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Optimizing Your Harley's Engine Efficiency

A large included valve angle... 

   

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A large included valve angle requires a high-dome piston, resulting in large piston-dome and chamber-surface areas. The Ironhead Sporty and Shovelhead engines have large hemi-like combustion chambers with high surface-to-volume ratios, which increase heat loss. Also, high piston domes can impede flame travel and airflow during valve overlap, result in a heavier piston, and usually require more ignition advance.

Compression RatiosCylinder pressure is crucial to achieving a powerful and optimized engine. Increasing volumetric efficiency with big cams, higher-flowing cylinder heads, or larger induction are a few ways to increase cylinder pressure, but raising the engine's mechanical compression ratio (also known as static compression) is another. Up to a point, raising the compression ratio increases thermal efficiency because heat and combustion pressure are increased for greater gas expansion. Therefore, assuming an optimized exhaust valve opening, more potential energy can be extracted from the expanding gases. Since the speed of combustion increases as the square of combustion pressure, doubling combustion pressure through higher compression increases the speed of combustion by a factor of four. This has several implications for the optimum ignition timing and the required fuel quality.

An engine with aluminum heads can usually support a compression ratio of roughly 1.0 to 1.5 points higher than an equivalent cast-iron head engine without incurring detonation. This is because the aluminum acts as a heat sink and pulls heat from the combustion chamber. From a power standpoint, this means that all things being equal, including the compression ratio, an iron head engine will make more power than an aluminum head engine because more heat will remain in the combustion chamber, thus producing higher cylinder pressures. To make up for lost heat and power, an aluminum head engine must run a higher compression ratio than an equivalent iron head engine. As an example, it takes roughly a 15.5:1 compression ratio with an aluminum head Evolution engine to match a 14:1 compression ratio with an Ironhead Sportster. Another factor that is affected by cylinder head material is fuel octane. For a given compression ratio, an aluminum head engine requires less fuel octane to avoid detonation than an equivalent iron head engine.

Combustion chamber design, thermal barriers, and ignition timing all affect thermal efficiency. Although increased compression raises power over the entire rpm range, it is particularly effective for low and midrange power increases. Though many variables are involved, the maximum mechanical compression ratio is determined by the engine's detonation threshold. This threshold is primarily influenced by fuel octane, volumetric efficiency, combustion chamber efficiency, and cylinder head material. Engine compression is usually discussed in terms of mechanical, corrected, and dynamic ratios. We'll take a brief look at mechanical and corrected ratios.