By Jason Weigandt and Aaron Hansel
There’s always a lot of talk about the altitude at Thunder Valley and how it affects engine performance. For example, you probably heard about the 450s having to start in first gear about 983 times over the weekend, and everyone is always quick to point out how challenging it is for the teams to get bikes running right at Thunder Valley’s mile of elevation. But is it really that hard? We asked around in the pits over the weekend, and it turns out today’s technology has made dealing with elevation much easier.
When it comes to altitude, there are two major hurdles wrench spinners face: a lack of power and boiling fuel. The loss of power comes from Thunder Valley’s thinner air—the higher you go the less oxygen there is in the air. Since engines won’t run without oxygen, it makes sense that less air equals less power. There’s a bit more to it than that, but that’s the basic concept.
When there’s less air you must also reduce the amount of fuel going into the engine in order to achieve an ideal fuel/air mixture. There aren’t any other races on the circuit as high as Thunder Valley, so when Lucas Oil Pro Motocross first started going there this process was a big challenge that involved manually changing carburetor jets. Nowadays, thanks to electronic fuel injection, getting bikes in ripping form is usually as simple as plugging in last year’s settings and fine tuning from there.
“It was a lot worse because we’d never been here [Thunder Valley] before,” Monster Energy/Pro Circuit Kawasaki team boss Mitch Payton says. “We were racing at close to sea level and this was a big change. We used to always do press day in the old days to come get as much time as we could on the track. There’s still some dialing in now, but fuel injected bikes compensate for altitude. If you really want, you can put a Lambda sensor on the head pipe and ride around or do a practice and you can find out if it’s still a little rich or if there’s something you can do to make it a little better.”
If you’re wondering what a Lambda sensor is, it’s an oxygen sensor, and in this case it’s used to precisely gauge the fuel/air ratio in exhaust gases. If it’s not right, tuners can adjust the EFI accordingly. But even with a perfect fuel/air mixture bikes will still be down on power at elevation, thanks to the thinner air.
“Everybody is down on power. When you come up this high you lose whatever percentage it is, we think it’s sixteen, eighteen, percent,” Payton says. “We can map, or do whatever we want, and make it run as good as we can, but wherever you brought your bike from, it’s still going to be down that much.”
There’s no doubt that Pro Circuit had the jump on everyone in the carburation days. The Thunder Valley National started in 2005, and the Pro Circuit team won the 250 class the first seven years the race took place there!
Over at Yamalube/Star Racing Yamaha, the team was working with a new EFI map courtesy of the ignition and mapping experts at GET. That crew had actually discovered some things during some MXGP races at altitude last year—like Argentina—so they presented that data for the Star team to try here. But team manager Brad Hoffman, a long-time engine man for the Star outfit, was cautious about using it without some testing.
“You can find maps and get more power with fine tuning, but you always have to be careful from a reliability standpoint,” says Hoffman. “We can do a lot with mapping, but we always want to make sure every part can handle that.”
Mapping can make more power, but power always adds stress to an engine, and teams won’t go all-in on a new ignition map until they know every piece of hardware inside that engine can handle it. So Star rode press day and experimented, and they liked what they saw. Still, Hoffman says they backed the settings down a little bit just to stay on the safe side.
“That was something I was a little worried about today, just because it was new,” he says.
The other hurdle—boiling fuel—is slightly more complicated. Boiling liquid requires heat, right? Well, Thunder Valley has had some hot years, but typically isn’t as hot as some of the other rounds. So why would boiling fuel be such an issue? Well, heat isn’t the only thing that will cause fuel to boil. A lack of atmospheric pressure will do the same thing. If you stick a normal, room-temperature glass of water in a vacuum chamber, the water will boil. Crazy, right? At sea level, atmospheric pressure—the same force that’s preventing your glass of water from boiling—is roughly 14.7 PSI. At Thunder Valley it’s about 11.3 PSI, which is a significant difference. Throw in a hot day and engine heat, and that fuel’s going to have bubbles in it. In the carbureted days that was a real problem.
“When we first started coming here there were a lot of hot races and the fuel would boil inside the carburetor and the bikes would bog and run rich at the end of the moto,” Payton explains. The big difference now is that fuel-injected engines require a fuel pump, and the pressure created in the tank by the pump reduces the chance of the fuel boiling. “Fuel injection has pressure regardless, it’s pressurizing the system,” says Payton. “It’s not just relying on atmospheric pressure like a carburetor. If the fuel boils inside the float bowl it raises the floats like it’s rich.”
Thankfully the issue of boiling fuel at Thunder Valley is virtually a thing of the past, and getting the engines to run stronger in thin air is simpler, too, thanks to modern technology. While racing here does pose its own unique set of challenges, getting the bikes just right is much easier than it used to be.
