May 2009

Monday, May 04, 2009

DynoTech : Dyno Calibration



  Our SuperFlow 901 engine dyno absorption unit has a capacity of 1,000 HP at up to 10,000 RPM. To enable us to properly test relatively small engines it’s been helpful for engine control, ease of operation, and minimizing losses from inertia of the heavy bronze absorber rotor, to reduce absorber speed by 33.33% compared to engine speed. At DTR we use a 50mm wide Gates GT2 toothed belt drive with 1.5/1 reduction. The toothed belt sprockets are mounted on 1.5” diameter steel shafts, which are supported by four synthetic greased Sealmaster NP24 1.5” pillowblock bearings.

  Like any gear reduction, this drive system converts a certain percentage of the engine power into heat from friction. This loss has been accurately measured, and compensated for in our dyno calibration process. 

  Since HP loss from friction increases exponentially as the square of engine speed, the following percentages must be added to raw calibration torque numbers to be correct at the RPM where peak horsepower occurs. These percentages were determined by dyno testing a snowmobile engine—first, direct drive to the dyno absorber (1/1) then within 30 minutes installing the (1.5/1 or 33.33% speed reduction) toothed belt drive. Engine torque/ horsepower losses at every RPM were noted. Also figured into the friction loss is a GMN470 overrunning sprag clutch that is used in the electric start system on the engine drive shaft. The following percentages must be added to the raw calibration torque to be correct at the engine’s peak HP RPM:


7500 = 7.2% added (to compensate for 6.8% HP friction loss @7500 RPM)

8000 = 7.8% added (to compensate for 7.3% HP friction loss @8000 RPM)

8500 = 8.7% added (to compensate for 8.1% HP friction loss @8500 RPM)

9000 = 9.9% added (to compensate for 9.0% HP friction loss @9000 RPM)

9500 = 11.0% added (to compensate for 10.0% HP friction loss @9500 RPM)


The SF901 torque calibration arm is 3’ from absorber shaft center to the hang point of the calibration weight. The net weight value of the arm itself is 17.5 lb. The blue steel 72 cu ft full scuba tank we’ve used for calibration since 1987 weighs exactly 40.3 lb. The raw torque value of the hanging scuba tank is 40.3 x 3’ + 17.5 = 138.4 lb/ft. That is reduced by the 33.33% belt drive speed reduction to 92.3 lb/ft. The blue 40.3 lb scuba tank should have a hanging torque value as follows to be correct at these engine speeds:


7500 = 99.0 lb/ft   

8000 = 99.5 lb/ft

8500 = 100.4 lb/ft

9000 = 101.4 lb/ft

9500 = 102.5 lb/ft


We have 10 lb. weights which can be added to the hanging scuba tank to ensure proper calibration for higher torque engines. Each 10 lb added weight has a value of 20 lb/ft-- (10 lb. x 3’ = 30 lb/ft then reduced 33.33% for drive reduction). Add desired number of weights to the 92.3 lb/ft scuba tank. Then increase by the percentage necessary to correct for peak HP RPM.

With the mechanical drive issues addressed, what remains for accurate corrected horsepower measurement is:
1) proper observed barometric pressure measurement--not "corrected" baro used by weather stations and airports.
2) lots of clean, exhaust-free air to engine air intake. Inadvertent EGR costs lots of HP.
3) engine coolant temp typical of expected operating temperature.
4) adequate compliant media in engine to dyno connection to prevent engine damage from impact-gun-like torque spikes by absorbing and releasing those torque spikes. Those torque spikes that contribute to HP might be ignored by computer data acquisition so it's best to smooth them out so the computer will measure properly, while protecting the engines.

It's just math--accurate torque x accurate RPM/ 5252 = HP