This article was first published in 1999.
The first question must be: how accurate is the measuring system? When was the dyno last calibrated? What calibration weights were used for the calibration? Are their weights accurately known? Was the effective length of the torque calibration arm measured accurately? If the arm was assumed to be 610mm from the centre-line of the dyno main shaft to the hang-down point, but was actually only 603mm, the power reading would differ by about 1 per cent. Was the dyno measurement system checked for linearity? Some dynos, particularly those using dashpot-pressure torque read-outs, having varying accuracy depending on the applied torque.
Is the measured torque affected by the temperature of the measuring load cell? On some dynos the temperature of the load cell is affected by the absorber and/or exhaust system temperatures. This can lead to inaccuracies.
With a water-brake dyno, was the residual torque reading zero with the engine stopped? Drag from the water supply and discharge plumbing can create a significant torque offset error.
Were the reported torque and power figures corrected for atmospheric conditions? What correction standard was used? Just changing the correction standard between SAE and DIN can result in a 0.7 per cent change in power for exactly the same engine. How accurately was the barometric pressure, air temperature and relative humidity measured? Where was the air temperature measured? A 5ºC inaccuracy in air temperature can lead to a 0.9 per cent change in power figures. A 3mB (normal range 900 to 1050mB) inaccuracy in barometric pressure gives a 1 per cent change in power.
Was the engine tested with the alternator, fan, airconditioning compressor, power steering pump etc connected? This is very important when comparing power figures from different dyno shops.
Was the engine tested in the same tune and conditions on the dyno as when it was returned to the customer? What fuel was used when tested on the dyno? Was the ignition timing "tweaked" on the dyno, but retarded again when delivered? Was the same exhaust system used on the dyno as was used in the vehicle?
What oil and water temperatures were used for the test? Lower water and higher oil temperatures can produce higher power from an engine, but what of the long term effects on the life of the engine?
If using a chassis dyno, what gear was used?
Was the engine tested to its peak reported power, or was the peak power figure extrapolated (read guessed) from lower speeds on a "speed challenged" dyno unable to run at maximum engine/vehicle speed?
On forced-induction engines, was the boost limit as tested the same as that when delivered?
The peak power figure for a given engine is only one measure of its performance. One engine may have a higher peak power compared to another, but may be slower on the road or track. A better indicator of performance is the average power over the normal speed range of the engine.
For example, a "peaky" engine might produce say 300bhp at 7000 rpm, and an average power form 5000 to 7000 of 260 bhp. A less peaky engine might produce 280 bhp at 7000 rpm, but an average of 270 bhp over the same speed range. Clearly an engine is at 7000 rpm for only a brief time in the real world, so the "less powerful" 280 peak bhp engine will perform better on the road, and also be a nicer engine to drive.
Many aspects of engine performance testing can influence the "bottom line" power figure and its relevance to overall vehicle performance. The quoted peak power figure is only a part of the total picture.
*Arex Computing, dyno software development specialists. Contact:
+613 9751 1142