If you plot the engine’s torque vs. rpm curve and the clutch’s torque vs. rpm curve on the same graph, the two lines will cross. At that point the clutch and engine are in equilibrium. The equilibrium point defines the engagement rpm. At lower rpm the clutch slips because the engine produces more torque than the clutch can accommodate. At higher rpm the clutch is locked up because it can handle much more torque than the engine can produce. If you modify the engine by changing cam, jets, bore, stroke, valve position, fuel type et cetera, the shape of the engine’s torque vs. rpm curve will change. If you modify the clutch by changing spring position, the shape of the clutch’s torque vs. rpm curve will change. When either curve changes shape, the equilibrium point moves. The table is merely an estimate of where that point occurs for various combinations of engines and springs. See Spring Chart for Estimated Engagement RPM.

Spring Chart – Estimated Engagement RPM