Tuning For High Altitude
The general formula for power loss with increases in altitude is 3 percent for every 1,000 feet above sea level. If you race in Colorado at 5,000 feet instead of in California at sea level, you can expect to lose about 15 percent of the engine’s potential power output, if the engine is tuned properly.

At high altitudes we face a problem: how do you maximize engine performance and speed in the thinner air? Internal-combustion engines require three things to produce power - fuel, oxygen and heat -- and they need them in specific ratios. Running at 70mph is nothing special at low elevations; the denser air at sea level contains more of the oxygen an engine needs to produce power. At higher altitudes, the thinner air means less available oxygen, so an engine will not be able to burn as much fuel as it does at lower altitudes. Less fuel means less power, and that means slower speeds.

What can you do to minimize the effects of altitude? Some equip their cars with a higher gear ratios to recover some of the rpm lost because of the reduction in power. Others improve airflow, by lengthening or shortening their tuned pipes until performance improves.

But can you do anything to the engine itself to compensate for thinner, high-altitude air? Compression creates power, and because the air at high altitude is less dense, there is less fuel/air mixture available to compress.
To recover some of the lost compression, you can:
• Use a fuel containing a higher percentage of nitromethane.
• Reduce engine-head clearance to increase the compression ratio.

To compensate for higher altitudes, you need to adjust your needle valve to a leaner setting to maintain the proper fuel/air ratio. The opposite is true for lower altitudes. The idea is to maintain the optimum ratio of fuel and oxygen by adjusting the fuel volume to compensate for ambient air pressure.

Elevation complicates the situation. A venturi such as a model engine carburetor's works on the vacuum principle: as air rushes through the venturi, it accelerates and creates a vacuum at the spraybar. This vacuum draws fuel from the spraybar and fills the crankcase with a mixture of air and atomized fuel; at very high altitudes, less air accelerates through the venturi and past the spraybar. This reduces the vacuum at the spraybar, so less fuel is drawn through it, and the mixture becomes too lean.
There is a way to compensate for this:
• Switch to a venturi with a smaller area.

When you have a smaller venturi area, the airflow velocity through the venturi is increased. This also increases the vacuum at the spraybar, which, in turn, draws more fuel.

If you don't want or can’t buy a separate carb just for high-altitude running, the easiest way to increase fuel flow is to open up the needle valve to richen the mixture setting. A richer needle-valve setting requires less of a vacuum to draw in a given amount of fuel. Ultimately, this won't be quite as effective as having a smaller venturi area.

Obviously, you have to experiment to learn which combination of these high-altitude tuning techniques best suits your model.

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