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Discussion Starter · #1 ·

Every racer / mechanic understands that, without horsepower, cars go nowhere. As dynamometers are the only tool specifically designed to measure engine horsepower, it's no surprise that top racers / mechanics want their own. This examines things to consider before selecting and using these expensive tools.

Like most test equipment, a dynamometer (or dyno for short) helps isolate and quantify a particular parameter (in this case the engine's power output) from overall vehicle performance. Why do you need to do that? Racers (that don't dyno) often rationalize "I only test on the track ...where it counts"! They infer that power output is good if lap / quarter times are low. But, that fails to isolate the contribution of a sharp driver good handling vehicle from a strong engine!

Many hot-up modifications only help at high rpm, actually reducing power down low. Even with days of track testing you might condemn some new mod unless you test a bunch of other changes too. What if you need to match the fuel mixture / ignition too? Add up those exponentially increasing combinations, and thoroughly track testing stretches in to years! Dynamometer owners get pointed in the right direction with just a couple of 20-second "runs".

Using a dynamometer also helps you avoid discounting "insignificant" 1-% gains from modifications. Just because you can't "feel" a single 1-% power increase does not mean you want to forego ten such tricks! Combining small improvements is how pros win trophies. Ten 1-% tricks ad up to 10-%.


I’ll assume you are a serious engine builder and want to start in-house dynamometer testing. What do you need? First, to measure engine torque, your dynamometer system must provide a load. Automotive engineers refer to this loading device as an absorber or a "brake" (since early dynamometer absorbers used a drum and band brake to load the engine). Absorbers do not actually absorb the power. Rather, they convert it to another form of energy, like heating water or air.


Currently there are several commercially available absorber choices for engines.


Professional engineers, with Fortune 500 budgets, often use electric DC generators or retarders with computer controlled field excitation to load and regulate their engines. The engine's power is typically dissipated as heat in the armature area or wired to remote heating elements. If the test engine's operating rpm is low enough, it can be directly coupled to the armature with a short drive shaft. 6,000+ rpm engines will need a gear reduction drive to match them to these low rpm generators / retarders.

The main advantage of electric generator / retarder systems is that they can be readjusted anywhere from zero load to full load in microseconds. This allows the engineer to regulate engine speed within a couple of rpm (even while changing throttle settings). Then you still need to the data acquisition system. If your engine runs at high rpm, you need the required gear reduction. Reduction transmissions add still more, complexity, and parasitic drag.

The DC generator or retarder dynamometers have another shortcoming. They have a high polar moment of inertia. That's a fancy way of saying that the generator's or retarders armature feels like a giant flywheel to a small engine. High inertia means a lot of horsepower is required to accelerate the armature or rotors. Likewise, a lot of stored horsepower will be returned when dropping down in rpm. This really skews the test data whenever rpm is changing. So, while generator or retarder dynamometers are great for steady state and ramp control, they are not ideal for testing rapid acceleration transient conditions on small engines.


Eddy current brakes are similar in operational characteristics to electric DC generator / retarder absorbers. The main difference is that the eddy current brake does not actually generate electricity. Rather, you use an electrical power supply to charge its electromagnetic coils. The brake's input shaft spins a metallic rotor inside that resulting magnetic field. When the dyno operator increases the current supply to the coils, the rotor shaft becomes harder for the test engine to turn. Like the DC generator, an eddy current brake's advantage is its lightning fast response to the controlling computer's loading instructions.

These eddy current brakes dissipate the engine's power as heat input to the rotor. This rotor must be cooled or it will eventually melt. Air-cooled eddy current brakes have cooling fins on a big iron rotor, making them look like automotive disk brake rotors. These big rotors have too much flywheel mass though, and dominate the rotating inertia of a typical kart dynamometer installation.

Water-cooled eddy current brakes are available that have significantly lower rotating inertia (at least compared to air cooled eddy current and DC generator systems).


Let's look at lower cost absorbers. The simplest and earliest forms of brakes were just that, brakes. A rotating drum with a friction brake pad was used to apply drag at the engine's output shaft. These looked like old truck brakes. To measure torque, some sort of calibrated scale linkage was inserted at the brake pad anchor points to display the applied drag load. Problems with friction brakes included much difficulty in accurately regulating the load and brake pad cooling.


