Forget the fact it’s the first gravel round of the season. Forget that it gets hot inside the cars. Forget, even, the potentially damaging concrete culverts that cross the roads. The challenge of Rally Mexico is all about the altitude.
The host city of Leon sits at 1815 metres and the stages in the Sierra de Lobos and Sierra de Guanajuato mountains climb to nearly 2800 metres.
That’s not enough to induce altitude sickness in drivers and co-drivers, but the thin air at that height does have a similar effect on World Rally Cars, which suffer a power drop of about 25 per cent.
We’ve all heard how the thin air effects performance, but what exactly happens? We thought we’d fish out the school science books and also enlist the help of M-Sport’s engine expert Nigel Arnfield to find out.
Well, the higher you climb, the more the atmospheric pressure decreases and the air becomes thinner. That means there is less oxygen and less air available for the engine to breathe.
Even though World Rally Cars are turbocharged and air is pressurised into the engine, that air must be forced harder because the process starts with less of it than there is at sea level.
“You have to work the turbo quite a bit harder to achieve the same pressure you could at sea level,” said Arnfield. “As you reach really high altitudes you arrive at a point where you can no longer increase the speed of the turbo because you are at its limit.
“You can only run the turbo within its safe limits and you have to accept you can’t generate the same boost pressure as at sea level,” he added.
Arnfield said engines also experienced the pressure difference in the exhaust system which affected the mapping programmes used by engineers. So there we go. But how does that affect competitors?
You will often hear a driver, particularly an altitude rookie, complain that his or her car lacks power and there must be an engine problem. Well, nine times out of 10 it’s simply the altitude. On the 10th occasion, of course, it’s time to pack the car into a crate and head to the airport...
The crucial aspect for drivers is to ensure they carry maximum speed through corners. Let the speed drop or make a mistake and it takes much longer than usual to regain it on the following straight.
Before we finish, let’s not forget that thinner air means a car’s cooling systems must work harder. Engines and brakes will run hotter because the air that cools them is considerably less dense and overheating is another common complaint.