A chassis dynamometer accommodates an entire vehicle and allows operation of its powertrain while the vehicle remains stationary. The car is attached to the dyno with tie-down devices and its drive wheels sit on a metal roller or a pair of rollers. The rollers are connected to whatever measuring system the dyno uses to test drive wheel output.

The chassis dynamometer was developed in 1938 by Clayton Industries. It was termed a "hydrokinetic" dyno because it used a water brake to absorb power. Since then, chassis dynos have been built with water brakes, oil brakes, eddy-current brakes, generators and other devices which actively absorb power.

While brake-type chassis dynos are great pieces of equipment, most are expensive, a good part of which is the power absorption system and its controls. A chassis dyno suitable for testing today’s high-tech performance cars can cost upwards of $150,000. Another problem with some brake-type chassis dynos is they lack the accuracy and repeatability demanded by many performance-aftermarket manufacturers and tuners.

"They (Dynojet Research) towed it (trailer-mounted demo unit) down here behind a truck," Bell told us. "We ran a bunch of cars on it. I compared the data to my engine dyno tests and drag strip tests and, no question, it’s accurate.

"Then I took the one of the vehicles I ran here–this was a 410hp supercharged car we owned and built–over to K&N Engineering and ran it on their Dynojet. It was right on the money–within one horsepower of what it ran here.

"They left it (demo unit) here about a month. We fired the dyno up, oh–once a week, ran the same car and that thing was right on the money, every time. Then, we tried the same car on a third one and it was dead-accurate, again. This wasn’t some slipshod test, either. We put the data logger on it and read intake temperature, the temperature out of the supercharger–I knew the engine coolant and oil temperature, the transmission temperature, the rear end temperature–we knew everything. There were no variables.

" I was convinced. The Dynojet is 100% accurate."

Physics Lesson

At the core of a Dynojet 248H is a pair of metal drums joined by a shaft and riding on antifriction bearings. The drums weigh about 2700 lbs apiece, are four feet in diameter and have knurled surfaces to enhance traction. Because 5400 lbs. takes time to slow once the dyno run is complete; the drums have air brakes, similar to those on rail cars, operated via a button the dyno’s hand-held controller.

A combination of two laws of physics, force equals mass times acceleration and work equals force times distance, gives us this equation: W=m X a X d. "W" is the work, in pounds-feet, the rear wheels are doing, "m" is mass equivalent (the drums), "a" is acceleration (increasing drive wheel speed) and "d" is distance (drum circumference). Once we have the work, we can find horsepower. One horsepower is 550 pounds-feet of work done in one second so, we divide the work number by the length of time measured, then divide the number we get from that by 550. To simplify: we get horsepower by multiplying the mass, acceleration and the distance, then dividing that product by time multiplied by 550. This can be expressed by: hp = (m X a X d) ÷ (t X 550).

Torque can be figured by multiplying the horsepower by a constant, 5252, then dividing that product by the speed at which the thrust force was measured. Generally, with rear wheel numbers, axle ratio is not considered in the torque computation. For comparison purposes, this makes more sense. The computer factors out the axle ratio by using engine speed data in the torque derivation.

In the real world, the measurements and computations are not quite that simple, but the complex methods Dynojet Research uses to apply these laws of physics and their mathematics to accurate measurement of rear wheel power is a proprietary secret.

Computers Make it Easy

While all this math and physics stuff may bring back horrid memories of high-school, computer power makes it quite simple and this simplicity is the beauty of inertia dynamometers.

The newest Dynojet peripheral hardware is intended to be used with a personal computer running the exclusive "Windows Performance Evaluation Program" or "WinPEP" software (current release: v6.03) under the Microsoft Windows 95 or 98 operating system. WinPEP not only captures dyno measurements and makes the computations, but it’s also a powerful analytical tool. It can superimpose power/torque graphs of up to 12 runs or "passes.". Using it, you can zoom-in on specific parts of the power or torque curves. You can set custom preferences for graph axis choices such as power, torque, rpm and wheel speed. You can choose a variety of different corrections. You can display peak power and torque values, atmospheric data and other information pertinent to each run as text on the graph.

WinPEP data can also be printed or exported to a Windows Metafile or .bmp file. As WinPEP is fairly new, many dyno operators will have the older, "PEP" v4.x software, a DOS application not quite as powerful, but sharing some of WinPEP’s analytical tools. PEP can only graph three dyno runs and it has a lesser number of corrections. Additionally, the interface is not as intuitive as that of WinPEP.

The Dynojet CPU is intended for connection through a serial port to a PC having at least a 100 Mhz., Intel Pentium processor, 16 Mb. RAM and an 800 Mb. hard drive. In world of 750-Pentiums that seems a bit low-tech, but the needs of WinPEP are satisfied by that level of computing power.

This CPU and additional peripherals are called the "DynoWare EX+". It consists of: the CPU Module, a Dynamometer I/O Module (sends and receives data from the dyno and its hand-held controller); a RPM Module, (processes RPM data from an inductive pick-up clipped on a plug wire or other engine speed sensor) and an Atmospheric Sensing Module (measures absolute pressure, air temperature and humidity and transmits that data to the CPU). These four modules stack together, allowing easy upgrades or expansion with options such and Air/Fuel Ratio Module, for exhaust gas analysis, or an Optical Pick-up, for engine speed input. Additionally, WinPEP supports both the Pi and CDS data acquisition systems popular in motorsports which allows graphing of parameters such as boost pressure, fuel pressure, intake air temperature (IAT) and spark advance against the power and torque data.

