Truth, Lies, and Dyno Runs
03-19-2008, 11:31 AM
#1
5th Gear Member
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Join Date: Mar 2007
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Posts: 2,257
Truth, Lies, and Dyno Runs
A popular thread that continues to surface in various enthusiast forums is related to the
results attained from various chassis dynos. In the following paragraphs, I’ll attempt to
explain the various types of chassis dynos and their operating principals, how they’re
used in the field, and how to interpret the results.
I know that you’re thinking, “oh boy,
the VP of Dynojet is going on a sales pitch”, and that couldn’t be further from the truth.
I’m an avid automotive enthusiast, and no, that doesn’t mean I just plop down cash for
the latest exotic car. I’ve made thousands of dyno runs, tuned 1000’s of automotive
ECM’s, designed and fabricated my own turbo kits, and made hundreds of passes at the
strip. I hope to educate the sometimes confusing subject of chassis dynos and how
they’re used in the field.
There are generally two types of dynos that are used for performance verification and
tuning, “inertia type loading”, such as the Dynojet 224x or 248, or “electric type
loading”, such as a Mustang 250, 1100, Dyno Dynamics, or Dynojet 224xLC / 424xLC.
Traditionally Dynojet has offered the inertia loading dynos, whereas Mustang
Dynamometer and Dyno Dynamics have been electric type loading dynos.
The major differences in the two types of dynos are their principals of operation. A true
inertia dyno (such as the Dynojet 224x or 248) uses large steel rollers that contain mass.
This mass is fixed, it can never change, and for those that remember high school physics,
Force = Mass x Acceleration. Based on the time that is required to accelerate a mass (the
steel drums in this case), you are effectively measuring force. Ok, so now that we have
force, how does an inertia dyno come up with horsepower? Simple, force (lbs) multiplied
by speed (ft/sec) effectively yields horsepower. Since every Dynojet dyno on the face of
the earth has a mass that has been precisely quantified using a proprietary process, and
that value is stored in the dyno software for each dyno, not only are the horsepower
numbers consistent every morning, noon and night, but each and every Dynojet is relative
to one another. Go ahead, take your car to 25 different Dynojet dynos, run it up, and I
personally guarantee the horsepower will repeat to within 1/2 HP (no one else would dare
make that claim). What if the numbers aren’t the same between the 25 Dynojet dynos,
well, it’s quite simple to explain, and you need look no further than the vehicle. Most
modern powertrain management systems have a lot of authority when it comes to how
much power they ultimately put to the wheels. Capturing OBD2 parameters such as
spark advance, engine coolant temperature, inlet air temperature, mass air flow, and other
critical PID’s provide the insight required to determine why the vehicle did, or did not
repeat.
So let’s take a step back before we move on and look at this from a practical perspective.
As an enthusiast, you may be wondering, “what are the variables that exist when I dyno
my car on a Dynojet, there has to be something”. I would respond, “
A Dynojet dyno
operator CAN NOT change the data that determines what the power reading will be
(remember the physics thing?). What ever power is being put to the drums will be
measured and displayed, period.”
[font=times-roman]Once that power has been measured, there are two
ways to look at it, corrected or uncorrected. Since every Dynojet dyno is equipped with
electronics that measure the atmospheric pressure, temperature, and humidity, the results
are able to be analyzed as a “corrected value”. This allows an “apples to apples”
comparison when testing in regions that are at different altitudes, which affects the
atmospheric pressure, and different temperature. This is also important for comparing
results at the same dyno shop, for example, here in Las Vegas our atmospheric pressure
stays relatively consistent, but our temps vary from 38 degrees to 118 degrees. So the
same car with no changes will certainly make less power on the hot day compared to the
cool day, but applying the SAE CF allows us to make an “apples to apples” comparison.
If you take anything away from the last few sentences, it would be to ask your dyno
operator to make sure that he is showing you SAE corrected HP figures, and before I get
off my soap box, I will say that there are certain Dynojet dyno owners who apply the
“STD” correction factor, and that’s not proper to do. The STD CF is an older standard
that yields slightly higher HP numbers, so don’t allow yourself to fall into the trap (just
use the SAE CF!!).
