3. These two engines produce...
3. These two engines produce the same amount of peak torque at the same rpm, but Engine #2 has a boarder, flatter curve for more area under the curve. For best street riding, strive for the broadest, flattest torque curve within the engine's most important rpm range.
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.
4. Torque for this engine...
4. Torque for this engine peaks at an early 3,000 rpm, and then drops off rapidly because the engine runs out of air due to a restricted induction and/or exhaust system. Like torque, horsepower will also drop quickly because cylinder fill is dropping faster than rpm is rising.
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.
5. Increasing the engine's...
5. 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. This graph is located either separately below the horsepower/torque graph or included along with the horsepower graph (Chart 7). 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
Chart 2 shows the horsepower curve for two different engines. Let's assume both power curves are for a Harley-Davidson Big Twin with stock ratio gears. Let's also assume the engine is up-shifted at 6,000 rpm and rpm drops down to about 4,500 after each shift. Which one of these engines would be faster? 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.
6. his example shows that...
6. his example shows that power peaks at approximately the same rpm as in Chart #4 but improves vertically across the entire rpm range. Adding displacement and/or compression typically improve power in this fashion.
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 (reference Chart 3). 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.