Overlap: How much is needed?Overlap can have both positive and negative effects on engine performance. The ideal amount of overlap is related to engine displacement, rpm, and intake/exhaust flow characteristics during the overlap period. Exhaust system tuning also plays an important part because it has a major influence on airflow during overlap. Generally, the greater the engine displacement, the more overlap required for a given rpm band. Additionally, the higher the engine's intended rpm band, the more overlap required for optimized performance. Low-lift valve flow is another factor influencing overlap because the valves are only slightly off their seats during the overlap period. More overlap is usually beneficial when the cylinder head has poor low-lift flow, the combustion chamber has poor cross-flow during the overlap period or the valve opening acceleration rate is slow. An engine with a small carb or large lazy-flowing intake ports generally responds well to more overlap. These conditions generally work best with a tighter LSA.
The position of the intake lobe in relation to the exhaust lobe determines the valve overl
Long overlap typically is what gives an engine the cool-sounding lopey idle that many riders desire. If matched properly to the engine combination, overlap can improve cylinder scavenging and power. Yet excessive overlap can over-scavenge the cylinder, reducing power and fuel economy. At low engine speeds, a large amount of overlap can allow reversion of exhaust gases into the combustion chamber and intake manifold, causing rough idling, poor throttle response, and increased exhaust emissions. Increased overlap also reduces vacuum in the intake tract at low-engine speeds, which can result in poor fuel atomization and throttle response. The benefits of overlap are maximized when combined with a tuned exhaust system. A long overlap period and poorly tuned exhaust system can produce significant dips throughout an engine's power curve.
Engines such as the Shovelhead, which have a high-dome piston and obstructed combustion chamber, typically have poor combustion chamber cross-flow and require a cam with a greater amount of overlap for optimized performance. If you were to compare the amount of overlap for Shovelhead, Evolution, and Twin Cam engines, you would notice that Shovelhead cams generally have the most overlap, while cams for the Twin Cam engine have the least. The reason for this is that differences in combustion chamber design along with intake and exhaust flow characteristics have a significant effect on the optimum amount of overlap. Greater overlap usually improves performance when the intake charge cross-flow is obstructed, or the intake and exhaust ports are extremely large for the engine combination and inlet flow is lazy and slow moving. Less overlap is needed when port velocity is high or the exhaust-to-intake flow ratio at low lift is high. Additionally, for maximum fuel economy, reduced emissions and best low-rpm torque, keep overlap to a minimum.
Another consideration is that the benefits of overlap are very rpm-specific. Consequently, it is easier to design a cam for a race engine that operates over a narrow 2,000-rpm band than it is for a street engine that operates throughout a 5,000-rpm or larger band. A long overlap is typically associated with long duration and is best suited for high-rpm power and tuned exhaust systems. In the case of a street engine, a long overlap accompanied by long duration and a late closing intake valve will kill low-speed performance. In general, when selecting a cam it is best to keep overlap to the minimum that will achieve your power goals, and avoid cams with excessive amounts of overlap.