Rally Guanajuato Mexico’s high altitude stages present teams with a complex engineering challenge as they try to limit engine power loss caused by thinner air.
Friday’s El Chocolate stage climbs to 2,781 metres above sea level and is the highest point in the WRC season. The rest of the route never drops below 1,800 metres and the combination reduces the power of a typical 320hp World Rally Car by up to 30 per cent.
The high altitude Sierra de Lobos and Sierra de Guanajuato regions compound the effect of the regulation FIA air intake restrictor on the 1.6-litre turbo engines - creating an effect Volkswagen engineers liken to ‘jogging whilst breathing through a snorkel’.
There are two key problems. The high-altitude air sucked through the 33-millimetre opening contains less oxygen in Mexico than anywhere else in the season, leading to less efficient combustion. Secondly, the less oxygen the air contains, the harder the turbo charger has to work to produce the same power.
“On one hand, the air pressure sinks with altitude, resulting in a reduction in the oxygen content in the air. On the other hand, so does the air resistance in the turbo charger, which achieves higher speeds than at any other rally,” explained Dr. Donatus Wichelhaus, Head of Engine Development at Volkswagen Motorsport.
Limiting the turbine speed is one way engineers can avoid overstraining the turbo. But even with the unit spinning at its maximum, the engine will the lack the grunt it has on an event like Rally Sweden which boasts almost perfect conditions.