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onto the dyno and tested without having to remove the rear wheel or the engine from the chassis. This makes it perfect for the shop that services engines or for the racer who has only a few days between races and needs to do some quick testing. It also makes it possible to test the efficiency of the bike's drivetrain. In the automotive world, rear wheel dynos are used mostly for final tune-ups instead of R&D testing, but in the motorcycle environment, both types of testing are prevalent. Rear wheel dynos are relatively low cost, convenient and designed for motorcycles, automobiles, go-karts and other types of vehicles. Dynojet was the first to introduce a rear wheel dyno for motorcycles, and it is the most extensively used dyno in the motorcycle marketplace today.
In its most basic form, a rear wheel dyno is a fixed inertia-only type device in that it does not actually measure an engine's torque output using an absorption unit. Instead, the motorcycle's rear wheel accelerates a heavy roll of known mass and inertia. The dyno measures the time and rate of acceleration to a given engine speed at wide-open throttle (WOT) conditions. Torque and horsepower are then calculated by software from the time and acceleration rate. The more rapidly the heavy steel drum is accelerated to a given rpm, the greater the engine's horsepower.
The advantage of a rear wheel dyno is that engine performance can be tested as installed. There is no need to remove the engine from the chassis or remove the rear wheel. This design results in quick and cost effective testing while factoring in drivetrain power losses. However, accurate and repeatable testing is more difficult to achieve than with an engine dyno because several factors such as drivetrain losses, along with tire temperature, wear, and traction influence the results. With the appropriate options and computer software, a rear wheel dyno can be made as sophisticated as an engine dyno, but many are not. Nevertheless, the rear wheel dyno has become the de facto standard for dyno testing motorcycles due to relatively low cost, ease of use and high availability.
It should be noted that horsepower and torque would be higher when measured directly at the crankshaft than the rear wheel or a jackshaft. When power is measured at the rear wheel, it is reduced because some power is lost through the vehicle's drivetrain, which includes the primary and secondary drives (clutch, transmission, chains or belts, and rear wheel). The exact power loss is dependent on the efficiency of the drivetrain, but in most cases, you can assume 10 to 18 percent lower than crankshaft power. Be aware that most factory power ratings are taken at the engine's crankshaft, as are some aftermarket power figures. When comparing dyno charts, always verify that each dyno pull measures power at the same location-at either the crankshaft or rear wheel.
Dyno Testing Variables
Atmospheric conditions: Atmospheric conditions have a significant effect on the power an engine makes. Ideally, every dyno pull would be conducted under identical atmospheric conditions. But this is not the case, so a correction factor (CF) is used to compensate for barometric pressure, temperature and humidity. The more oxygen in a given volume of air, the more power an engine will make. Lower altitudes increase barometric pressure, consequently, oxygen content and power increase. Lower temperatures increase air density and the oxygen content for a given volume of air, thereby increasing power. Humidity or water vapor in air displaces oxygen, reducing power. To compensate for weather changes, a CF that takes into account barometric pressure, temperature and vapor pressure is applied to dyno-generated uncorrected power data. Either an SAE (Society of Automotive Engineers) Standard or DIN CF is applied. SAE is typically used by Detroit automakers, Standard by the auto racing industry and DIN in Europe. When comparing dyno charts, be sure they were all generated using the same CF.
Repeatability
Repeatability is crucial for accurate dyno testing. A CF is accurate only to a point because it does not take into account variables such as engine and oil temperatures, air quality or fuel specific gravity. For accuracy, cylinder head temperatures should be monitored and kept consistent, and oil temp should be kept ideally between 200 degrees and 220 degrees F for maximum power. A dyno room needs lots of clean, cold air for consistent results. Exhaust contaminated air contains less oxygen and reduces power. Just having the door open doesn't guarantee clean air. Even venting crankcase oil vapor into the air cleaner can change the air/fuel ratio, giving bogus results. Fuel specific gravity changes with different brands of gasoline and different temperatures. Colder gas is denser and requires smaller jetting. For consistent air/fuel ratios, use the same brand of gas and monitor its temperature. The dyno's air inlet temperature sensor should be mounted near the carb or throttle body, but not so it is unduly affected by heat from headers or fuel standoff. For rear wheel dynos, the model and air pressure of the rear tire, the force used when strapping the bike down and drivetrain efficiency can significantly affect power readings. For accurate dyno results, make sure the dyno operator keeps these variables constant.
Gear ratios
When conducting "roll-on" power tests using a rear wheel dyno, the transmission gear used and final drive ratio can affect the power readings. For example, a power reading recorded in Fifth gear is generally higher than one performed in Fourth gear. Additionally, a lower final drive ratio such as 3.15:1 will often show a higher power reading than a higher 3.37:1 ratio. However, power differences may diminish as engine displacement increases. Generally, the lower the transmission gear or final drive ratio, the greater the engine loading and higher the power reading will be. Since the lower gear ratio slows the engine's acceleration rate, less power is required to accelerate the rotating and reciprocating parts. Furthermore, the air/fuel mixture has more time to stabilize within the intake tract similar to a step or steady-state test, thus resulting in a higher power reading.