A more controllable load device is the hydraulic oil pump. These are occasionally seen on low rpm, moderate horsepower engine dynamometers. A positive displacement oil pump acts as the brake, and an adjustable oil discharge orifice valve sets the load. They can have a lower inertia than the DC generator and eddy current units if the pump is small, but sometimes require gear reduction units and couplings. Like many absorbers, the oil pump units convert a test engine's power into a fluid's temperature rise. Since the oil can't be just freely discharged, a cooling system (typically an oil to water heat exchanger) must be used to keep the oil's temperature within safe limits.


When low cost, low inertia, high rpm limits, and race engine horsepower capacity are all requirements, the most prevalent choice for an absorber is the water brake. These have been the favorite of professional automotive engine builders for decades. Water brakes are another form of hydraulic pump absorber. These pumps typically have one or more vane'd rotors spinning in between pocketed stator housings. Load is controlled by varying the level of water in the brake with adjustable inlet and/or outlet orifices. Raising this water level increases the rotational drag of the pump's rotor, applying more resistance to the engine turning it. Interestingly the water brake is, by design, a very inefficient pump. It uses up your engine's horsepower output by making "instant hot water"! Since the discharged hot water is clean, it can either be allowed to just run off, or it can be air cooled and recirculated.

The power capacity vs. size of water brakes is startling. A water brake can be made to handle over 3000 continuous Hp at 12,000 rpm! By comparison a 3000 hp eddy current brake would need multiple retarders for this power rating and is only good to 5,000 rpm. It is no wonder that water brakes are virtually the only choice for testing 2,000+ horsepower drag car engines. Modern water brakes have low enough weight and inertia that they can be directly mounted on the engine's crankshaft. Direct mounting eliminates the inertia and parasitic drag of drive shafts, u-joints, pillow block bearings, etc.

All of the above absorbers can be controlled manually by the operator (with a simple knob), or under computer control. Manual valve water brake load control is not as responsive as the electric DC generator or eddy current controls but, with good electronic servo valve controls, you can close the gap a lot.


In discussing the pros and cons of various absorbers I keep mentioning problems with high inertia. To illustrate just how much power flywheel energy can mysteriously "absorbed" let's "talk about" a crude, dirt-cheap dynamometer with no brake at all! This will be an "inertia dynamometer" because the engine's power output will go into "winding up" a heavy flywheel.

This example uses a flywheel that is large, in relationship to the engine, so accelerating the combination from idle to peak rpm takes several seconds. A fast data acquisition system logs the time periods and rpm changes. From that information we calculate the torque and horsepower the engine supplied to accelerate that known flywheel mass. The formula for determining the torque is:

Torque = JM * rpm per second / 9.551

Where JM represents the Polar Moment of Inertia of our inertia dyno's flywheel.

If we don't know the Polar moment of Inertia for the flywheel (and our flywheel has a constant thickness cross-section) we can calculate it with the formula:

JM = (W * r ^2) / 32.16 / 2

Where W represents the flywheel weight in pounds and r is its radius in feet.

Once you have the torque, it is easy to calculate the horsepower with the standard formula:

Hp = Torque * rpm / 5252

Keep in mind that the rpm in the last formula must be the average rpm during the sampling period.

Say our example uses a 10-pound flywheel, 8" in diameter (thus it would have a Polar Moment of Inertia of .017 foot-pounds-second2). If the engine was able to accelerate this flywheel from say 4,800 rpm to 5,200 rpm in 2/10 of a second (a rate of 2,000 rpm per second) that would represent a torque of 3.6 pound feet. Since our above example had an average rpm of 5,000, it produced 3.4 Hp during the test. That's all here is to it. Unfortunately, inertia dynamometers alone are useless for doing the steady state testing needed for methodical development of porting, pipes, etc. You cannot adjust the load to hold the engine at a given rpm point, it must always be accelerating. Still, inertial testing is handy for working out acceleration and drivability problems.

