Ever wonder why different dynos yield different numbers?
There is a wide variety of dynos in use, designed with different capabilities, ratings and purposes. Some are more repeatable than others. In many ways, the type of dyno an engineering company uses defines its capabilities. It also says a lot about their standards.
By John Stewart
When it comes to diesel power production, knowledge is everything. Being able to test, calibrate, monitor and analyze diesel engines and entire diesel truck powertrains is central to our business. In the automotive world, and in diesel racing, the instrument most often used for performance testing is the dynamometer.
There are a wide variety of dynos in use, designed with different capabilities, ratings and purposes. In many ways, the type of dyno an engineering company uses defines its capabilities. It also says a lot about their standards.
At Gale Banks Engineering, we have multiple dynos in operation. Currently, there are two engine dynos, one chassis dyno, and a third engine dyno in the works.
Engine Dynos for Gas and Diesel Performance
Engine dynos measure power characteristics at the flywheel or output shaft. A good engine dyno will tell you not only how much power you are making, but where the power is in the rpm range. It’s a crucial tool for research and development, allowing us to evaluate modifications one at a time, or as a complete system.
Engine dynos also can be useful to ensure that a race engine is right before race day. It can be used to develop the engine, break in the engine, and make a quality audit before you bolt it into the car or truck. All of our engine dynos are hydraulic water brake type.
A properly engineered, fully instrumented dyno cell costs at least one million dollars. An engine dyno cell with full emissions equipment costs around $2.5 million. Dyno test cells at this level of accuracy are found at GM, Ford, Daimler-Chrysler, CAT, Cummins, and International to name a few. Because of the cost and the need for extensive wiring, plumbing, venting and electronics, aftermarket companies rarely have this level of equipment.
At Banks, we require multiple engine dynos because we want to get the most accurate results, considering the characteristics of the engine.
For example, one of our engine dynos is a Superflow SF-901. This is the dyno we use for high-performance gas engines, typically small blocks or NASCAR-type high-revving engines. A mildly tuned Duramax can actually overwhelm the SF-901, but for anything up to 1000 lb-ft, and 1000 horsepower, turning 11,000 rpm or less, the SF-901 is ideal.
For diesel engine testing, we have a stouter Taylor TD-3100. It’s the tool we use for diesel endurance, range and durability testing, including the extreme duty NATO-cycle for military customers. It doesn’t rev to radically high speeds, but it can handle large amounts of torque. With the TD-3100, our operators can test engines continuously up to 3000 lb-ft of torque at 1500 rpm, and up to 1000 hp, from 1750 to 4000 rpm. Even higher speeds and loads can be tolerated on an intermittent basis. It’s a dyno that has been instrumental in developing our matched diesel component bundles, custom crate engines and diesel racing powerplants. For example, the SuperTurbo high-performance Duramax has seen more than 6200 hours on the dyno. Senior Program Engineer Matt Hill estimates that Bank Power engineers have logged more than 17,000 hours of dyno time with the Duramax engine alone.
Recently the Banks Engineering campus in Azusa, California, took delivery of a third dyno, a SAJ SH-750. This dyno is capable of testing at both high-rpm and high torque. It’s a high-speed diesel development device that has become necessary to keep up with diesel engine advancements of the last 4 or 5 years, and handle the kind of powerplants now on the drawing boards at OEM engine development facilities around the world. The SH-750 can run numbers on engines that produce up to 1000 hp, 2000 lb-ft, and turn 9000 rpm.
These dynos allow us to do constant-speed, full-load engine testing. Worldwide, this is the accepted method of developing an engine’s power and torque ratings. The power curve is developed by running the engine until all the iron, aluminum, steel, plastic, coolant, lubricants, turbocharger and charge-air cooler are at stable operating temperatures. Then, the horsepower and torque curves are developed by running full-throttle at several constant rpm points from idle to redline. These points are used to plot the horsepower and torque curves.