Although no formal standard exists, fourth gear is normally the de facto standard for conducting roll-on dyno pulls. Some tuners elect to use Fifth gear because they prefer to load the engine more believing that hard to detect problems may be identified. Astute engine builders have been known to deliberately perform rear wheel dyno tests in Fifth gear to maximize power readings. However, that doesn't necessarily mean the power readings are wrong. Instead, it only illustrates that rear wheel dyno charts cannot be accurately compared when using different gear ratios. A Dynojet output report lists engine rpm per one mph. The exact rpm for each mph is dependent on the transmission gear, overall drive ratio and rear tire diameter. When comparing rear wheel dyno charts, make sure all roll-on dyno tests are made using the same transmission gear.
Baseline Tests
You must know where you are starting from in order to know where you have to go. Before making any changes to the engine or testing different parts, an accurate baseline power reading should be established to provide a solid starting point to work from, and a point to go back to if you get confused. If testing spans more than one day, a baseline power reading should be established at the beginning of each day. The baseline parts combination and tuning specifications should be documented for future reference.
Increasing the engine's ability to breathe will move the torque peak-and horsepower-horizontally across the chart to a higher rpm because cylinder fill is dropping more slowly than rpm is increasing. Higher-flowing induction and exhaust parts and a more potent cam will improve cylinder filling. The baseline should include a minimum of two power runs and preferably three runs made close together. The baseline must be representative of what the engine's power normally is; otherwise, the results cannot be valid. If there is an unusual power gain or loss, repeat the baseline tests to verify accuracy. Once you have a valid baseline, each tuning adjustment or modification should be tested with a series of three power runs.
Dyno Reports
Dyno reports include a multitude of information, including a two-dimensional graph-horsepower and torque vertically down the sides and rpm horizontally across the bottom. Dynos with an air/fuel ratio module will also include a horizontal graph of the air/fuel ratio throughout the engine's rpm band. Graphs that substitute rpm with mph (aka mph graphs) have specific and/or limited uses, so if you are paying for a dyno pull, insist on an rpm graph.
When comparing dyno charts, the first thing to do is verify that you have the correct charts, the right parameters were entered, and the test procedures were consistent. If you are not present during testing, it can be difficult to verify certain information. Moreover, if the input data is incorrect or if there are inconsistencies in the testing procedures, the output data will be incorrect and your conclusions will be wrong. The acronym "GIGO" sprang from the computer world and best describes the importance of accurate input data-garbage in, garbage out. Other things to check are that each chart is for the correct engine and accurate barometric and vapor pressures were entered. Also, verify whether the chart represents an average of multiple runs or is an individual run. If the chart is for an individual power run, it's helpful to know if the run was the first, intermediate or last run of a test sequence. Once you are sure the input data is correct and you have an understanding of how the tests were performed, you are ready to look at the power data. Note that some of the charts included here are generic in nature. Moreover, the dyno operator has various options for formatting charts.
Reading Power Curves
At dyno shoot-outs, everyone is typically chasing after maximum horsepower as with engine number one. But this engine only makes about two more horsepower within a narrow 250-rpm band at the very top end of the power curve. However, engine number two makes more average horsepower in the working rpm range and would be the fastest in a drag race. It would also be more pleasant to ride on the street.
Unless you're competing in a dyno shoot-out, there is more to focus on than just peak horsepower because the shape and location of the torque and horsepower curves are critical to building a winning and satisfying engine combination. The objective should be to maximize torque within the engine's working rpm range, regardless of whether you have a street or race engine. In other words, always strive for the flattest and highest power curve within the engine's working rpm range. A horsepower curve that rises and falls very quickly demonstrates a "peaky" engine-one that makes high power over a small rpm range but requires lots of shifting to keep the engine working within the narrow rpm band near the power peak.
The best way to compare two different dyno power charts is to average the area under the torque or horsepower curve for each engine, and then compare them. However, overall averages can be misleading because one engine may produce more low-end power, while the other is better on top. Be sure to identify the rpm range that is most important to your application and compare only within that range. The engine with the largest area under the curve in the most important rpm range will generally be the best.
Examining Power Curves
For pleasant street riding with a Big Twin engine, peak torque rpm should be no higher than 4,500 to 4,800 rpm (maybe as high as 5,000 rpm). However, for maximum acceleration, peak torque rpm should be equivalent to or slightly lower than the bottom of the shift rpm. Since the bottom of the shift rpm is 5,500 and peak torque rpm is 4,500, the engine combination should be changed to move peak torque rpm closer to 5,500 rpm. Once peak torque rpm is near 5,500 rpm, any further component changes should always move peak torque and peak horsepower rpm in unison.
Summary
Remember that the optimum power curve depends on whether your priority is dyno shoot-outs, racing, or street performance. But regardless of the dyno curve's shape, first and foremost, you want an accurate dyno chart. And for that, you need an honest dyno operator who is knowledgeable in tuning your engine combination, whether it is carbureted or EFI. For EFI engine, that means your tuner must be experienced using your fuel/ignition software program or any add-on EFI module. When done properly, dyno testing can save you time and money and has become a vital tool for optimizing EFI fuel and ignition maps. [Editor's note: The numbers and graphs used were for demonstration purposes.
writer: William Denish
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