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EFI Tuning Fundamentals: Continuously Variable Valve Timing

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Continuously Variable Valve Timing


To learn more about Variable Cam Control Tuning Click Here

00:00 - The point in the engine cycle when the intake and exhaust valves open and close can have a dramatic effect on the shape of the engine's torque curve.
00:07 As a guide, normally we want to advance the cam timing so that the valves open and close earlier in the engine cycle at low RPM but in order to take advantage of the inertia of the high air velocity at high RPM, we may want to retard the cam timing, meaning that the intake and exhaust valves will open and close later in the engine cycle.
00:27 In a conventional engine the cam timing is fixed so we must make a compromise between cam timing that improves low RPM power and torque and one that's best suited to high RPM power and torque.
00:39 With modern EFI though, continuously variable cam timing, where the ECU can mechanically advance or retard the cam timing as the engine is running has become commonplace.
00:49 This allows the ideal cam timing to be selected based on the current engine RPM and load, providing a much wider torque curve.
00:57 The dyno graph you can see here shows what you could expect from a properly tuned variable valve timing system, the two lines show the engine power with the cam control disabled when the intake cam is run in its most retarded position.
01:11 The overlay shows the result of optimising the cam timing throughout the rev range and you can see the gain in power shown, particularly in the low and mid RPM range.
01:21 Depending on the engine, this sort of system may be referred to as VVT, VCT, VANOS, Vario Cam or others.
01:30 I should point out here that it's important to not confuse this sort of continuously variable cam control with the much simpler switched cam timing systems where the cam can only run at maximum advance or maximum retard but nowhere in between.
01:44 Cam control systems use a special cam wheel that includes internal chambers as shown.
01:51 By directing high pressure engine oil to one side of these chambers, the camshaft can be retarded in relation to the cam wheel.
01:57 Supplying oil to the opposite chamber will advance the camshaft.
02:01 The ECU provides a pulse width modulated output to a solenoid which controls the oil flow to these chambers.
02:09 At the same time a camshaft position sensor will provide the ECU with feedback about the current camshaft position.
02:15 Tuning a cam controlled engine does add another parameter that the tuner has control over and must optimise.
02:22 Some engines are fitted with variable cam control on only the intake cam, while others are fitted with variable cam control on both the intake and the exhaust camshafts which of course adds further complexity to the tuning process since we now can control the cam timing on both cams.
02:38 This can be a little daunting as at any point in the load and RPM table, the ignition timing, fuel and cam positions need to be optimised.
02:47 To make matters worse, every time the cam position is moved, the fuel and ignition may need to be adjusted to match the changing volumetric efficiency of the engine.
02:56 To summarise this module, with continuously variable cam timing, the ECU can mechanically advance or retard the cam timing as the engine is running.
03:06 This allows the best cam timing to be chosen, based on the current engine RPM and load, allowing for an improved torque curve throughout the engine RPM range.
03:15 These systems add another layer of complexity to the tuning process but once mastered, continuously variable cam timing can be harnessed to greatly improve the drivability and performance of your vehicle.
03:27 If you want to learn more about variable cam control tuning, then our Variable Cam Control Tuning course dives deep into this subject in much more detail.