Since most people
don't have dynos in their driveways, we have to rely
on others, usually on those who are selling us something,
to test our cars. Sometimes these dyno operators are
sloppy, sometimes wishfully optimistic, and occasionally
downright dirty. To give you a chance of spotting
the unrealistic dyno results, here are the easiest
ways to lie with a Dynojet:
Other modifications (Oh, that
car was turbocharged?)
This is perhaps the oldest trick in the book, and
a favorite for the liar with a conscience. Say you
are quoting power output on an exhaust system. It
is natural to expect that someone buying an exhaust
would also have some other performance parts, so it
is not too far fetched to test a slightly modified
car. So you stick a stock exhaust on that modified
car, test it, and then put your exhaust on and test
it again. This is still an honest assessment of what
the exhaust did, but if that car happens to be a Civic
DX, and the other modifications happen to be a 15-psi
turbo system, the 30-hp gain now claimed for the exhaust
will be more than a little unrealistic.
Hot vs. Cold
Most cars make more power on their first Dyno pull
while everything is still nice and cool. If you allow
a reasonable cooldown period between runs (only a
minute or two is needed on most cars) all the subsequent
runs should be consistent with each other. The natural
inclination of anybody trying to show a power gain,
though, is to use a lower, hot run as the baseline,
and a nice, strong cold run to showcase their part.
I have seen people do this with no malicious intent,
just elation that their part made so much power. "As
soon as I put the power pulse super-mega-flagenator
on the car, it made 17 hp on the first pull!" Never
mind that it made only 13 hp on every subsequent pull.
Because there are so many different
levels of cold runs, the only realistic data, unfortunately,
is to compare consistent hot runs. Whenever possible,
we actually monitor coolant temperature and make each
run within a five- to ten-degree window. Our Nissan
Sentra SE-R, for example, is always tested between
185 and 195 degrees.
Different Gears
Back in the May, 1998 Technobabble, I dyno tested
a Sentra SE-R in every different gear. Second, third
and fourth were very similar, but first and fifth
were quite a bit lower. Testing a car in either first
or fifth is a little odd, and would probably be noticed
by the car owner if they were observing the test.
The differences are much more significant on a turbocharged
car, though, as the higher gears slow the engine's
acceleration and allow the turbo to spool up earlier
and stronger. If you do a second gear pull with a
turbocharged car, the engine will often outrun the
turbo--by the time the turbo is spinning fast enough
to make boost at 3000 rpm, for example, the engine
will already be going 4000 rpm; by the time the turbo
is fast enough for 4000, the engine is at 5000.
This is why turbocharged cars
feel so strong in higher gears when pulling up a hill,
or whenever they are heavily loaded. If you compare
a turbocharged pull made in second gear to a pull
made in fourth, the fourth gear run will almost always
be stronger. We typically dyno naturally aspirated
cars in third gear, and turbocharged cars in fourth.
Another difference that can occur
with gearing comes when there is a 1:1 gear ratio
available. Most rear-wheel drive gearboxes will have
a gear that is 1:1--typically this is fourth gear,
but in six-speed gearboxes it is often fifth. In the
1:1 gear in many gearboxes, power is not actually
transferred through a gearset; the input and output
shafts are just locked together. This eliminates the
losses through the gear teeth, and results in a higher
power output in that gear.
Random Variation
There is always a small variation from run to run.
Some of these variations, like the difference between
a hot engine and a cold engine, can be predicted and
controlled. Others, like the temperature of the transmission,
the alignment of the wheels on the roller, and the
slight difference between the actual intake temperature,
and the dyno's correction temperature as measured
elsewhere in the dyno room, are harder to control.
Cars that tune themselves while you are testing can
make this problem even worse. Run-to-run variations
of less than 1 percent are normal, but if you take
the lowest baseline run and compare it to the highest
test run, you can easily make something look better
than it is. The best way to avoid being snookered
by this one is not to split hairs. A 1 or 2 hp difference
is not really significant enough to feel, even if
it is real and repeatable, so why worry about it?
Corrected or Uncorrected
Dynojets can read corrected or uncorrected horsepower.
Because different weather conditions can result in
different air densities and different oxygen concentrations,
the weather can have a significant effect on power
output. The SAE has a standard set of correction factors
that can be used to normalize all power outputs to
what they would be at sea level, on a 60 degree day,
with 0 percent humidity. Every Dynojet has a small
weather station built in to feed the appropriate temperature
and barometric pressure readings to the computer so
it can calculate this factor. The difference between
0 percent and 100 percent humidity is about a seven
percent correction. A temperature change from 60 to
90 degrees, on the other hand, will have an effect
of about a 2.8 percent. A difference in elevation
from sea level to 5000 feet is worth a whopping 20
percent!
If you use uncorrected data, the
changes in power output due to weather conditions
could be misconstrued as being a result of something
else. For example, if you have a dyno sheet showing
that a header made a 15-hp gain, but the runs were
done on different days and the plot was made with
uncorrected data, you can't be sure if that is really
a 15-hp header, or an 8-hp header and a 7-hp weather
change.