The DynoWare EX+ is capable of some pretty accurate measurements. Timing accuracy is ±1 microsecond or .000001 sec. It takes vehicle speed twice per drum revolution with an accuracy of ±.01 mph. It measures engine speed within ±0.1 rpm, temperature within ±0.15°F, pressure within ±0.04 inches of mercury (in/hg.) and humidity within 1%.

Correct–then Compare

OK. You ran your car on a dyno and its computer spit out a bunch of power and torque numbers. In fact, you have two sets: uncorrected, or "raw," numbers and corrected numbers. Gearheads love to compare numbers. Each dyno is subjected to different atmospheric conditions. To facilitate comparison, there has to be a "standard" to which dyno results conform or are "corrected." In the United States this is usually Society of Automotive Engineers (SAE) standard J1349 Rev JUN90. Correcting to SAE J1349 alters the data to make it seem as if it was taken when the atmospheric pressure was 29.23 in/hg., the temperature 77° F and the humidly zero.

The degree to which temperature, pressure and humidity affect power output is constant. If we accurately measure those parameters at the dyno location, we’ll know the difference between the atmospheric conditions at the time and location of our test and those of SAE J1349. That difference is applied to the raw data and the result is corrected power and torque which we can compare to other data taken anywhere in the world that is also corrected to J1349.

Cans/Can’ts and Pluses/Minuses

A Dynojet can give you the rear wheel power and torque output of your car. Well–duh! A test run can done in two ways: 1) wide open throttle in one gear or 2) wide open throttle through multiple or all gears.

Besides simplicity, accuracy and repeatability, a great feature of inertia dynos is they get performance data with less abuse to the car than does a brake dyno. A Dynojet can be used to evaluate the effects of modifications. It can be used to calibrate fuel injection systems and spark curves. It can be used to diagnose engine problems, such as detonation or mismatches between various modifications. It can help in solving driveline noise and vibration troubles. It can be used to evaluate the parasitic losses caused by different types of transmission and rear axle lubricants.

Since inertia dynos have one roller-per-wheel, overheating the tires and the tendency of cars to try and jump off the rollers are eliminated. Also, vehicles do not need to be loaded down against the rollers which also reduces heat build up and increased frictional losses through the tires.

There are a few things a Dynojet can’t do. Most importantly, it can’t duplicate a drag strip, thus it cannot directly predict a car’s e.t. and speed in a quarter mile pass, however, it can predict trends in a car’s performance. That is: if the car runs well on an inertia dyno, it’s most likely going to run well on the race track.

There are exceptions to this and they come with vehicles that use load tables in their engine controls calibrations to set the air/fuel ratio. It has been difficult to get specific information on which cars have this and which don’t, however, our limited research indicates this is more typical of Ford and Diamler-Chrysler products than it is of GM vehicles. It is also a problem most likely to occur if the mass of the vehicle (or combination of vehicles , if the vehicle in question is towing) is significantly more than the mass equivalent of the dyno drum.

A standard Dynojet 248H has no capability to load the vehicle with more than the drums’ mass equivalent, however, an option called "Dynotrac," which adds computer control to the dyno’s air brake, can load the vehicle to any wheel speed, engine rpm or percentage of braking force. While Dynotrac enables loading, currently, the dyno cannot measure torque output in real time because Dynotrac is simply a proportional air brake controller. At this writing, Dynojet Research has a strain gauge, necessary to measure torque in real time, under development. When it becomes available, the 248H will be able to function as a load cell dyno.

One complaint we hear is about the RPM pickup. It seems to cause some users problems with waste-spark ignition systems. For that, Dynojet suggests the user make sure the dyno computer is grounded to the vehicle and that it’s set to read spark data every 360° of rotation rather than every 720°. The RPM pickup is also incompatible with newer ignitions that have no spark plug wires or short, inaccessible wires. The optical RPM pickup is offered as a solution for that but when Jim Bell interviewed for this story, he told us both optical and magnetic pickups are time-consuming to set-up such that they provide accurate data. Clearly, a more simple way of setting up and obtaining RPM data is something Dynojet Research needs to look at.

What Kind, How Much, Who Uses Them?

Dynojet makes two inertia dynos capable of testing four-wheeled vehicles. The 248H, can handle up to 1200 horsepower and drive wheel speeds up to 200 mph. Its maximum axle weight is 3000 lbs. so it can be used with virtually all cars and some light trucks. Also available is the 224, a smaller unit with 500 hp. and 160 mph limits. There are 248Cs around, too. That was the company’s first dyno capable of testing four-wheeled vehicles and is now out of production.

Several years ago NASCAR’s Winston Cup Technical Director, Gary Nelson, discovered the Dynojet 248H as a tool to help him keep all the teams competing equally. It’s now the officially licensed chassis dynamometer of NASCAR. Also, most major teams in Winston Cup Racing have their own units. With most of them, no car goes on the truck for a race if it hasn’t run on the dyno before it leaves the shop.

K&N Engineering, Lingenfelter Performance Engineering, B&B Fabrication, Doug Rippie Motorsports, Dutweiller Performance, Kenne-Bell, Flowmaster, Second Street Speed, Motorsport Technologies and Borla Performance Industries are only a few of the aftermarket manufacturers and performance tuners whose names often appear in GMHTP and which use Dynojets in product development and validation.

An inertia chassis dynamometer can be a tuner’s secret weapon. It also can tell much about manufacturers’ claims about products. Jim Bell told us, "I look at the Dynojet as the lie detector of the industry."

It’s simplicity, accuracy and repeatability make comparing and contrasting different performance modifications an easy and sometimes very revealing process. The Dynojet is one hell of a piece of test equipment. Now, all I gotta do is get one installed in my garage.

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