The real reason for the above math exercise is to illustrate how much power it took to accelerate that small flywheel. If you buy an absorber with a polar moment of inertia in the same rage as our flywheel example above, don't expect to perform sweep acceleration testing. Even accelerating at just 200 rpm per second would consume 10-% of our sample engine’s power! Fortunately, high end computerized data acquisition systems provide composition algorithms to back out the effects of absorber (and crank-train) inertia from acceleration data. On a high inertia dynamometer, compensation is required even for fairly low rate sweep testing.

see part 2

862 Posts
Discussion Starter · #2 ·
part 2


Assuming you settle on a nice low inertia brake to load the engine's torque output, how do you measure that torque? Some DC generator and eddy current dyno's use in-line rotary torque transducers because they measure engine torque before the influence of the high inertia rotor!

To get torque data without a rotary transducer, the brake's outer housing must be mounted free floating (i.e. in trunion bearings). Housing rotation is then prevented with a form of "torque arm" protruding radialy from the housing. Some stationary support linkage holds the end of the arm. The arm is called a torque arm because it "feels" 100% of the engine torque trying to rotate the loaded brake. Inserted somewhere in this anti-rotation torque arm linkage is a calibrated scale or "load cell transducer". This transducer converts any applied force into a usable torque signal that it supplies to a gauge or data acquisition unit.

Beware that, some oil pump "dyno's" skip the expense of a load cell and try to use discharge oil pressure (usually in conjunction with a look-up chart) as a crude estimation of power output. This is unsuitable for performance engine testing. No matter what type of absorber you select, get a transducer that can directly and accurately measure torque, not "guesstimate" it.

An electronic display or data acquisition system expects to interface with an electrical strain gauge bridge load cell. This type load cell has a metal cross section with a hairline electronic wire grid glued to its surface. As this cross section is compressed, tensioned, or bent (depending on the linkage and load cell design) the attached wire grid is likewise deformed. The almost infinitesimal deformation of the wire grid changes its electrical resistance some tiny amount. The electronic circuit acts like an ohmmeter to read the resistance change; only it is calibrated in pound-feet. This same principle is used in everything from $500,000 dynamometers to $19.95 digital bathroom scales.

Calibrating the torque display for accuracy is usually straightforward. Typically a certified weight is hung off the end of the horizontal torque arm while you observe the torque display. Multiply the distance from the center of the brake out to where you hung the weight, and it must match the pounds-feet of torque displayed. If the reading is off, the data acquisition system will provide some means to recalibrate it for the deviation.

Once you have a system that is accurately measuring running torque, you only need a calibrated tachometer to calculate horsepower. Horsepower specifies the rate at which your engine is capable of producing a given level of torque (see the earlier horsepower formula).


On old-fashioned dynamometers, an observer must record the simultaneous tachometer and torque gauge readings with a pencil and paper. Today, most dynamometers replace the observer’s notes with computerized data acquisition electronics. You would not believe how often everyone watching a test gets so excited by the noise and thrill that no one records the data! Or worse, the readings are "rounded up" by the biased engine builder. A good computerized data acquisition system should be considered mandatory for any real testing, period. Fortunately, today it is possible to get recording, control, and playback capabilities in a hand held package that years ago would have cost the price of a house and filled a small room.

A suitable computerized data acquisition system should have a fast sampling rate, especially for testing 4-stroke, single cylinder engines. By fast I mean at least 100 samples, of all sensor channels, per second (100Hz). A 200Hz logging rate is a bit better still. Why? Understand that, between spark plug firings there is a measurable drop in the instantaneous crankshaft torque and rpm. The crankshaft gets accelerated in the moments after combustion, and then begins to slow until almost two revolutions later the plug fires again. You can't feel these rapid highs and lows when driving around the track (with all that vehicle inertia), but the dynamometer will!

If you sample at only 50Hz, that’s only a single torque and rpm sample every other revolution (at 6,000 rpm)! Periodically, a series of samples will fall in synch with the firings of the plugs, while at other times sampling will fall in synch with the lower power compression strokes. By using a fast acquisition system to read each firing cycle multiple times, enough data is captured to average out this phenomenon. While experienced dyno operators see the same power curve in both graphs, inexperienced operator's would expect that it would be smooth "publication quality" line.

