The Red Bull X-Fighters riders estimate that 2,240-meter high altitude robs their engines of about 25 percent of their usual power at sea level --which means that even a routine jump or trick becomes an extraordinary challenge of skill and courage. Their mechanics are working overtime to try to help the engines compensate somewhat for the reduced compression in the thinner high-altitude air but there is nothing anyone can do to alter the fact that the ambient air pressure in Mexico City is so much lower than at sea level. The riders are also doing what they can to adjust their runs and timing to the reduced engine power.
"It's such a shock because you're used to having a certain amount of power on the ramp and now the bike feels more gutless, less powerful and not as snappy," said Australia's Josh Sheehan after his first eye-opening training runs in the Monumental Plaza de Toros bullring ahead of Friday's 2013 season opener in front of a capacity crowd of 38,000. "The first time down the ramp was a little daunting. Normally when you pull the throttle on the ramp you feel a hit of power before you take off. But here it's much flatter, less powerful. It's just something I'm going to have to get used to."
The higher altitude means less oxygen which in turn means less power, says Red Bull X-Fighters Sport Director Tes Sewell. "The altitude is always a problem because it robs you of the oxygen you need to make the combustion in your engine. It's not a new challenge. But it's something the guys have to get used to here." He said that teams of local mechanics are helping to make fine-tuning adjustments to the carburetor settings, changing the jetting to allow in more oxygen. They are also reducing the octane level of their fuel.
"It just freaks you out the first time," said Sewell, who like the riders and crew members admit that the high altitude makes it more strenuous physically to simply walk up the stadium steps. "It's an alien feeling. It's not what you expect when you twist the throttle. "
France's Tom Pages threw down a pair of near-perfect flatspin 540 flairs in training on Thursday. But the 2012 Red Bull X-Fighters World Tour runner up said the thin air was definitely a major challenge for the 2013 season opener and that he would have to drop a few tricks from his repertoire here because they would be impossible in the thin air.
"I knew the oxygen would be a problem because I've been riding here before," he said. "We don't have as much power because with no air, no execution. The bike doesn't really work well. My goal is to try to have fun and take part with no pressure."
Martin Koren of the Czech Republic reckons that there's about 10 percent less power for the engine for every 1,000 meters in altitude. "The whole range of power in the engine works differently at this altitude," he said. "If you go up the ramp and want to snap the throttle you don't get the power as fast as you're used to and the bike will tend to choke up. But the more practice we get here, the more natural it's going to feel. I'm sure by Friday night we'll all be dialed in."
The four-stroke bikes have Electronic Fuel Injection (EFI), a computer chip that somewhat helps adjust for the drop in atmospheric pressure but riders such as defending world tour winner Levi Sherwood who prefer the two-stroke bikes with their carburetors are feeling the full effect.
Sherwood's mechanic Chad Geib worked hard to make adjustments to the engine, giving it a larger bore cylinder. "We made it basically a 300cc engine instead of a 250cc to give it some compression back because you lose so much power up here. Then we leaned out the carburetor as far as jetting goes. We're also running on fuel that's oxygenated. When you go up in altitude you want to go lower octane, more oxygen because you lose so much air going into the motor up here.