Altitude and Engine Performance

Pressure equivalents to: 1.0 atm = 29.9 in. Hg. = 760 mm Hg. = 101.3 kPa = 1013.25 mb

Atmospheric Pressure = 14.7 psi & 13 cubic feet of air = 1 pound

Air Density decreases at a rate of 2.9% - 3.0% for each 1000 ft. of elevation above Sea Level. See Standard Atmosphere below for background information.

Naturally Aspirated: Atmospheric Pressure 14.5 psi (It's hard to ride at sea level 14.7 psi)
Atmospheric Pressure @ 9000 feet = 10.5 psi
Pressure Loss = (14.5 - 10.5) = 4.0 (4.0/14.5) = 27.58 % @ 9,000 feet

Atmospheric Pressure = 14.5 psi, Boost = 10 psi, Total Pressure = 24.5
Atmospheric Pressure @ 9000 feet = 10.5 psi + Boost of 10 psi = Total 20.5 psi

The power loss due to altitude is much less with the Turbo. The critical difference is that you can flip the switch on the Turbo to 15 lbs boost and get your sea level HP!!

Turbo considerations: As altitude is increased the turbo fan must increase rpm to maintain a constant boost pressure. With large displacement engines (read 1000cc 4-strokes) the turbo fan may have to spin faster than is efficient. The result is slower acceleration. The cure is a larger turbo or lower elevation. Mountain sleds will challenge the best tuners ability to compensate for temperature, altitude, and changing snow conditions!

In simple terms, density is the mass of anything divided by the volume it occupies. Scientists usually measure air density in kilograms per cubic meter. At sea level, if the air is completely dry, and the temperature is 0 degrees Celsius, a cubic meter will have 1.275 kilograms of air in it. In other words, the density is 1.275 kilograms per cubic meter.

The air's density depends on its temperature, its pressure and how much water vapor is in the air. We'll talk about dry air first, which means we'll be concerned only with temperature and pressure.

The molecules of nitrogen, oxygen and other gases that make up air are moving around at incredible speeds, colliding with each other and all other objects. The higher the temperature, the faster the molecules are moving. As the air is heated, the molecules speed up, which means they push harder against their surroundings. If the air is in a balloon, heating it will expand the balloon, cooling it will cause the balloon to shrink as the molecules slow down. If the heated air is surrounded by nothing but air, it will push the surrounding air aside. As a result, the amount of air in a particular "box" decreases when the air is heated if the air is free to escape from the box. In the free atmosphere, the air's density decreases as the air is heated.

Pressure has the opposite effect on air. Increasing the pressure increases the density. Think of what happens when you press down the handle of a bicycle pump. The air is compressed. The density increases as pressure increases.

Altitude and weather systems can change the air's pressure. As you go higher, the air's pressure decreases from around 1,000 millibars at sea level to 500 millibars at around 18,000 feet. At 100,000 feet above sea level the air's pressure is only about 10 millibars. Weather systems that bring higher or lower air pressure also affect the air's density, but not nearly as much as altitude.

We see that the air's density is lowest at a high elevation on a hot day when the atmospheric pressure is low, say in Denver when a storm is moving in on a hot day. The air's density is highest at low elevations when the pressure is high and the temperature is low, such as on a sunny but extremely cold, winter's day in Alaska.

The standard atmosphere can be thought of as the average pressure, temperature and air density for various altitudes. It is useful for engineering calculations for aircraft & mountain sleds. It also shows in a general way the pressures and temperatures to be expected at various altitudes. The standard atmosphere is based on mathematical formulas that reduce temperature and pressure by certain amounts as altitude is gained. But, the results are close to averages of balloon and airplane measurements at various altitudes.

The table below uses metric units, which scientists use. Scroll down for a version using feet, fahrenheit temperatures and pressures in inches of mercury.

This table give density in slugs per cubic foot because it uses the American system of altitude in feet, pressure in inches of mercury and temperature in degrees Fahrenheit. While people often use pounds per cubic foot as a measure of density in the U.S., pounds are really a measure of force, not mass. Slugs are the correct measure of mass. You can multiply slugs by 32.2 for a rough value in pounds.(Pounds per cubic foot have been added to Chart)