If you like fiction, you'll like this adage, " It makes
no difference where I test my motorcycle, the air
density correction factors will make my engine numbers
comparable no matter where the testing took place."
Look closely at all of the various accepted air density
correction formulas. I can count four that we work with
on a daily basis (DIN 70020, EEC 80/1269, ISO 1585, SAE
J1349), and who's to say which one was used and with
which brand of dyno?
They all state if a variance is greater than 3 percent
of what you're trying to correct to, it is suspect. So,
say you want to correct to SAE J13490150–atmospheric
conditions to air temperature of 87 degrees, 29.235
inches-Hg (990mb) actual pressure and 0-percent relative
humidity, you're testing in Sturgis and you want to
compare results with a test done in Daytona. The only
prize you win is being close to 87 degrees. The rest of
the corrected numbers are as far off scale as Daytona is
from Sturgis.
On the other hand, if you're in Sturgis and you want to
compare power and torque numbers on your dyno on a
day-to-day basis, then your testing is probably valid
because it's unusual for there to be a significant
atmospheric variance between the tests day to day. In
this case, it works.
2. Fluctuating CO Levels
It's
important to control what your engine is breathing
during testing.
It takes about 10 parts-per-million (ppm) of carbon
monoxide (CO) to give you a headache and a lightheaded
feeling. At about 6 ppm, your engine also feels the
difference. I've lost count of how many times I've
witnessed a dyno session where CO continued to build up
in the engine test cell from small exhaust leaks or
crossover with discharged exhaust being sucked out of
the exhaust duct and being drawn in the intake duct.
Everyone wonders, "Why doesn't the engine repeat? Why is
it down on power? What is happening here?"
Clean air is paramount, so in order to get accurate and
repeatable results testing must be done in a test cell
that exchanges the air every second and also have
neutral cell pressure ( no vacuum or pressure).
Testing your motorcycle in a open shop area is not going
to give accurate results, your just wasting your time
and money.
At the other extreme, most shops that strive for the
most accurate results will have spent ten's of thousands
of dollars on their test cell's in controlling the air
their engines breath. They demand consistency.
Without consistency all test results are inaccurate and
useless.
3. Inconsistent Engine Temperatures
Always control the engine temperatures. The secret to
obtaining repeatable results is beginning each and every
"pull" at the same engine oil temp.
Oil temperatures can heavily influence how well an
engine repeats. We use a window of 2 degrees. If the
engine's oil temperature is outside the 2-degree
tolerance, the test doesn't begin.
Obviously, in these cases, we may be looking for a gain
as small as 0.25 hp, but we will actually find that
difference and know it's real–and not attributable to a
difference in oil temp from one test to the next.
We all know how much money can go into engine
development and testing and it is very time consuming
and costly chasing "temperature ghosts," so why should
you want to waste time and money on testing when the
facility is unable to monitor your oil temperature and
start their test at exactly the same temperature every
time.
Also, have you ever looked at the temperature of your
fuel when testing? It can and will make a huge
difference.
4. Differing Acceleration Rates
What kind of a test is being run on the dyno? Is it a
controlled rate of acceleration test or a step test or
just an inertia test ( inertia test is what most shops
use on chassis dyno's)?
It really doesn't matter if you're testing with an
engine dyno or a chassis dyno. The point is that you
can't compare a test run at one rate of acceleration
with another one at a different rate.
It seems obvious, but many times people will compare one
test with a controlled acceleration rate of 200 rpm/sec
and another test done at 500 rpm/sec (or, in the case of
the chassis dyno an uncontrolled inertia-only test).
In the end, you come up with two totally different sets
of torque/hp numbers/graphs. Why? Because it requires
power to accelerate mass. Engines have rotating mass.
Chassis dyno's have engine rotating mass and driveshaft
mass and wheel and tire mass. Accelerating that mass
requires power. So, you always see less power to the
flywheel or the chassis rolls when you have higher rates
of acceleration. If you want to see your highest power
numbers, just do a steady state test or step test and
log data at each step (the best test is a steady state
test allowing 5-10 seconds to stabilize and then sample
for 5 seconds at each rpm that you test at, we like to
test at 250 rpm increments to assure fuel curve and
ignition timing accuracy).
In this step, the engine doesn't have to accelerate from
one rpm to another and the power numbers for a given rpm
will be higher than when a test is done using a high
rate of acceleration ( 500 rpm increments) at the same
given rpm point.
5. Using Unverified Weights
Finally, be sure to calibrate the load cell with a set
of verified weights.
Oftentimes, dyno operators can't remember the last time
they calibrated the dyno. In most instances, chassis
dyno's can not be calibrated
and this is why there are so many inaccurate results and
inconsistencies when you have your motorcycle tested.
Like I said, the answer to
success is in the details.
Make sure that when you have your motorcycle tested that
the facility that is doing the testing is able to meet
all the requirements that we have discussed. If
they can't then the test results are false and
inaccurate.
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