As the air thins out, the 1.6-litre turbo engines fitted to all World Rally Cars lose around a quarter of their performance.
The turbocharger plays a key role. As well as the loss of high-altitude performance caused by a lack of oxygen and reduced air pressure in the inlet charge, the thinner air allows the turbine to spin so fast that it could burn itself out.
Ever since Mexico made its WRC debut in 2004, each manufacturer has developed engine settings to minimize power loss in this unique event. Ahead of its Mexico debut this week, Volkswagen is the most recent to look into the problem.
Last November, before its competitive debut, Volkswagen was free of the WRC restrictions that prohibit testing outside Europe and was able to run the Polo R in Mexico.
As well as this on-stage testing, the manufacturer also completed trials in its climate and altitude chamber in Germany to accurately simulate the challenge ahead.
“The altitudes encountered at the Rally Mexico affect the engine more than any other part of the Polo R WRC,” explained Dr. Donatus Wichelhaus, Head of Engine Development at Volkswagen Motorsport.
“On the one hand the air pressure and oxygen content of the air decrease the higher you go. On the other hand, the air resistance in the turbocharger also decreases, resulting in higher revs per minute than at any other rally. In order to guarantee its stability, whilst at the same time losing as little performance as possible, we have run various simulations to prepare for Mexico.”
Volkswagen says it has developed a detailed engine map for Mexico that limits loss of performance without endangering the stability of the turbocharger.
But there are limits to what can be achieved, and Wichelhaus estimates that at the rally’s highest point, the 2669 metre summit of Friday’s El Chocolate stage, the Polo R will lose about 28 to 30 per cent of its output compared to Rally Sweden.