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Dyno Debate - Real Wheel Horsepower

The great dyno debate rages on... Which one reigns supreme.

Justin Cesler Mar 21, 2012
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Well, here we are with another story about dynamometers and we've tested three of them at a time in one article. Already interested in skipping ahead to see which one is the "best?" Don't bother, we'll come right out and say it: They are all great. Disappointed? Sorry, but it is the truth. Several years ago, the debate raged between companies, with each brand recruiting a hoard of loyal zealots who ran to Internet forums and announced "X-brand is better than Y-brand because Y-brand is terrible!" Owners of such dynamometers rallied behind this too, posting long threads about how their dyno of choice was far superior to those "other brands" and real tuning can't be done on anything but the one they have. Of course, we all know this just isn't the case, since fast and well tuned cars come out of different shops all across the country, some with Brand X, some with Brand Y, and some with no dyno at all! So, what is the real deal here?

Truthfully, we've said it before and we will say it again--a dynamometer is a tool, nothing more and nothing less, and a tool is only as good as the person using it. Just like a TIG welder, a torque wrench, a lathe, or a dragstrip. Give an idiot any one of those tools and you're probably going to end up with garbage results. Give a skilled technician one and you're most likely going to get form and function worth more than you paid. This same idea holds true for a dynamometer, and a competent tuner with proper training should be able to nail down a great tune and accurately record horsepower and torque results on almost any modern day dyno. Now, that's not to say that these machines don't have their differences; they certainly do, but the main point we're getting at here is that no dyno is "better" than another. They are just different and it's important to understand that, whether you're looking for a new shop or you're interested in comparing one set of numbers to another.

To that last point, my friend, that is where we get into murky waters. Comparing dyno numbers, even within the scope of the same brand at different shops is usually a semi-educated guess at best. At worst, well, it's almost futile. As you will see here, each major brand of dyno has its own methods for recording horsepower and torque. They also all use different systems, with different weight rollers, different control schemes, and different recording methods. Add to that the fact that all manufacturers keep the "math" behind their results proprietary even though they all use the same correction factors and you've got a recipe for confusion. As you will see on the following pages, the same car on the same day in the same location produced different results on different chassis dynamometers. This shouldn't really come as a surprise, but it does serve as an interesting experiment into the "different numbers" you may see while reading magazines, researching parts, or arguing with people on the interwebs. And while you may be tempted to gravitate towards your favorite brand and claim the other two are simply wrong, we encourage you to look at the results below with an open mind and record the differences not as errors, but just as they are, which is to say--different.


If you've been around high-performance cars for any length of time, you've probably seen a Dynojet in action. Initially built as an inertia style dynamometer, the Dynojet 224x relies on a couple laws of physics, a fixed weight and diameter set of rollers (the big round things your tires sit on) and the idea that Force = Mass x Acceleration. Understanding that the drum has a fixed mass (M) the Dynojet software measures acceleration (A) of the drums, multiples the two (MxA) and outputs Force. If these look familiar, that's because we are working with Newton's 2nd Law, which you may or may not remember from your time in high school physics. You know, the class you slept through or otherwise spent daydreaming about cars in? Knowing the Force applied to the drum, the Dynojet software then multiplies Force by Distance (or, how many revolutions the rollers made during the run) to calculate Work, which it then divides by Time to get Horsepower. Simple, right? Well, to an engineer it is and in all honesty, it is in fact a pretty simple system for finding horsepower and calculating torque (HP = (TQxRPM)/5252), which makes it fairly difficult to mess up.