Ok, lets move on to “electric type load chassis dynos”. These types (Mustang, Dyno
Dymamics) of dynos utilize rollers that usually have very little mass, hence they’re not a
true inertia chassis dyno. In order to present any type of physical load on the vehicle,
there needs to be a PAU (power absorption unit). Typically this PAU is in the form of
electrical coils that utilize eddy current technology (think of this as a big electric brake).
Now having a big electric brake is all very nice, but we need to use a chassis dyno to
measure power at the end of the day. Since the mass of the rollers isn’t known (like a
Dynojet), there needs to be a means to measure power, so how this is done is with a
torque cell, or sometimes referred to as a strain gauge. As the big electric brake absorbs
power, the strain gauge is actually measuring torque. Now here is the kicker, the strain
gauge really doesn’t know the difference between 2 ft-lbs or 200 ft-lbs until you calibrate
the device, so in doing this, you’re introducing a potential margin of error and
inconsistency. This would apply to any "loading type dyno" that uses eddy current load
control and a strain gauge, even our own 224xLC which is a hybrid inertia / electric load
style dyno. The strain gauge needs to be calibrated to make it consistent and repeatable,
but there are other user defined (by the dyno operator) variables that makes things very
interesting. Once the strain gauge is calibrated properly, the dyno operator must enter
certain parameters about the test vehicle that determine the rate of acceleration the dyno
will allow, and are ultimately directly responsible for the power and torque readings that
are displayed on the graph (WOW!). At this point you may have connected the dots and
are either appalled, or still rather comfortable with the dyno graph you have in your
hands. The question that pops up, “my car made 280 rwhp on a Mustang dyno, what
would it make on a Dynojet”, should be countered with the statement, “I’m not sure, but I
do know this, you’d most likely get 10 different numbers on that same Mustang dyno, let
alone another Mustang dyno that’s installed in the field”. Now this isn’t meant to bash
[left]Mustang dyno or Dyno Dynamics, but some dyno owners just can’t resist
03-19-2008, 12:19 PM
#5
5th Gear Member
Thread Starter
Join Date: Mar 2007
Location:
Posts: 2,257
RE: Truth, Lies, and Dyno Runs
Just so you guys know, I didnt write this. 
The VP Dan Hourigan did. He is actually tuning my car this weekend so we have been talking back and forth. We got into the different dyno discussions and he sent me this. He said it was ok to post up so I did
The VP Dan Hourigan did. He is actually tuning my car this weekend so we have been talking back and forth. We got into the different dyno discussions and he sent me this. He said it was ok to post up so I did
03-19-2008, 06:23 PM
#9
4th Gear Member
Join Date: Jul 2007
Location: East Moline, IL
Posts: 1,852
RE: Truth, Lies, and Dyno Runs
Devil's advocate here, but I've heard basically the same arguement from someone talking about the Mustang dynos, but just the opposite. I guess they both have their pluses and minuses. If there was a dynojet here, I'd love to compare my numbers. I used a DYNOmite AWD dyno and it seemed really consistant between both my S197's. They both have similar mods. The '05 has X pipe which gives a few more ponies, but the '07 has charge motion delete plates, and according to my many passes on the DYNOmite, they're within 3HP from each other.
Also, I like the electric brake on the DYNOmite. I was standing next to a 750+ HP Talon when it slid off the dyno (due to the downpipe burning through the tie-downs), and while the car may not have gone anywhere, I felt quite a bit safer when they were able to stop the rollers very quickly.
And please don't take that as an arguement. I'm not meaning it to as such. I just have not been able to have experience with a Dynojet, and seem to have come out with good results from a competing dynamometer.
Also, I like the electric brake on the DYNOmite. I was standing next to a 750+ HP Talon when it slid off the dyno (due to the downpipe burning through the tie-downs), and while the car may not have gone anywhere, I felt quite a bit safer when they were able to stop the rollers very quickly.
And please don't take that as an arguement. I'm not meaning it to as such. I just have not been able to have experience with a Dynojet, and seem to have come out with good results from a competing dynamometer.
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