FROM MAHA HEADQUARTERS:
With unrivalled precision, MAHA has produced measurements with a variance of merely 0.6% for the test vehicle provided by Porsche on three unconnected dynamometers – at three different levels of altitude.
When the specialty magazine “Sport Auto” carried out a performance test on a Porsche 911 Turbo S in early 2014, its findings differed from the manufacturer’s specifications (560 HP) by more than 8%. The test readings from the roller dynamometer indicated that the car had much more power than the manufacturer claimed.
Porsche asked the magazine’s staff to take another look at this topic, this time in more detail. Working with MAHA’s test bench experts, they delved into some of the unexplained issues relating to the performance test. The measurements taken in three new tests were carefully analyzed and published in the latest edition (1/2016) in a report spanning several pages. This resulted in a reproducible, and therefore reliable, measurement that provides precise verification of the Porsche’s power.
What went wrong in the initial tests? This was the question that the new measurements set out to solve. “In the performance test, there are all kinds of factors that can have a significant influence on the final measurement and distort it,” explains Michael Pleinies, trainer for test bench technology at MAHA.
The engine power is calculated by adding the measured wheel power to the towing power. The engine power is fundamentally dependent on a large number of measurement variables. The tire temperature and the air pressure need to be correct to ensure that the measurement is not impaired by a faulty grip between tire and roller. A further criteria for a reliable measurement is the type of fuel used and its temperature. For example, the measurement would be distorted by a low-octane fuel and a high temperature. Ideally, a full tank of 102 octane fuel would be used. The test vehicle should be thermally conditioned to operating temperature before the test is performed, as the engine will otherwise not be able to achieve its maximum level of power.
The quality of the air in the test room that is fed to the vehicle is also important – the air supply on the test bench should simulate conditions on the road as realistically as possible. It is therefore crucial for the relevant vehicle components to be cooled throughout the entire test procedure. Cooling must be provided in each case to ensure high-precision reproducibility.
All of these measurement variables were taken into account and implemented correctly as a matter of course in the initial test. The engine power, calculated on the basis of the measured wheel power and towing power, is normally corrected in accordance with the applicable directive (80/1269/EEC). This allows climatic differences such as the air pressure, inlet air temperature and humidity to be taken into account and offset.
All in all, it is clear that the measurements can vary greatly depending on the nature of the individual relevant factors. On top of the test bench’s basic measurement tolerance of 2%, the tolerance is also affected by altitude. For example, an additional engine tolerance of 7% can apply if the altitude increases by 2296 ft, and in some vehicles this number may be even greater. This is because engine power is lower at greater altitudes. MAHA dynamometers are equipped with an environment module that automatically records environmental data during the performance test. This allows the engine power to be corrected automatically in accordance with the applicable directive. This standardized engine power (Pstandard) can then be compared with the manufacturer’s values.
“In the initial 2014 test of the innovative Porsche Turbo engine in Haldenwang (at an altitude of 2483 ft), it was decided that the test should be performed with extrapolation, as it was not clear at the time whether extrapolation should be applied for this vehicle,” says Mr. Pleinies. In other words, a correction factor was applied as prescribed by the EEC directive. This meant that the measurements were effectively corrected twice over, and this is what made them differ from the manufacturer’s specifications. The three subsequent measurements of the same vehicle at different altitudes – on one double-roller and two single-roller dynamometers – then provided reliable and reproducible results, as MAHA expected. The average measurement difference for the new tests was 0.6% (measurement 1: 567.0 HP / measurement 2: 563.4 HP / measurement 3: 564.1 HP). Even Porsche was surprised that three near-identical measurements were taken on the three unconnected MAHA dynamometers.
In conclusion, the modern Porsche engine automatically corrects altitude and environmental air pressure by means of a pressure sensor, up to a height of 3280 ft. The boost of the engine remains constant, as the variable turbocharger geometry and appropriate engine management automatically compensate the difference in altitude, and the engine power and torque remain stable. In modern turbo engines(such as those with VGTs) with corresponding adjustment from the engine control unit, environmental influences are compensated independently and reliably.
This eliminates the need for a correction factor to be applied as prescribed by the EEC directive – provided, of course, that the tests are performed using modern testing equipment.