Frequently Asked Questions

What is a water brake dynamometer?

A water brake dynamometer is a device used to measure mechanical power by absorbing energy output from an engine. Water brake dynamometers are also known as fluid or hydraulic dynamometers. They absorb energy with water and viscous losses in rotor/stator combinations.

What is brake power?

Brake power is the power measured at the engine crankshaft. It is the power generated inside the combustion chamber due to burning fuel (combustion), less the power due to mechanical friction and parasitic loads, such as the oil pump.

Brake power is also defined as the indicated power multiplied by the mechanical efficiency of the engine.

What are the water requirements for a Stuska dynamometer?

Stuska recommends a minimum of 10 GPM per 100 HP between 45 psi to 65 psi.

For the XS-19 or up to 200 HP, a garden hose is sufficient. At 200 HP, Stuska recommends a 1″ diameter ID supply line.

For the XS-111 or up to 800 HP, a 1.75″ ID supply line is recommended.

For the XS-211 or up to 1,600 HP, a 2.5″ ID supply line is recommended.

How can one correct problems with dyno control and load fluctuations?

1) Verify that the supply water pressure is constant. Fluctuations in water supply pressure affect dynamometer load. A water pressure regulator and/or an accumulator will correct most problems with inconsistent water pressure.

2) Measure the dynamometer water exit temperature. Water vaporization inside the dyno will cause dynamometer control problems. Stuska recommends that the exit water temperature does not exceed 130° F.

3) If using a manual control valve, inspect the valve disc and seat for any signs of wear. A worn disc and/or seat can cause dyno control problems.

What are correction factors and why are they needed?

A correction factor is an estimate to correct engine horsepower under different atmospheric conditions. Correction factors are needed because the same engine will produce more or less power under different atmospheric conditions. For example, a volume of air will increase as temperature increases (pressure constant) according to the Ideal Gas Law i.e. air becomes more dense. Therefore, there are less oxygen molecules in a fixed volume of air at a greater temperature than that of a lower air temperature and the amount of oxygen significantly affects combustion.

How can one correct problems with dynamometer test repeatability?’

Different atmospheric conditions such as air, temperature, barometric pressure, humidity, and altitude affect engine performance.

Note: Correction factors are estimates. Various engines behave differently under the same atmospheric conditions.

Varying sweep rates and inertia effects can also result in repeatability issues.

How does one produce the most accurate dynamometer test results?

To insure that the power measurements on the dynamometer will be the same as in-vehicle results, simulate the in-vehicle conditions as best as possible. For example, using the same exhaust system on the dynamometer as in the vehicle ensures that the exhaust back pressure will be the same and using the same fuel system ensures the fuel pressure will be the same. These are key functions of engine performance.

Also, make certain the dynamometer torque system and all other equipment is calibrated properly.

Why am I getting a HP reading when the water to the dyno is turned off?

Windage is the condition that occurs when the water to the dynamometer is off, yet horsepower readings are still being obtained. Windage increases exponentially with RPM and cannot be eliminated.

Inspect the load control valve for any signs of leakage.