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| Car Bibles |
The lobe separation angle (LSA) of a camshaft is defined in the Figure below. LSA is an important performance characteristic because it controls the interaction between the intake and exhaust valves.
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| mooregoodink |
Here the LSA are 106 and 110 degrees, respectively. In a narrow LSA situation (i.e. valve overlap increases), the intake valves are opened earlier that draw more air into the cylinders. While the burnt-air is moving through the exhaust valves, the pressure gradient drags more fresh air via the intake pipe. In addition, the Exhaust Valve Opening (EVO) is delayed before reaching Bottom Dead Centre (BDC). These three effects build up cylinder pressure and produce peak torque at lower rpm range but the engine output falls off quicker too. It favors to give a torque graph in a triangular shape.
However, the cylinders are required to clear the burnt-air as quickly as possible at high rpm range. Opening the exhaust valves earlier is one of the promising ways to remove the exhaust gases effectively. But then a widening of LSA is expected. Despite the maximum torque decrease, this design allows the engine to run faster at high rpm range.
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| Measurement data |
If the intake and exhaust lobe centerlines are shifted under the same LSA, it also affects the dyno performance. Increasing the intake lobe centerline from 106 to 114 degrees is considered as retardation. Retarding the camshaft leads the intake valve to open and close later which weakens cylinder pressure and reduces the low-speed performance of the engine. Retarding the camshaft weakens the engine output at the high-end region because the Exhaust Valve Opening (EVO) is delayed.
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| Measurement data :int CL: intake centerline, ext CL: exhaust centerline |