Camshaft Selection Guide for Four-Stroke Engines: A Comprehensive Playbook

Camshaft selection for four-stroke engines is a critical process that involves several measurable and quantifiable factors to determine the performance and suitability of a camshaft for a specific application. These factors include engine displacement, compression ratio, fuel system (carbureted or injected), RPM range for power desired, camshaft duration, lobe separation angle (LSA), lift, and rocker arm ratio.

Understanding Camshaft Duration

Camshaft duration is a crucial specification that determines how long the valves remain open during the engine cycle. It is usually expressed in degrees of crankshaft rotation and can be measured at two points: advertised duration and .050 inch duration.

  • Advertised Duration: The angle between the point where the valve starts to open and the point where it returns to its closed position, measured at a very small lift (typically between .003 inch and .006 inch off the seat).
  • 0.050 Inch Duration: The angle between the points where the valve lift reaches 0.050 inch on the opening and closing strokes. This measurement is more relevant for comparing camshafts, as it provides a better indication of how the camshaft will operate in the engine.

For example, a camshaft with an advertised duration of 260 degrees and a 0.050 inch duration of 220 degrees would have a longer duration and potentially higher lift compared to a camshaft with an advertised duration of 240 degrees and a 0.050 inch duration of 200 degrees.

Camshaft Lift and Rocker Arm Ratio

camshaft selection guide for four stroke engines

Lift is another essential specification that determines the maximum height the valves can be opened. It is calculated by multiplying the lobe height at its highest point from the base circle by the rocker arm ratio. The rocker arm ratio increases the effective lift of the camshaft lobes, allowing for greater valve opening and potentially higher engine performance.

For instance, a camshaft with a lobe lift of 0.500 inches and a rocker arm ratio of 1.6:1 would result in an effective valve lift of 0.800 inches (0.500 inches x 1.6 = 0.800 inches).

Lobe Separation Angle (LSA)

The lobe separation angle (LSA) is the angle between the centerlines of the intake and exhaust lobes on the camshaft. It significantly impacts the engine’s performance by influencing the overlap period, during which both the intake and exhaust valves are open simultaneously.

  • Larger LSA: Improves low-end torque and idle quality.
  • Smaller LSA: Enhances high-end power and throttle response.
  • Excessive LSA: Can lead to reversion, where the exhaust gases flow back into the combustion chamber, reducing engine efficiency and power output.

For example, a camshaft with an LSA of 110 degrees would have a longer overlap period and better low-end torque compared to a camshaft with an LSA of 104 degrees, which would have a shorter overlap period and better high-end power.

Factors to Consider in Camshaft Selection

Camshaft selection also depends on the engine’s intended use and operating conditions. Factors such as transmission type, stall speed, vehicle weight, final gear ratio, tire height, cylinder head and valves, head flow, manifolds, rocker arm ratios, aspiration, and emissions test passage must be considered when selecting a camshaft.

For a street performance engine, you might choose a camshaft with a larger LSA (110-114 degrees) to improve low-end torque and idle quality, while for a high-revving race engine, you might opt for a camshaft with a smaller LSA (104-108 degrees) to enhance high-end power and throttle response.

Camshaft Selection Checklist

When selecting a camshaft for your four-stroke engine, consider the following factors:

  1. Engine Displacement: Determine the engine’s displacement in cubic inches or liters.
  2. Compression Ratio: Measure the engine’s compression ratio.
  3. Fuel System: Identify whether the engine is carbureted or fuel-injected.
  4. RPM Range for Power: Determine the desired RPM range for maximum power output.
  5. Camshaft Duration: Evaluate the advertised duration and 0.050 inch duration.
  6. Lobe Separation Angle (LSA): Choose an LSA that suits your engine’s intended use and performance goals.
  7. Lift: Calculate the effective valve lift based on the camshaft lobe height and rocker arm ratio.
  8. Rocker Arm Ratio: Determine the rocker arm ratio to be used with the camshaft.
  9. Transmission Type: Consider the transmission type (manual, automatic, or CVT) and its impact on engine load and RPM range.
  10. Stall Speed: Evaluate the stall speed of the torque converter (if applicable) to ensure compatibility with the camshaft selection.
  11. Vehicle Weight: Factor in the vehicle’s weight, which can affect the engine’s power requirements.
  12. Final Gear Ratio: Determine the final gear ratio of the drivetrain to optimize power delivery.
  13. Tire Height: Consider the tire height, as it can impact the engine’s RPM range and power delivery.
  14. Cylinder Head and Valves: Assess the cylinder head and valve specifications to ensure compatibility with the camshaft.
  15. Head Flow: Evaluate the cylinder head’s flow characteristics to ensure the camshaft selection is optimized for the engine’s breathing capabilities.
  16. Manifolds: Consider the intake and exhaust manifold designs, as they can influence the engine’s power curve and response.
  17. Aspiration: Determine the engine’s aspiration method (naturally aspirated, forced induction, etc.) and its impact on camshaft selection.
  18. Emissions Test Passage: Ensure the camshaft selection meets any applicable emissions regulations for your specific application.

By considering these factors, you can select the most suitable camshaft for your four-stroke engine, optimizing its performance and efficiency.

Reference:

  1. Camshaft Selection Guide
  2. Camshaft Selection Factors
  3. How to Choose a Camshaft