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

Honda/Acura VTEC
VTEC is Honda/Acura-speak for Variable Valve Timing and Lift Electronic control. (VVTALEC just isn't animal enough, is it?) First introduced aboard the groundbreaking NSX in 1991, VTEC is now available across the Honda/Acura product line, right down to the gas/electric Civic Hybrid. VTEC has evolved to suit those various engine applications, but its basics have remained the same.

The heart of the VTEC system is the design of the camshaft, with three lobes for each pair of intake and exhaust valves, plus the corresponding rocker arms that actuate the valves. At low- to midrange engine speeds, the valves are opened and closed by the rocker arms following the two outboard lobes. These outboard lobes have been ground to provide relatively low lift and short duration. Once a certain engine-speed threshold is crossed (varying from around 6,000 rpm in the NSX, RSX, and S2000 to 5,600 rpm in the Prelude), the VTEC computer sends a signal to a valve that uses engine oil to pressurize small pistons in the rocker arms. This locks the two outboard rocker arms to the center arm, which is aligned with a higher-lift, longer-duration cam lobe. The valves now open farther and stay open longer to feed the engine the increased air and fuel it needs for high-rpm, high-horsepower operation.

In the 10-plus years since VTEC's introduction, the corporation has developed some interesting variations on the theme. The current Civic Si and 160hp RSX engines are fitted with a simplified rocker system, with two arms that adjust the valve timing on the intake side of the engine only. Below 2,200 rpm, incoming air moves primarily through only one intake valve, giving the charge a strong swirl that improves low-speed combustion. Above 2,200 rpm, the second rocker arm engages the first, so that both intake valves open for the same lift and duration, which greatly increases the amount of air for the engine to burn. The 200hp motor in the RSX Type-S uses the traditional three-arm rocker setup on both the intake and exhaust valves.

The Civic Si and both RSX models also share the new "intelligent" VTEC system, called i-VTEC. The "i" is really a less cumbersome way for Honda to say it has added Variable Timing Control (VTC) to VTEC. VTC is similar to BMW's VANOS system, which adjusts the phasing of the intake camshaft to continuously match the engine's air and fuel needs. Here's how it works: The VTC actuator monitors a number of engine inputs--cam position, ignition timing, throttle position, and more--and then sends pressurized oil to chambers inside the cam's drive sprocket to advance or retard the cam's position relative to the crankshaft. So when you're sitting at a stoplight, for example, the cam is almost fully retarded to provide a smoother idle and reduce NOx emissions. Crack open the throttle, and the cam will advance to open the intake valve sooner and build in more valve overlap, which improves midrange responsiveness.Honda plans to have added this "i" technology to all of its four-cylinder engines by the time you read this., but does this mean Type-S cars for everyone? Hardly. While the VTC system does help the engine make power, that's a side benefit to the corporation's real target: efficiency. VTC reduces emissions and improves fuel economy to help Honda meet the ever-stricter engine regulations here and in Japan.

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