For the past 40 years, we’ve checked the accuracy of our dyno cells by obtaining lab test engines from Detroit manufacturers. Then, we compare their test data to ours on the same engines. That’s why we can be sure that we can synch our performance numbers directly with OEM baselines, within a very small margin of error. When you are supplying engines to government or OEM customers, it’s essential that the output is the same on both dynos–ours and theirs.
Chassis Dynos: Why Numbers May Vary
We also use chassis dynos. They’re great for drivetrain development, but you really have to know what you’re doing to extract correct information. Wheelslip, clutch or transmission slip, any changes in fluid temperatures, tire temps, inflation pressure, and the overall mechanical condition of the drivetrain will all impact the ability to compare the results with previous runs or with other dynos.
To get an honest result, we measure these items on each run, ensuring they are proper and have not changed. We also ensure that road-speed air velocity and volume are provided to the engine radiator and charge-air cooler on each test. It takes at least a 15hp fan to do this correctly; one fan we use on our chassis dyno can simulate road speeds up to 100 kpm.
There are two types of chassis dynos in common use these days. The first is a continuous-load-capable “real” dyno that loads the wheels with a power absorption unit. These load dynos reasonably duplicate the work environment of the diesel engine in a truck. Load dynos are also capable of acceleration tests, accurately duplicating the inertial resistance a truck has to overcome to accelerate.
Load dynos can also accurately account for aerodynamic load as speeds increase, which becomes tremendous at highway or drag strip speeds. Overall, the possibility for accurate testing with these dynos is very high. Our chassis dyno is a Mustang MD-750. We have two 20-inch rollers, each one rated at 750 hp. The Mustang MD-750 uses bi-directional, precision machined and dynamically balanced rollers, with a heavy-duty restraint system to keep the test vehicle solidly in place. The power absorbers are like very large, internally vented disc brakes. They are air cooled and electrically actuated, so the dyno loads instantly. The bearing set is good for speeds as high as 200 mph-NASCAR stock car territory-and the double-roller setup can handle heavy trucks up to 33,000 pounds. This dyno has proven ideal for drivetrain tuning development on just about all the sport and commercial vehicles we’ve thrown at it.
There is another type of chassis dyno, the inertia dyno, which is one we don’t use. They are simple and easy to use, and certainly they are far cheaper. But they can do only one thing-measure the time it takes to accelerate or spin a weight (usually a large steel drum) from one rpm to another. Horsepower data is then estimated by computer. These dynos can’t take into account the weight of the vehicle, or variations in inertia from one vehicle to the next. And they can’t take aerodynamic drag into consideration, which can make a huge difference as speeds increase. So the numbers are incorrect, almost always on the optimistic side. While they can be useful for back-to-back comparisons on the same vehicle if properly used, inertia dynos can’t be used to study steady-state engine operation, or to simulate vehicle operation on a course, because they have no dynamic load control. So these dynos don’t meet our needs. At Banks Engineering, we can’t afford to use dynos that generate numbers that can’t be compared to anything else.
A Craftsman’s Tool
A dyno, like any other tool, is only as good as the skill and knowledge of the user. It’s a “garbage-in, garbage out” situation. Careless testing results in random numbers. The weather alone can throw off results up to 10 percent. It’s essential to account for air temp, barometric pressure, and relative humidity and the instruments you use to measure those factors have to be equally accurate. By accounting for those and using known, standardized correction factors, we can get repeatable results whenever we test. So we have a broad test opportunity window, which is important for a company that does as much testing as GBE. If we’re testing for mileage, it’s also necessary to look at fuel temperature, because fuel density is a factor. Other fluid temperatures that we standardize are coolant, engine oil and transmission fluid. On the chassis dyno, we’re also looking at tire temps and inflation; we use a separate set of fans to cool tires if need be.
In short, even on the most accurate dynos, the results are only comparable if the operator is honest and meticulous about applying the exact same conditions, before and after. If any of the many variables are not accounted for accurately and honestly, the results can be manipulated in a predictable way.
For more information about dynos, click here.