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Over and over we hear that internal-combustion engines are just air pumps, and that’s largely true. But what happens when you take that pump up to high altitude, or run it on a hot day? And for that matter, what happens to the other air pump in the equation—you, the rider?
Most of us know that with a carbureted engine we need to adjust the jetting at higher altitudes. More correctly, though, we need to adjust the jetting because of changes in air density, and altitude is only one of several factors influencing air density, although it’s probably the most important. At high altitudes and on hot days, there’s simply less air in the air—a cubic foot of the invisible gas has fewer molecules than it does at sea level on a cold morning. Note that there’s not just less oxygen, there’s less of everything: Air is about 78.09 percent nitrogen, 20.9 percent oxygen, 0.93 percent argon and 0.03 percent carbon dioxide no matter where you go on planet Earth (well, maybe except behind that smoke-spewing metro bus). And our engine wants about 14.7 parts of air to every one part of gasoline (by weight; by volume, it’s more like 20,000 to one). Changing your jetting lets your carburetor get close to this perfect ratio, while fuel-injected engines automatically compensate for changes in altitude.
Neither fuel injection nor revised jetting will recoup the horsepower lost with altitude; they just eliminate additional losses from running too lean (too much air in the mix) or too rich (too much fuel). How much power do you lose at, say, a 12,000-foot pass in Colorado? First, we need a baseline: The Society of Automotive Engineers has a standard based on an atmospheric sample representing sea level at 77 degrees Fahrenheit, per percent relative humidity and a barometric pressure of 29.235 inches of mercury. This is what the horsepower in a “corrected” dyno chart is corrected to. There’s another commonly used standard in aviation called the ISA (International Standard Atmosphere); 29.92 inches of mercury at 59 degrees Fahrenheit; 14,699 pounds per square inch of pressure. So much for geek speak: What this means to you is that for every 1000 feet in elevation gain, if the temperature holds steady, you’re going to lose about three percent of your engine’s horsepower. That 250-horse engine in your pickup is going to be putting out only 160 ponies at 12,000 to 17,000; now you’re getting only 57.4 percent of your sea-level horsepower—your 250-horsepower truck is down to 143 horsepower. Approaching this another way, if you take your ATV out to the deer stand on a 35-degree winter morning, then visit the same stand in the summer when it’s 105 degrees, you’ll see a drop in power of about 10 percent from the rise in temperature alone, even though the geographic altitude stay the same.
There’s a little bit of good news in all of this. In general, the higher we go, the cooler the air gets, to the tune of about 5.5 degrees Fahrenheit per 1000 feet of elevation—that’ why there’s snow on the mountaintops but not down below. And that cooler air is denser, helping gain back a little of the power we’re losing by climbing so high. But of the two, large gains in altitude rob more power than what the drop in temperature returns, and the higher you go, the less power your engine will make.
As mentioned, neither jetting nor fuel injection can compensate for this, although they both will minimize any further losses from an inefficiently running engine. Only turbo charging or super charging—stuffing more of that thin air into the cylinder—can compensate. This is why virtually every high-performance internal-combustion aircraft engine from World War II used one of the two systems. And it’s also why all those guys in the mountain states choose ATVs with big engines.
So much for your ATV—now what about the ATV rider?
If the internal-combustion engine is a relatively simple air pump, then human physiology is much more complex. Drive up to our 12,000-foot pass and you’ll feel winded or short of breath. Again, most of this is because the air is less dense—there’s less oxygen in every breath you inhale.
And just how much are you inhaling? Lung capacities vary with a person’s age, physical size, gender and health, but the average human has a tidal capacity (the amount he inhales and exhales with each breath) of about 500cc—think of yourself as the human equivalent of a Honda FourTrax Foreman. Total lung capacity is about five liters (5000cc) for most humans, but we use much less under normal respiration. The normal respiration rate at rest is between 23 and 15 breaths per minute, although we all know we can rev up our respirations under exercise.
But there’s another big difference waiting for you at altitude, and that’s what the relative lack of pressure does to other parts of your body, right down to the cellular level.
Humans are incredibly adaptive creatures, and we all know that we can get used to higher altitudes given enough time. Mountaineers call it acclimatization, and here’s an example: Take a person from sea level, put him in an aircraft and fly him up to 29,000 feet. Now, open a window: within four minutes or so that person will be unconscious, and within 15 minutes he’ll be dead.
Let’s take the same person, outfit him with some climbing equipment and set him to work scaling Mount Everest—also 29,000 feet above sea level. Hundreds of people have made it to the top, many without any supplementary oxygen. Why? The air’s just as thin, but their bodies have had time to acclimatize physiologically. Think of it as a natural, biologic re-jetting.
There’s still a lot that’s not fully understood about the body’s ability to adapt at high altitudes, but in general we know that most humans plateau in terms of acclimatization at about 16,500 feet. Naturally you can go higher, but your body won’t compensate for it, and you’re just losing ground in terms of fitness. Unless you’re a serious mountaineer, that’s not an issue—the highest point in the continental United States is about 14,500 feet, and few roads go over 12,000 feet here.
Below 5,000 feet few people show signs of altitude-related discomfort, but above that we begin to see symptoms—elevated respiration rate, shortness of breath, more rapid pulse and headaches. Above 10,000 feet, there’s a thickening of the blood and a greater loss of fluids due to urination. You may have trouble sleeping and experience a loss of appetite. And really, at any altitude above 8,000 feet you need to be on the lookout for something called AMS—acute mountain sickness.
AMS victims commonly complain that they feel like they’ve got a hangover—headache, loss of energy and depressed appetite or nausea. Severe cases show a loss of coordination. None of these is irreversible, though; just descend to a lower altitude and everything should be fine. Above 14,000 feet , however, two far more serious health problems appear: high-altitude pulmonary edema (HAPE) and high-altitude cerebral edema (HACE).
You may have noticed that at high altitudes, your hands, feet and face feel puffy. These are external signs of mild edema—the accumulation of fluid in the soft tissues between the cells in the body. In your hands and feet it’s no big deal, but when these fluids accumulate in the lungs (pulmonary edema) or start pressing against the brain (cerebral edema), it’s time for a rapid descent to lower altitudes.
The edemas are caused less by the reduced oxygen available at altitude and more by the lowered pressures on the cell walls themselves, although the physiology is complex. Acclimatization and hydration are two of the keys when it comes to staying healthy. But how to acclimatize? Here are some guidelines:
Few people have difficulties up to altitudes of 5,000 feet, even if they live at sea level. Above that a very conservative approach would be to raise the altitude at which you sleep by 1,000 feet per is the general rule of thumb for travel higher than 10,000 feet. Drink lots of fluids, treat headaches with aspirin, get plenty of sleep, avoid alcohol (coffee and tea may or may not help the symptoms; listen to your own body here) and avoid overexertion for the first couple of days. Finally, if your condition deteriorates or fails to improve, descend.
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