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How to dyno correctly
No matter what type of dyno you select, getting consistent results still depends on the operator and facility. The operator must control the test conditions to be as repeatable as possible.
Controlling the dyno room is extremely important. Room air temperature, exhaust removal, and humidity control are just some of the variables that need to be constant during a dyno session. Having an inconsistent room can produce inaccurate dyno information. If you are “open room” dynoing be sure to choose the day closest to the standard dyno correction factors as possible. Choosing a day with these parameters will keep the dyno from needing a larger correction factor and producing less accurate results.
Step one for any dynoing is to ensure the dyno is calibrated correctly and reads zero torque when not running the engine.
The torque reading is produced by a strain gauge, this type of load cell has a metal cross section with a small electronic wire grid mounted to its surface. As this cross section is compressed, pulled, or bent (depending on the linkage and load cell design) the attached wire grid is likewise deformed. The almost immeasurable deformation of the wire grid changes its electrical resistance a tiny amount. The electronic circuit acts like an ohm-meter to read the resistance change, only it’s calibrated in pound-feet.
With the dyno calibrated you are ready to start the engine and begin engine stabilization. This includes having the water, exhaust, and head temperatures at the same operation level the sled will have in the field. The correct operation temps ensure the testing duplicates how the engine will perform in everyday conditions.
The operator is the next variable in a dyno pull. For this example we used a sweep pull. The operator starts the load at a chosen rpm and with a desired sweep rate. Making 2 to 3 initial dyno pulls is normal as you will want to overlay them and be sure the engine is producing the same repeatable information for each pull.
Getting to understand the engine is necessary as output will change from the ever-changing variables of the sensors. The vehicle’s factory computer can correct for outside conditions and internal changes so making multiple initial runs on the machine is important.
Water temperature, humidity, and intake air temperatures, are a few of the constantly changing parameters an engine’s ECU makes corrections for. After these baseline runs have been completed, deciphering the information and what it all means is of the utmost importance before you can make any educated changes.
Dyno testing can be subjective and without understanding both the baseline information and the data you collected, dynoing is of little value. Deciphering the data produced through the data acquisition system is what separates a knowledgeable dyno operator from a hack. The use of a high-quality data acquisition system is a must. Your output data is what all the performance changes will be based on. Today’s data acquisition systems not only gather data, they can control the entire dyno pull with 99% accuracy. This provides very repeatable data.
Dynos now typically have data acquisition with an auto weather station that includes air temperature, barometric pressure, and humidity. The SAE (Society of Automotive Engineers) is the most commonly used correction factor in engine dyno testing. The base parameters modern dynos correct to are 77º F, 29.23 absolute pressure, and 0% humidity.
These parameters are what the data acquisition system uses to create a corrected torque number. The dyno produces “raw” uncorrected data and with the use of the SAE parameters the torque is then corrected up or down, depending on the information. Corrected horsepower is used to create an equal baseline for a level field for all dynos.
If you are at a higher altitude (such as in mountains) typically you will create less horsepower, but the barometric pressure would be lower and the data acquisition system would add a percent of increase in horsepower to compensate.
Standard data inputs a dyno should include are rpm, torque, horsepower, BSFC (brake specific fuel consumption), water in, water out, and EGT (exhaust gas temperature). There are many other bells and whistles available on dynos, but these inputs are typically the minimums for any dyno facility. Additional data acquisition features include air/fuel, mass air, volumetric efficiency, oil temp, brake mean effective pressure, and more.
Reading a dyno sheet can be confusing.With the large mass of information presented, we suggest taking it one step at a time. The rpm, torque and horsepower are standard information already discussed. The water in and out, is the temperature of the water entering the engine and leaving the engine. This water is regulated to help stabilize the engine and should be in a separate tank controlled electronically to manage the engine’s water temp. The EGT is a probe inserted into the exhaust to measure exhaust gas temps leaving the engine. EGT is used to help in tuning the amount of fuel to add or subtract to produce optimum horsepower.
The last item used in most dyno facilities is BSFC (brake specific fuel consumption). This is the amount of fuel burned per hour to produce a given amount of brake horsepower in a reciprocating engine. Normally BSFC is expressed in pounds of fuel burned per hour for each brake horsepower the engine develops. BSFC is more commonly used in 2-stroke applications, but as 4-stroke engines have become more prevalent in the market BSFC has started to be replaced by the air fuel ratio. This is the percent of air vs. fuel; typical engines will produce maximum power from 12.8 to 13.2 to 1.
Engines today, with emission limits becoming more prevalent, have forced the manufacturers to produce engines with a higher air to fuel ratio in the 13.5 to 14.2 to 1 range. This keeps them compliant with the EPA and produces better fuel economy. The performance enthusiast today typically requires a lower air fuel ratio, as this produces the max power.
For dyno customers Dynoing is a necessity with ever-changing technology. Engines now have more precise advancements and less available area for improvements. This means the aftermarket must be even more precise and accurate in its testing in order to produce quality products and provide the customer with accurate information.
When dynoing your personal machine, testing or buying parts, we suggest you also stick with one dyno center, and don’t get hung up on the top horsepower numbers too much as it’s extremely common to have dyno facilities produce different horsepower numbers depending on all the factors given above.
Most importantly, the dyno center is a place to prove performance gains, or correct drivability issues. If your machine creates 150 hp at one dyno and 145 at another, yet the end result is both dyno’s show a 5 horse increase, that is what’s important.
See additional dyno info from this story on www.amsnow.com