Variable Valve Timing: What Is It? Who Needs It?

Nissan CVTC
Nissan's VQ V-6 engines, which began life in the Maxima and are now found in the new Altima, the Infiniti G35, and the 350Z, feature a valvetrain with Continuous Variable Timing Control (CVTC). Similar to the VTC and VVT-i setups already discussed, CVTC is a cam-phasing system. A vane, housed within the cam-drive sprocket, rotates to advance or retard the intake camshaft's timing to maximize the engine's efficiency and power output based on engine and driving-condition information it gets from the engine's control monitor. Advancing the cam timing boosts low- to midrange torque, while retarding the timing improves high-rpm performance.

Unlike the other technologies, though, CVTC doesn't have a corresponding system to alter the valves' lift or duration. So like the hot rod cam that is advanced or retarded during an engine buildup, CVTC doesn't change the character of the valve-opening event, just its timing. But unlike old-school hot rod cams, CVTC can move both ways, at will, to affect power throughout the rev range.

Mitsubishi MIVEC
The Mitsubishi Innovative Valve Timing and Lift Electronic Control (MIVEC) system went into production in 1993 and was used on several of Mitsu's home-market vehicles from the early to mid-'90s. Applications ranged from 1.6L four-bangers in the Mirage Cyborg, Mirage Asti, and Lancer to a 2.0L V-6 for the FTO (an Eclipse-like coupe also known by its full name, Fresh Touring Origination) and even a 3.0L V-6 version for the high-line Diamante. MIVEC is old tech now, though, as Mitsu has phased it out in favor of other engine-efficiency models, such as direct gasoline injection.

So why are we talking about mid-'90s Japanese technology? These were pretty respectable machines in their day. The 1.6L MIVEC motor peaked at nearly 175 hp, which at the time was the "highest unit power output for a non-supercharged production engine anywhere in the world." The other two motors weren't bad, either--the 2.0L made just shy of 200 hp, and the 3.0L churned out a tick over 265 horses. In box-stock form. No wonder there are rabid MIVEC fans all over the Internet, telling tales of MIVEC engine swaps and 7-second 0- to 100-mph sprints. As with VTEC, the MIVEC system used camshafts with low- and high-speed lobes, dual rocker arms, and a hydraulically controlled lever that would engage either rocker depending on engine speed. The threshold for MIVEC was 5,000 rpm. Below that the low-speed lobes actuated the low-speed rockers and the car felt like any other. At five grand, though, the lever engaged the high-speed rockers, allowing the high-speed lobes to use their higher lift and longer duration, and the motor would scream right up to its 8,000-rpm redline.

Yet peak power was only part of the MIVEC story. For those who were seeking optimal fuel economy (an important consideration in Japan), a second version of MIVEC came with a modulated-displacement (MD) mode. At low engine loads (in urban driving conditions, for example), the lever connecting the high- and low-speed rockers in cylinders 1 and 4 would disengage, closing the valves and turning the four-banger into a two-banger. Not the most exciting thing for an engine to do, but testing by Mitsubishi showed that at a steady 60 km/h, the 1.6L MD-mode MIVEC engine delivered 26 km/L (or 61 mpg at around 37 mph). Keep in mind that the MD mode didn't negate any of the top-end power available to the MIVEC motor from its high-lift cam lobes. So this one engine could go from gas sipper to pocket rocket with a simple stab of the gas pedal.

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