Many folks feel the camshaft should be the first component selected when planning an engine build, however, the engine’s airflow potential is crucial for guiding camshaft choice. Selecting the right cylinder head and supporting components is vital for producing power in the desired rpm range.
Understanding Cam Qualities & Valve Overlap
Some enthusiasts focus on achieving that iconic choppy idle when selecting a camshaft—yet they often overlook the impact of an aggressive cam’s increased valve overlap (the period when both valves are open simultaneously). Excessive valve overlap can lead to exhaust gases flowing back into the intake, a phenomenon known as reversion. At low rpm, reversion slows the air velocity in the intake runners, leading to sluggish throttle response.
While more overlap can enhance cylinder pressure and power at higher rpm, it reduces manifold vacuum, making for poor power brake performance.
An engine with excessive overlap and insufficient static compression is considered over-cammed,
resulting in that desirable lopey idle but lacking performance, especially at low rpm.
An aggressive camshaft also requires proper differential gearing and a high-stall torque converter to enable the engine to reach its optimal rpm range more quickly. The camshaft lobes control intake and
exhaust valve timing, and their characteristics are determined by three key factors:
- Lift
- Duration
- Lobe Separation Angle (LSA)
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3 Key Camshaft Terms You Should Know
Lift
Lift is measured in inches and determines how far the valve is raised from its seat. There are two common lift measurements: lobe and gross.
- Lobe lift is measured at the nose of the lobe, minus the base circle.
- Gross lift can be calculated by multiplying the lobe lift by the rocker arm ratio. This calculation represents the actual lift at the valve.
Lifting a valve beyond the cylinder head’s capability is counter productive.
Duration
Duration describes how long the valve is open and is measured in degrees of crankshaft rotation. The measurement comes in two forms: advertised and at 0.050″ lift.
- The advertised duration will always be larger since it starts lower on the lobe, closer to the base circle.
- When comparing camshafts, using duration at 0.050″ lift is a more accurate comparison because there is no standardization for measuring the advertised duration.
Short-duration cams boost low-speed torque and throttle response, while long duration cams enhance high-rpm power.
Lobe Separation Angle
Lobe Separation Angle (LSA) measures the angle, in degrees, between the peak lift of a camshaft’s intake and exhaust lobes.
While lift and duration provide details about individual lobes, LSA illustrates how those lobes relate to each other. A wider LSA is ideal for broad torque curves in trucks or street cars, while a narrower LSA is better for higher rpm and a tighter torque band in performance and race engines.
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