The inherent simplicity of the Dynojet system, with its fixed weight roller, non-adjustable load (on standard 224 systems) and simple mathematics, makes the Dynojet reliable, accurate, and, most importantly, repeatable from run to run and dyno to dyno. In fact, during our testing, we recorded within 1 hp from our in-house Dynojet 224xLC when compared to the unit we tested with in Orlando. However, this simplicity does present some disadvantages, the largest of which has to do with load. Unless you're driving a very lightweight and extremely efficient car, we doubt that a standard Dynojet will produce the same acceleration rate in 4th gear as it would on the street or track. This is usually seen in high horsepower runs, where the dyno pulls take literally just seconds to complete. On a standard car, this makes it somewhat challenging for tuners to replicate the effects of additional load, which makes it difficult to perfectly dial in certain aspects of the tune. With turbocharged cars that rely on load to build boost, this "light" load can also affect overall boost and, more importantly, the boost curve, which can and will effect your street session. Luckily, Dynojet has addressed all of these issues with its Load Control equipped models (look for the LC after the model name), which use an Eddy Current absorption unit to apply additional load, allowing tuners to accurately model real-world conditions on the chassis dyno any time they need it. With user-adjustable load or the company's "wind drag simulation" feature, almost any situation can be modeled, which is perfect for tuners and drivers looking for that additional accuracy.

In practice, operating the Dynojet 224x is a fairly straightforward procedure, and for our testing we simply backed up onto the large roller and strapped the STI Killer down to the dyno using two forward straps (wrapped around the front control arms) and two rear straps (wrapped around the rear axle). An inductive pickup is placed around a spark plug wire to acquire an RPM signal from the car and tire pressure was checked to verify consistent pressure from run to run. Starting a test requires the operator to "drive" the car up through 4th gear (1:1 ratio on a T56), hit the green button on the pendant and apply wide-open throttle. Once the maximum RPM is reached, the operator releases the gas pedal (clutch in) and hits the red button on the pendant, which applies the Dynojet brake and brings the test to a complete stop. Alternately, the operator could also set a "start" and "stop" RPM in the software, to automatically control the run, although we typically see this done using the easy to operate pendant.

Mustang Dyno

Instead of calculating horsepower based off of the acceleration of a heavy fixed mass roller, Mustang Dynamometers (ignore the name, these can and are used on all cars, not just Mustangs!) rely on a highly sensitive torque cell to directly measure torque applied to the unit, while the actual acceleration of the run is controlled by an Eddy Current Power Absorption Unit (PAU). To make sense of this, let's first take a look at the rollers, which are only 12.625-inches in diameter on the company's MD-AWD-500 model. If one were to load a car on such small rollers and make a pull, without any additional load, the actual run (the time it takes for the car to accelerate from 3000-6500 rpm) would take almost no time at all and no real data could be gained, as the test wouldn't replicate any type of real-world situation. To solve this problem, Mustang uses the PAU, which is essentially a frictionless, air-cooled electromagnetic brake connected to the rollers that uses electricity to control how much load is applied to them.

With an eye on "true road simulation" the Mustang Dynamometer software relies on two factors to determine load, including the vehicle's weight and the amount of horsepower the vehicle would require to maintain 50 mph on a flat, windless road. That latter number is actually garnered from our friends at the Environmental Protection Agency (EPA) and varies (obviously) depending on a vehicle's aerodynamics. Knowing these two factors, the Mustang Dyno software actually creates a load profile, which it outputs to the PAU to generate the desired amount of load during a full-throttle test. If you've ever seen a Mustang Dyno test performed, this would explain why they seem to "take so long," although if you were to test the time of a 4th gear pull on the dyno versus one on the road, you may find that they are surprisingly similar in regards to the acceleration rate. And really, that's what it's all about on the dyno, getting that acceleration rate (and load) as close to the real world as possible. Downsides? Some. For one, it is up to the operator to correctly input all of those values, which can sometimes be time consuming to obtain. Additionally, if you've modified your car's aerodynamics, or, more commonly, its weight, that can skew the results. Because the software accepts many inputs, it could be done incorrectly--either by mistake or on purpose--which just means you need do your homework and find a quality tuner whom you can trust if you want accurate results.

