Dynamometer runs are a great way to find problems and see how your truck’s performance measures up but the way the dynamometer operator runs the test can influence the results.
Before getting a dyno run done keep these 4 things in mind.
1. Start bringing on the load when the engine is at maximum rpm: For example a 500hp 12.7 Series 60 “BK” engine has a peak horsepower output of 500 horsepower. Anyone who’s used Detroit Diesel’s ECM program can see that this program is listed as [email protected] If the operator starts the test below the 2110 rpm limit peak horsepower will never be seen. So in order to get an accurate reading the test needs to be started when the engine is on the governor.
2. The testing duration can affect results: You can never get enough air though the charge air cooler and radiator during a power pull. Some dyno operators force the fan to run but when they do this horsepower is lost and any time the fan is engaged during the test your horsepower reading gets less accurate. On the other hand if a power pull is done too fast your engine won’t have time to build boost and the engine never gets a chance to make peak horsepower. Ideally a power pull should be as slow as possible without overheating and without engaging the fan.
3. Dynamometers need calibrated regularly and honestly: If a dyno isn’t calibrated regularly the torque reading gets less and less accurate. During calibration weights are hung on a dyno’s calibration arm to ensure the dyno is reading accurately. If this isn’t done honestly the dyno will report that you are making more torque and more horsepower then you actually are depending on how far off the measurement is.
4. Roller torque vs. flywheel torque. Besides horsepower everyone wants to know what flywheel torque is but the reality of it is that a chassis dynamometer is the wrong tool for the job. People try and estimate a drivetrain loss of 10 to 15 percent but the big problem here is the dyno doesn’t know what gear you’re in, what size tires you’re running, what your rear end ratio is, and what the torque loss is between the tires and the flywheel. Your transmission is a torque multiplier so any flywheel torque calculation can be easily cheated if the truck is in any gear other than direct during the test. Many ecms have a number they broadcast in the data link as a calculated flywheel torque number. As long as the ECM isn’t being cheated by a program or aftermarket device this number is more accurate than a calculated flywheel torque number from a chassis dynamometer. When you talk to your dynamometer operator about what you want listed in your dyno report ask for the wheel torque and roll speed numbers as well as the data link broadcasted torque number. This information will give you a more accurate flywheel torque number.
Written by Fernando DeMoura, Dieselcontrolservice.com. Phone 412-327-9400