The ability of the acquisition system to average and dampen the data is mandatory for other reasons. At 200Hz you're getting 2,000 lines of data for even a ten-second dyno run. Who wants to always wade through 40-pages of data for a ten second run? Averaging both eliminates transient "noise" and produces more practical printout.


A computer that only logs horsepower, torque, rpm, and time may be all your testing requires. It will certainly put you several notches ahead of those without in-house dynamometers. But, for more advanced engine development there is much more you'll want to capture.

Weather data, meaning air temperature, barometric pressure, and humidity are something that needs to be noted for each dyno test session. As you are aware, lower barometric pressures, higher air temperatures and humidity will lower an engines power output (and vice versa). Without doing atmospheric correction, data taken under other conditions cannot be directly compared. Dynamometers often come with the atmospheric correction tables found in many engineering handbooks. These tables have factors for the various weather conditions, which you multiply against your observed torque data. "Corrected" data is a closer estimate of what the engine would have produced had it been tested under, for example, "standard" atmospheric conditions. Good data acquisition software should allow entering or recording these conditions and automatically calculate the correct data.

Exhaust, cylinder head, water, air and oil temperature thermocouples, are good to have. They provide a safety check and insight into what is happening inside the engine. Monitoring the EGT readings is a nice security blanket when you start leaning her out! On air-cooled engines, special spark plug thermocouples are equally important. Some dyno software even lets you program safety limits that will shut down the test if things get to warm!

Block mounted thermistors let you monitor temperature variables that might inadvertently influence engine power. For getting repeatable test data you want to test at consistent temperatures. Thermistors data also lets you check the engine's sensitivity to cooling system alterations.

Airflow metering turns the dyno and data acquisition system into a dynamic flow bench. Small turbine type transducers are available that simply clamp onto the engines inlet like an air cleaner. With the Static Cubic Foot per Minute numbers you can sort out combustion efficiency improvements from mass airflow gains. The software should combine the airflow info with horsepower data and provide a Brake Specific Air Consumption number. Having BSAC data let's you compare your engine’s efficiency with published dyno data from others. Such comparisons help guide you to areas where improvements are most likely to be had.

Like airflow turbines, a fuel flow turbine provides instantaneous fuel consumption and Brake Specific Fuel Consumption numbers. I like having BSFC numbers along with thermocouple temperatures to help me isolate fuel mixture issues from those induced by spark timing, etc. This add-on pays for itself in shortened test sessions many times over. Combined with airflow data, software can even track the engine's real-time air fuel ratio.

Another computerized data acquisition software feature, one that buyers may not think of until after running the system, is automatic triggering of data logging. Just as observers often fail to note gauge readings, busy dyno operators forget to toggle the data record button at the start and finish of important tests! It's frustrating pushing the print button and getting nothing, or, ending up with hundreds of pages of engine idling data! Better systems allow setting rpm and horsepower trigger points, which, once exceeded, automatically start logging. Similar algorithms should control the end of logging. This feature really makes a dyno operator's life easier.

The acquisition system should handle numerous types of ignition system rpm signals, have provisions for 2/4 stroke, distributor/ ditributorless, normal/wasted spark etc.


No matter what type of dynamometer you select, controlling the test conditions is vital to getting usable data. It's not enough for the dynamometer equipment itself to be accurate; you have to know that the engine's output is not being skewed by improper dynamometer procedures. For example, if you fail to start all your tests from a standard, stable engine and head temperature, there's no way to tell which variable is responsible for any measured power differences.

Likewise, poor cell ventilation can allow exhaust gas to be inducted into the engine, drastically reducing its power. I've actually seen dyno operators, squinting from the pain of exhaust fumes, trying to figure out why the engine suddenly lost 50-% of its torque!

In a typical installation a servo valve, under the data acquisition computer's control, adjusts the load rather than the operator trying to do it manually. Water brakes equipped with computer servo load control routinely hold the engine within 1-% of target rpm. That is much better than you should expect to do manually. Computer load control allows programmable rate sweep testing and automated step testing (i.e. running the engine at each even 250 rpm for a few seconds of settling time and then automatically logging a couple of seconds data). In fact, with the additional electronic throttle control on top of the electronic load control you can actually program an entire racecourse simulation and sit back and watch the dyno run the show.

special thanks to land and sea for some snippets!
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