Optimizing Your Harley's Engine Efficiency

Nooks and crannies in the combustion chamber, which are located farthest from the point of ignition and generally near the cylinder bore edges, are potential detonation traps. Portions of the intake charge are trapped in these areas and ignite spontaneously due to heat saturation before being ignited by the normal flame-front reaction, thereby resulting in detonation. Eliminate detonation traps as much as possible.

Nooks and crannies in the combustion chamber, which are located farthest from the point of

Mechanical CompressionAn engine's mechanical compression ratio can be defined as the ratio of the volume above the piston at bottom dead center (BDC) compared to the volume at top dead center (TDC). The Harley V-Twin engine responds exceptionally well to an increase in mechanical compression. In fact, many engines are poor performers because they lack sufficient mechanical compression.

The first thing to consider when determining the mechanical compression ratio is whether the engine will use 92-octane pump gas or high-octane race gas. If the engine will be run on pump gas, it will be limited to somewhere between 9.0:1 and 10.5:1 mechanical compression, depending on variables such as cam timing, combustion chamber design, ambient temperature, bike weight, and gearing. If race gas is the fuel of choice, the effective maximum compression ratio will be limited to roughly 17:1, again depending on several variables. Keep in mind, however, that once the compression ratio goes beyond about 16:1, thermal efficiency starts to drop and parts breakage potentially becomes a major problem.

A compact combustion chamber, relatively flat or slightly dished piston top, and centrally located spark plug will provide for short, unobstructed flame travel and rapid combustion. Additionally, a large tight squish band is important for proper turbulence. High turbulence will more thoroughly mix the air/fuel mixture into a more homogenous mixture that will burn more quickly and efficiently, thus improving power. A compact chamber will also require less ignition advance, run on lower octane fuel, and be less prone to detonation and pre-ignition.

A compact combustion chamber, relatively flat or slightly dished piston top, and centrally

Ambient temperature plays a big part in where detonation sets the engine's power limit. One reason a drag racing engine can run very high compression is that it only operates for a short period and doesn't get very hot. On the other hand, a street engine heats up significantly on a hot summer day, especially while idling for long periods in traffic in the late afternoon. Where a street engine may not encounter detonation during the cool spring and fall months, detonation may be rampant during summer months, especially when riding double and launching from a stop. If you are building a street engine, keep this point in mind when determining the engine's mechanical compression ratio.

General guidelines for conservative mechanical compression ratios on 92-octane pump gas with the V-Twin engine are as follows: With Twin Cam and Evo Big Twin engines, you can run between 9.5:1 and 10.5:1 mechanical compression without encountering detonation. The lighter weight Evo XL can handle between 10:1 and 11:1 mechanical compression. For the Ironhead XL and Shovelhead, expect 9:1 to 10:1 as the maximum mechanical compression ratio on pump gas. Fuel-injected models may work well with up to a half point higher mechanical compression than the values listed above. Several other variables, such as combustion chamber design and cam timing, will also influence the actual compression limit. Since cam timing affects cylinder pressure when the engine is running, cam events and compression must be closely coordinated to maximize power for a given parts combination. This is where corrected compression ratio comes into play.