How it works

toyota

Can it be that no two cars are created equal? JAKE VENTER explains why some cars may feel more powerful than others.

 

Seemingly identical cars often differ in the way they perform. It may be that the slower one needs a service, but one often finds that the difference cannot always be accounted for.

Sometimes it is due to small machining differences, but most modern manufacturing processes are now so well controlled that such variations are small. Engines leaving the assembly line seldom differ in output by more than few per cent. Toyota and Lexus are known have to some of the most stringent quality control checks in the world to ensure that each car leaving the production line is manufactured to the highest degree of consistency and quality.

Due to friction, there are small power losses in the drivetrain, but the main source of lost energy is the tyre/road interface. Different brands of tyre, as well as the various versions of the same brand, will show slightly different losses while transmitting power. Using different brands of tyres on the same car exacerbates the problem. For this reason it is always recommended that when it comes time to replacing worn tyres on your car, you swop it with the same brand and size as what it came from the factory, as Toyota and Lexus collaborate with various tyre manufacturers to determine the best combination of tyres to optimize the performance on your specific car model.

Furthermore, tyre pressure has a major influence on the energy loss percentage, to say nothing of the condition and type of road surface. The power loss on a good road between the driven wheels and the road could be anything from 14 to 17%. It will be worse on a poor road surface.

 

HIGHS AND LOWS

The performance of a car is also affected by the road gradient, while variations in air temperature and pressure affect the under-bonnet temperature. Also remember that normally aspirated engines, i.e. those without a turbo, lose about 1% of power for every 100m in altitude above sea level. This means a coastal car is theoretically considered noticeably more powerful than a car with the same engine performing on the Highveld.

We can get some idea of the changes in power output due to the under-bonnet temperature and pressure by applying the engine correction factors published in the SABS standards for dynamometer testing. The correction formula adopts the average sea level pressure of 99kPa and a temperature of 25ºC as reference standards.

Applying the formula for a naturally aspirated petrol engine, we find that, at a barometric pressure of 86 kPa, such as often occurs in Gauteng, the power loss would be 18%, compared with the coast, at the reference temperature of 25ºC. If the temperature in Gauteng dropped to zero degrees, then the power loss would be only 12.1%. However, if the under bonnet temperature went up to 42ºC, which it could easily do if the outside temperature is in the low 30s, the power loss would be 22%! Similarly, at the coast a temperature of zero degrees would imply a power gain of 5%, but a temperature of 42ºC degrees would mean a loss of 3.4%.

Lastly, let’s look at the worst case. Imagine a car that is supposed to have a 100kW engine, but is 4% down on power because it needs a tune-up, has a 20% power loss at the wheels, and is based in Johannesburg. On a day when the under-bonnet temperature is 42ºC degrees, this car will deliver 100 x 0.96 x 0.80 x 0.78 = 59.9kW to the road. The best case would be a 100kW Cape car at the coast that is 4% up in power, has only 15% power loss at the front wheels, and has an under-bonnet temperature of zero degrees. It will deliver 100 x 1.04 x 0.85 x 1.05 = 92.8 kW to the road.

So, not all cars are equal after all…