The goal of the following is presented as a 'FYI', to help better understand how certain actions/factors can alter dyno test results, and how some may use the dyno to mislead. Armed with this information, you can be better informed when viewing and believing dyno results as 'GOSPEL'.
The first dyno run when the engine is cool usually produces the biggest power. As heat builds in the engine, the peak power number usually settles at a lower level. This is especially true in turbo applications where heat rises. Intercooler efficiency and the placement of the cooling fans can play a role in the heating trend. The scheme here is to take a hot baseline and compare it to a cool product-test run. A lower whp baseline coupled with a higher-whp test makes the product appear to make more power than it actually does.
Performing back to back to back pulls without ample cooling time between runs can cause an unnatural buildup of heat. This will adversely affect the before and after testing of components. The goal is to conduct testing at consistent, normal engine operating temperatures.
Another way of misleading is to play the density intensity game by conducting baseline runs in the heat of the afternoon and run the after tests in the cool evening air. While some dynos can adjust for atmospheric conditions, the bottom line is denser air makes more power.
Dyno operators can reprogram the weight of the drum, which will allow the dyno to create inflated power numbers.
The dyno operator could elect to change the elevation value in the computer, which would have a critical impact in power generation.
Changing the load by tightening or loosening the tie-down between runs can compromise any comparisons made with the dyno.
Turning on the air conditioning during a baseline testing will cost effect horses and make the after test all the more impressive.
Swapping in a different graph in place of your car's baseline will make for big power gains later during the comparison.
When possible, have work conducted while the car is still strapped to the dyno after baseline testing. On some dynos the positioning of the tires on the rollers and the tautness of the straps can change the load characteristics seen during the testing.
Please remember Dynos are tools used to; assess where your car is, show how modifications effect you car, repeatability and performance gains, not just PEAK OUTPUT!
Principles of operation for a Chassis Dyno
An absorbing dynamometer acts as a load that is driven by the prime mover that is under test. The dyno must be able to operate at any speed, and load the prime mover to any level of torque that the test requires. A dynamometer is usually equipped with some means of measuring the operating torque and speed.
The dynamometer must absorb the power developed by the prime mover. The power absorbed by the dynamometer must generally be dissipated to the ambient air or transferred to cooling water. Regenerative dynamometers transfer the power to electrical power lines.
Dynamometers can be equipped with a variety of control systems. If the dynamometer has a torque regulator, it operates at a set torque while the prime mover operates at whatever speed it can attain while developing the torque that has been set. If the dynamometer has a speed regulator, it develops whatever torque is necessary to force the prime mover to operate at the set speed.
A motoring dynamometer acts as a motor that drives the equipment under test. It must be able to drive the equipment at any speed and develop any level of torque that the test requires.
Only torque and speed can be measured; Power must be calculated from the torque and speed figures according to the formula:
Where K is determined by the units of measure used as can be seen below:
To calculate power in horsepower (hp) use:
Horsepower = Torque x rpms / 5252
An engine dynamometer measures power and torque directly from the engine's crankshaft (or flywheel), when the engine is removed from the vehicle. These dynos do not account for power losses in the drivetrain, such as the gearbox, transmission or differential etc.
A chassis dynamometer measures power from the engine through the wheels. The vehicle is parked on rollers which the car then turns and the output is measured. These dynos can be fixed or portable.
Modern dynamometers can do much more than display RPM, Horsepower, and Torque. With a Wideband 02 Sensor, graphed along with RPM, the dynamometer has become the ultimate tool for tuning automobiles. Recent innovations from DynoJet Research have added the ability to add vehicle diagnostic information to the dyno graph as well. This is done by gathering data directly from the vehicle's ECU.
Because of frictional and mechanical losses in the various drivetrain components, the measured horsepower is generally 20-30 percent less than the brake horsepower measured at the crankshaft or flywheel on an engine dynamometer
'FOOD FOR THOUGHT'
Increasing Torque Output Produces More Useful Power!
Increasing the "Torque Output" over the entire rpm range produces more useful Horsepower and faster acceleration than increasing only peak Horsepower or rpm. All engines need to produce a lot of low to mid range Torque to quickly accelerate a vehicle BEFORE the engine can reach its peak Horsepower. The engine does not produce peak Horsepower from idle. Therefore, the more "Average Torque and Horsepower" the engine produces without increasing peak Horsepower or rpm the faster the vehicle will accelerate. A good example is the low rpm 440 Chrysler big block which produces about 100hp less than the high rpm 426 Hemi, but the 440 accelerates faster up to 80mph because it produces more "Average Torque and Horsepower".
Also, an engine which produces 300hp @ 4,000 rpm is almost twice as powerful as an engine that produces 300hp @ 7,000 rpm. This is because at 4,000 rpm the engine is producing almost twice the combustion pressure (Torque) than at 7,000 rpm which produces faster acceleration. Therefore, comparing the performance of engines by only their peak Horsepower are meaningless numbers without also comparing their peak rpm and "Average Torque and Horsepower" which is the most accurate way to determine how much 'USEFUL Horsepower' the engine actually produces.
To determine the "Average Torque or Horsepower" produced by your engine, add together the Torque or Horsepower produced at each 500 rpm increment from 1,000 to peak rpm and then divide the total amount of Torque or Horsepower by the number of 500 rpm increments from 1,000 to peak rpm.
The higher the "Average Torque or Horsepower" the faster the vehicle will accelerate regardless of how much peak Horsepower the engine produces.
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