Corrected numbers, on the other
hand, can be suspect in certain cases as well. Turbocharged
cars running at high altitude, for example, might
be more accurately represented by uncorrected numbers.
Say you are testing an FD3S RX-7 in Denver, where
the elevation is approximately 5,000 feet. Shiv Pathak,
master of our FD3S RX-7 project, reports that he always
sees higher boost levels at high altitude. The reason
is simple. The wastegate opens when boost is 12 psi
higher than the normal sea-level reference air behind
the wastegate actuator diaphragm (air that has been
stuck in there ever since the diaphragm was sealed
somewhere in Hiroshima). As the air density drops
at high altitude, the actual pressure in the boosted
intake manifold remains constant. The boost gauge,
though, reads pounds of boost over ambient pressure.
If the ambient air pressure in Hiroshima was 14.5
psi when that diaphragm was sealed, but it is only
13.5 psi when Shiv drives through the mountains, his
boost gauge will read 1 psi higher than normal.
The SAE correction factor used
by Dynojet assumes that lower air pressure at the
sensor box means lower air pressure in the intake
manifold, though, so at 5,000 feet the dyno is applying
a 20-percent correction factor to compensate for a
loss of air density that the engine never sees. This
is fine if you are doing all your tests in Denver,
but if you do one test in Denver and one test in New
Orleans (the highest mountain in New Orleans is 12
feet above sea level) uncorrected numbers will be
more accurate.
The SAE correction factors are
only accurate over a relatively limited range, and
the Dynojet software is smart enough to warn you when
two runs with wildly different correction factors
are being compared. The software in New Orleans can't
check your glovebox for that last dyno printout from
Denver, though, so you'll have to warn yourself.
Messing With the Weather
This is where the dyno operator has to be extremely
devious. If you mess with the readings that the computer
uses to calculate the correction factor, you can alter
the corrected output significantly. The one reading
that the built-in sensors do not take automatically
is humidity. The dyno operator has to enter the humidity
correction themselves. Since the humidity is manually
entered into the computer it is the easiest to alter.
While humidity numbers are obviously suspect, the
temperature can be faked pretty easily as well. We
actually decided to try this one, just to see how
easily it could be done.
Jackson Racing's Dynojet is set
up with the weather data box mounted on a perpetually-shady
portion of the dyno room wall. The temperature probe
hangs under the box in the open air about two feet
off the ground--right where most air intakes pull
their air supply. Oscar Jackson pointed out that he
has seen these boxes mounted where they were more
easily accessed, and has even seen the temperature
sensor hanging on a divider wall next to the computer,
or in a drawer on the dyno bench. In the drawer, an
unscrupulous dyno operator could put his or her hand
around the sensor before doing a run, bumping the
ambient temperature reading up into the 90-degree
range. With it hanging on a well-placed wall, the
sensor could be flipped from the shade into direct
sunlight, where it could slowly bake up to a nice,
warm temperature.
The wire on Jackson's sensor was
only about two feet long, so we couldn't get it into
the sun, and a shield prevented holding the sensor
in a warm hand from having much effect. Instead, I
cupped my hands around the sensor and blew on it.
Within 30 seconds the dyno was reading 95 degree ambient
temperatures even though our baseline run made a few
minutes earlier had been in 66-degree air. We made
another run with the engine breathing 66-degree air,
but the dyno correcting for 95-degree air. Our corrected
power jumped from 136 hp to 143 hp.
Air Conditioning
This seems almost too easy to mention, but the fact
is that it is one of the easiest and most effective
ways to make a dyno lie. Just run the baseline pulls
with the air conditioning on. The only ways you can
tell if the air conditioning is on from the outside
of the car is to look at the clutch on the air conditioning
compressor, look at the fans on the radiator (most
cars will kick on a cooling fan whenever the air conditioning
is on) or look at the face of the dyno operator. No
sweat? Be suspicious.
Since the Sentra SE-R we were
testing automatically disengages the air conditioning
compressor at full throttle, we actually had to adjust
the throttle position sensor so that the ECU never
noticed it was at full throttle. This alone has some
effect on power, but based on subsequent runs without
the A/C, the difference was only 1 or 2 hp. The difference
caused by the compressor, on the other hand, was10
hp.
Since many cars do disengage the
compressor at full throttle, this trick does not work
on every car. Jackson pointed out, however, that although
the Integra GS-R is supposed to be one of those cars,
he has seen the compressor run at full throttle on
multiple occasions.
So what can you do so you don't
fall for fake dyno figures? There is no foolproof
way, short of dynoing everything yourself (and not
making any of these easy mistakes on your own), but
a critical eye is always a good idea. Look at the
printout from the Dyno. Dynojets always print the
weather conditions and show the correction factor
(labeled as CF). Correction factors between 0.97 and
1.03 are pretty normal. Outside that range, you should
be on the lookout for large differences between the
runs you are comparing. You should also have some
sort of idea what to expect from each modification.
If you tweak your adjustable cam sprockets, don't
expect 15 hp across the board--4 or 5 hp over part
of the powerband is more realistic. How are you supposed
to know what to expect on every single part? Well,
I can think of a magazine you might want to read...
