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solely for cooling. A leaner mixture would give more power. Actually, water or, more accurately, the antidetonant (water/ alcohol) mixture is a better coolant than extra fuel. Therefore, water injection permits higher manifold pressures and a still further increase in power. 

In establishing the final curve for engine operation, the engine’s ability to cool itself at various power settings is, of course, taken into account. Sometimes the mixture must be altered for a given installation to compensate for the effect of cowl design, cooling airflow, or other factors on engine cooling. The final fuel/air mixture curves take into account economy, power, engine cooling, idling characteristics, and all other factors that affect combustion. 

Figure 10-47  shows a typical final curve for a float-type carburetor. Note that the fuel/air mixture at idle is the same 

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in rich and in manual lean. The mixture remains the same until the low cruise range is reached. At this point, the curves separate and then remain parallel through the cruise and power ranges. 

Note the spread between the rich and lean setting in the cruise range of both curves. Because of this spread, there is a decrease in power when the mixture control is moved from auto-rich to auto-lean with the engine operating in the cruise range. This is true because the auto-rich setting in the cruise range is very near the best power mixture ratio. Therefore, any leaning out gives a mixture that is leaner than best power. 

## Idle Mixture 

The idle mixture curve shows how the mixture changes when the idle mixture adjustment is changed. [Figure 10-48]  Note that the greatest effect is at idling speeds. However, there is some effect on the mixture at airflows above idling. The airflow at which the idle adjustment effect cancels out varies from minimum cruise to maximum cruise. The exact point depends on the type of carburetor and the carburetor setting. In general, the idle adjustment affects the fuel/air mixture up to low cruise on engines equipped with float-type carburetors. This means that incorrect idle mixture adjustments can easily give faulty cruise performance, as well as poor idling. 

There are variations in mixture requirements between one engine and another because of the fuel distribution within the engine and the ability of the engine to cool. Remember, a carburetor setting must be rich enough to supply a combustible mixture for the leanest cylinder. If fuel distribution is poor, the overall mixture must be richer than would be required for the same engine if distribution were good. The engine’s ability to cool depends on such factors as cylinder design (including the design of the cooling fins), 

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