Making a run on a Mustang Dyno requires a decent amount of setup, as the operator first needs to acquire the vehicle's weight and HP@50, both of which can be referenced in the accompanying software. Additionally, the operator must enter a "start speed" and "stop speed" for the test, which tells the dyno when to begin and end the wide-open throttle testing. These can either be entered as wheel speed, if you know what your top speed in gear will be, or by RPM if you have a solid Tach signal. With everything entered, we drove the STI Killer up onto the rollers and strapped it down using two straps in the back and two in the front, all of them cinched tight. Finally, the "start test" button is selected on the dyno computer and the run begins.

Dyno Dynamics

While Dynojet and Mustang represent the majority in America, Dyno Dynamics is the dyno of choice in Australia, where this popular portable dyno was designed and manufactured, and has been gaining followers here in the States for years. Built using small, lightweight rollers, Dyno Dynamics dynamometers rely on an Eddy Current Power Absorption Unit (PAU), often referred to as the Retarder by Dyno Dynamics' users, to measure and apply load to the rollers for tuning and data acquisition purposes. For our testing, we loaded up on the new DynoTech model, which serves as the company's newest "entry level" two-wheel drive unit, capable of recording up to 1,200 hp and holding 600 hp in a steady state. Much like other Eddy Current PAU units, the Dyno Dynamics dyno relies on a carefully calibrated load cell to measure Tractive Effort, which the Dyno Dynamics software can then convert to Horsepower and Torque. Unlike an inertia style dyno, which relies on the weight of the rollers to provide a load during traditional wide-open throttle runs, the DynoTech uses an electromagnetic brake (the PAU) to control how fast a vehicle can accelerate during the test, similar to the systems used on Mustang and LC-equipped Dynojets. This load, which is variable, can be dialed in by the tuner to help perfect the tune-up or it can be set automatically by the dyno software, depending on the operator's preference. Unlike other brands, Dyno Dynamics' operators simply enter a preferred acceleration per second number and the dyno does the rest.

Of course, any time you give the end user the ability to "simply enter a preferred acceleration rate" you're treading on thin ice. Without knowing the actual rate of acceleration that a vehicle would see on the street, which can be hard to do without either datalogging a street session or having years of experience with a particular type of car, getting the load just right can be a challenge. Too much or too little will skew the results, and, more importantly, changing these factors during another test--say, after you install a supercharger or any new speed part--would make comparing the results difficult if not impossible. As always, make sure you find a qualified technician and stick with them during the modifications to your car. Or just use the Dyno Dynamics "Shootout mode," which applies a fixed load on every dyno and records the load and information on the graph, so you can compare it to other runs later on.

Setup on the DynoTech was easy, with the dyno operator simply backing the car on the rollers, strapping down the rear axle (no front straps are needed on the DynoTech, which is great for lowered cars), installing an Incoming Air Temp sensor into the air stream (the lid on a fourth-gen) to acquire ambient temperature and setting up the software. [Normally a tach signal lead would also be hooked up at this time, but unfortunately this show model lacked one, which also meant we would not be able to record torque.] Unlike many other dynamometers, the DynoTech uses no large computer tower or stack to operate and everything is controlled through a laptop that the operator can keep within the car. With no pendant, runs are simply made by entering a start and stop RPM and the dyno does the rest once you go wide-open throttle. When the run is complete, the dyno operator simply backs off of the throttle and applies the vehicle's brake to bring everything to a halt. This is unique to the Dyno Dynamics brand, as the rollers are so lightweight and have such little base inertia that the vehicle's brake system can easily bring everything back down to speed without using a large air brake as seen on some other machines.


As you can see, each dyno that we were able to test on produced slightly different results, although it is impossible to say which one is "right" or "wrong." It is, we're afraid, not as simple as that and shouldn't be treated in such a way. What's important to understand is that each of these highly accurate machines can provide a safe and reliable way to accurately tune and record the power of your car. Just like cars themselves, each person will have a different opinion on which model is superior, and only you can decide which one you would prefer to work with. We suggest you clearly define your goals, find a tuner you trust, and spend time on that exact same machine throughout the life of your project. And next time someone posts a graph you don't agree with, let's try to help them understand it instead of just bashing them… after all, we're all here to learn a little something, right?



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