How to Degree a Cam: Fixed vs Continuously Variable Cam Timing
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Fixed vs Continuously Variable Cam Timing
03.05
| 00:00 | - Traditionally, engines have used what's referred to as fixed cam timing where the cam timing is determined by the engine manufacturer and is then set by way of the location of the cam and crank pulleys, along with a belt or chain that drives them. |
| 00:15 | In this sort of engine design, the cam timing is a compromise between low RPM and high RPM performance. |
| 00:22 | With the modern crop of performance engines however, often these engines will use what's referred to as continuously variable cam timing. |
| 00:30 | Manufacturers have a variety of terminology for this technology including VVT, VANOS, IVTEC and many others. |
| 00:40 | However the principle is that the cam timing can be advanced or retarded as the engine is running. |
| 00:46 | This is achieved by the ECU and the cam position can be mapped against engine RPM and engine load to provide optimal engine performance throughout the entire engine operating envelope. |
| 00:58 | A continuously variable cam timing system makes use of the high pressure engine oil to advance or retard the cam. |
| 01:06 | This is achieved with a special cam gear that uses internal lobes in chambers. |
| 01:11 | By supplying oil to one side of the lobes, the cam will be advanced and by supplying it to the opposite side the cam will be retarded. |
| 01:21 | The oil supply is controlled by a solenoid that the ECU pulses at very high speed, and this allows the cam timing to be moved incredibly quickly. |
| 01:31 | The difference between an engine that uses variable cam control and one with fixed cam control is that the variable cam control system provides a significantly wider torque curve and in particular we normally see a large improvement in low RPM torque without sacrificing high RPM performance. |
| 01:50 | The flipside of this however is that a variable cam control system usually limits the amount of duration that can be used, while still being able to achieve a reasonable amount of cam movement. |
| 02:02 | This limits the system's usefulness in an all out race engine since as we move to more aggressive cam profiles with more lift and duration, there's less ability to move the cam before we run into contact between the valves and the piston. |
| 02:16 | In a race application however a variable cam control system is less critical, since the engine is likely to operate over a much narrower rev range and hence the wide torque curve is less of an advantage. |
| 02:30 | For this reason, we often see variable cam timing systems removed and replaced with vernier adjustable cam pulleys in race applications. |
| 02:39 | Since a variable cam control system adds cost and complexity to an engine's design, in some factory engines, we see it only being applied to the intake cam, where we will see a greater effect on the engine's power delivery as the cam timing is altered. |
| 02:56 | In these engines it's common to add a vernier adjustable cam gear to the exhaust cam so that the exhaust cam timing can also be accurately controlled. |
