Performance Measurements - Results

This is the final part of the series. Matters such as advantages and disadvantages in test methods will be discussed, and also how test results are analyzed and presented.

Test & Measurements

Power output can be measured in two ways, at steady state or during acceleration. Both have benefits as well as drawbacks.


Car manufacturers are required to use steady state tests for certification of power output. It has an advantage as it is a well defined test that is easy to repeat and verify. However, it demands a high cooling capacity for the engine / car.

Steady state

The simplest way of testing is to let the engine / car accelerate against a fixed inertia load. Simple in one way most often means drawbacks in other ways. In this case it has numerous drawbacks, which affects the results. Also you are limited to just one type of test.

Acceleration with inertia load

A more demanding test for the dyno is to accelerate against the road load. Then the dyno has to simulate running on the road. This can be good when tuning a specific car. However the results will be affected by the vehicle set-up. Changing the weight of the car will produce different power readings even if it is the same engine.

Acceleration with road load

The problem with results measured during acceleration is that the acceleration itself has a considerable effect on the result. It is therefore important that the rate of acceleration is defined and controlled within tight tolerances. Only then can the test be repeated and verified.

Acceleration at constant rate

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Test Results

This is a comparison of steady state vs acceleration at a steady rate. The acceleration is started at 1500 rpm and stopped at 6500 rpm. The main difference is caused by the inertia of the drive train but also the fact that the engine is running in a different way when it is accelerating compared to steady state.

Steady state / acceleration at constant rate, Mini Cooper S

The influence is even more evident in the following figures. As the steady rate increases full torque is reached later and the torque available is decreased. The inertia of the drive train is reducing the available torque directly related to the acceleration rate [rpm/s]. However it is obvious that the difference is related to other variables as well, especially if you compare the fastest steady rate to the others.

Torque at different acceleration rates, VW Passat 1.8T

Power at different acceleration rates, VW Passat 1.8T

Three racecars from the same team were tested and compared. The racing class uses the same type of car and engine. Sealed engines are delivered from one source. If the seal is broken the car is not allowed to run. First tests were made at steady state and one of the cars was clearly making more power than the other two, close to 10 hp more.

Volvo S60 Challenge 2003 acceleration behaviour

The cars were also tested during acceleration. At steady rate a lower gear results in a higher acceleration, which in turn means more inertia losses. In other words, less power available on the wheels.

Volvo S60 Challenge 2003 acceleration behaviour
Using the data from the acceleration tests acceleration was simulated down a long straight. All cars were said to have the same exit speed, shift point and shift time. As one could suspect the "red" car is faster. The important question is: How much faster, and does it matter?

Swedish Volvo S60 Challenge racing cup 2003
Sealed "equal" engines 2.3 litre "275 hp"
Acceleration behaviour due to power discrepancies
"Starting point 0 sec" 3rd gear, full throttle, 5025 rpm
Volvo S60 Challenge 2003 acceleration behaviour

Converting the difference into distance instead makes it easier to understand how important it can be to have those last horsepowers. If the drivers are equal in driving, the driver in the red car will always win!

Volvo S60 Challenge 2003 acceleration behaviour
At Rototest we do a lot of testing ourselves. This means we can produce some interesting statistics. This figure shows a comparison of what the manufacturers claim in the engine specification and what we measured on the wheels. The average difference turned out to be about 7-9%, which can be said to be a good estimate of a normal transmission loss. However we have seen cars producing quite a lot more on the wheels than the engine spec says. At the other end there are examples of -25% difference too. If this were to be the losses then the oil would boil in the transmission.

Discrepancy between certified engine performance and measured drive wheel performance (404 cars)

True measurement™ Certificate of Performance, Steady state measurements with background facts on page 2, BMW M5 -02.

Certificate of Performance

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