Dyno Debate - Real Wheel Horsepower

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

Justin Cesler Mar 21, 2012 0 Comment(s)
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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.

Dynojet

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.

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