious
dyno users should strongly
consider having a
dedicated mercury
column barometer
located near their
test cell to enable
them to properly
calibrate the electronic
barometer within the
dyno system itself. Properly
calibrated and maintained, the
mercury column barometer is
extremely accurate and very
reliable. Barometric pressure
at sea level stays around 29.92
inches Hg. While it typically
reads 25 inches Hg. in Mile
High Denver, Colorado.
dynos, it is a calculated number based on
a formula for typical engines.
Where do the numbers
come from?
In most cases, various pressure
transducers, strain gages, LVDTs (Linear
variable displacement transducers) and
thermocouples produce discrete voltage
outputs which the computer interprets
and translates into usable test data.
How do we use
the numbers?
We use some numbers as a reference in
the case of RPM to denote where the
data was taken. Other numbers we use
comparatively when we compare one
EGT (exhaust gas temperature) to
another to analyze the fuel distribution
among the cylinders. The third way we
use the numbers is to set limits. Most
modern dynos have a safety table which
allows the operator to set limits for
virtually all of the values and program
certain functions ranging from a printed
warning all the way up to an immediate
full load shut down. Which, you would
really want for times when suddenly
something broke and you wanted to stop
the engine before any more harm was
done. Another way values are used is to
calculate correction factors. We refer to
these operations as secondary
calculations.
Standards
Since we are in the USA, most of our
measurements are still taken using the
English FPS (foot, pound, second)
system. For Temperature we gather data
in deg.F. We use inches of mercury for
barometric pressure.
Correction Factors or CF
Think of a correction factor as a data
weather calibration. This allows us to
test on days where the weather is
engine professional OCT-DEC 2009 23
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DYNO NUMBERS
BY BILL HANCOCK
markedly different, yet get the comparably correct end
results. By gathering weather data and inputting it into the
dyno system computer prior to a test, we can ensure that
our resultant performance data is very accurate. If you
should choose to ignore this, you will invariably end up
going out to the track with an inferior product. Get in the
habit of looking at the corrected numbers as well as the
raw numbers. If the corrected numbers seem too far off,
either you have a tornado coming in or you messed up the
data input for your CF.
The formulas for correction factors vary. Most of the
correction factors stem from the original SAE (Society of
Automotive Engineers) work. Some of the more popular
documents are designated as J607 and J1349.
Where do correction factors apply
Only normally aspirated engines run at WOT (Wide Open
Throttle). The correction factors do not apply to: Part
throttle, supercharged, or turbo charged engines. If you
apply the correction factor to these combinations, all you
will do is fool yourself and look stupid in front of the
customer, who probably already knows this.
TECH & SKILLS
REGIONAL
CONFERENCES
Getting Started
The first rule of dyno testing should be to keep good
records. Prior to each run or series of similar runs you
should enter the pertinent data such as changes and current
engine configuration and recheck the weather data to
ensure that the correction factor will be accurate. Don’t
forget to enter a value for the specific gravity of the fuel
you are using. The fuel flow transducer needs this value to
accurately measure the fuel flow in lbs. /hour.
Before you go out and look extremely dumb, use one
of the many fairly accurate engine performance computer
programs to predict the horsepower of the engine you are
getting ready to run. Divide the peak horsepower number
by 2 and the result should be the number of pounds of fuel
per hour that it will take to achieve the maximum
predicted horsepower. I like to go 10% more just in case I
get lucky! Make sure that your fuel flow system will flow
that much fuel AT THE RATED FUEL PRESSURE.
Save the Date!
Dyno Inlet Water Temperature
On a water brake dyno, this temperature number becomes
critical when the water is too hot and turns to steam when
it enters the absorber unit. The dyno immediately loses
load control and the engine over revs. Additionally, the
phase shift that occurs when water turns to steam inside of
the absorber causes severe wear on the dyno impeller and
stator. Monitor dyno inlet water temperature very carefully
and make sure that you never begin a test at a water
temperature too high and you’ll prevent this steam
condition.
SAT. NOV. 14
EPWI,
Dallas, TX
2009 Conferences
SAT. OCT. 10
Sunnen & Mahle,
University of NW Ohio
Lima, OH
Evaluating the Results
After your first full set of runs, it often helps to graph the
results so you can visually grasp the shape of the torque
curve. You will learn that the top tuners really focus their
work on shaping the torque curve. If a torque curve has
too many humps and dips, the driver will never be able to
24 OCT-DEC 2009 engine professional
500 COVENTRY LANE, SUITE 180
CRYSTAL LAKE, IL 60014
888-326-2372
WWW.AERA.ORG/CONFERENCES
EP 10-2009 14-33_Layout 1 10/8/09 1:45 PM Page 25
With SBI cylinder head parts,
you’ve got the job well in hand.
effectively pedal the car at the edge of traction and
ultimately will settle for a part throttle setting that is
controllable but not competitive. Ultimately you want the
driver to be able to hold the throttle wide open and have
the torque rise predictably and remain just below the
available traction limit. This will allow the driver to be
smooth and quick and focus on positioning the car versus
just keeping it off the wall or in the middle of the drag strip.
Driving a car with a
lumpy torque curve
is like trying to
dribble a football.
At SBI, our top priority is getting the exact quality
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So give SBI a call soon -- we make it our business
to see that you’re not left empty-handed.
The Numbers
The best way to get familiar with the numbers and how
they relate is to make several hundred dyno runs (you
would be surprised at how quickly this happens) and
carefully analyze them as you go along. Keep good notes
and take your time. Don’t base any decisions on one run,
try to make a series of runs until the engine settles down
and produces three runs within 1% of each other, then
average those three.
Keep track of each change and above all only make
one change at a time. Don’t fatten the fuel, change the
spark and advance the cam in one move. The end result
may be better, but which change made it better? Learn to
slowly and methodically work through a test plan and
pretty soon you will find yourself being able to better feel
what the engine wants. Keep a close eye on the internal
health of the engine by checking the oil filter and
continually running leak down and compression checks. If
you don’t do this, you will waste far more time and money
developing something that works well on an engine with
seven cylinders that leak over 20%.
Cautions
Never go directly to the track with a perfect max-power
dyno fuel and spark calibration. Always back off toward
the safe side and sneak up on the best car setting during
practice. Things like temperature, coolant flow and inlet air
flow are usually subtly different in the car at the track.■
S. B. INTERNATIONAL, INC.
2108 Utopia Avenue - Nashville, TN USA - 37211
Order desk: 800-843-7328 Fax: 615-248-6377
www.sbintl.com
Bill Hancock is a mechanical engineer and the founder and former owner of
Arrow Racing Engines for almost 30 years. He has presented numerous
papers and seminars on dyno testing and tuning. He and his longtime friend,
Harold Bettes, formerly the VP of SuperFlow Corporation, recently co-authored
a book entitled, “Dyno Testing and Tuning” (reviewed by Mike Caruso in
Engine Professional, July-Sept 2008) which is available in local bookstores or
from CARTECH Publishing, ISBN-101932494495.
© Bill Hancock 2009.
engine professional OCT-DEC 2009 25