The Discontinued AVTEC Technology: Honda’s Continuously Variable Valve Lift System

📝 Original by Honda誌 | 📍 JDM Car Enthusiast誌 | 📅 2025-03-18
🔖 Topic: #TechnologicalInnovationsInJapaneseCars

The VTEC currently used in the market originated from the B-series engine in 1989.

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Strictly speaking, the full name of the first-generation VTEC technology should be DOHC VTEC. Over the years, Honda engineers have continuously evolved various VTEC technologies for different applications and positioning. For example, SOHC VTEC system, which is the Single Overhead Camshaft VTEC; VTEC-E system, which focuses on fuel economy and environmental protection 🌿; three-stage VTEC which balances all aspects of performance; the i-VTEC introduced in 2001 integrates VTC (Variable Timing Control) syst...

Later came VTEC Turbo technology represented by K20C. However, in the midst of this historical development, a system called AVTEC appeared. It was claimed to be powerful but never implemented in actual vehicles, making it seem out of place. 🚗

In the AVTEC system, “A” stands for “Advance”, which roughly means a continuously variable VTEC system ⚙️. In the past, in order to change the valve lift at different RPMs, Honda engineers placed three cam lobes on the camshaft for each cylinder 🔧. The three cam lobes had different valve lift depths and timing durations, and different cam overlap angles. These lobes controlled low and high RPM valve lifts. A rocker arm mechanism was added between the camshaft and the valves. By controlling oil pressure t...

Honda has not significantly changed this working method in subsequent VTEC evolutions. The first evolution, SOHC VTEC, used three cam lobes on a single camshaft to control either intake and exhaust or just the intake rocker mechanism. The second evolution, VTEC-E, was a downgraded improvement 🌿 to enable each cylinder’s two intake valves to operate as either one half-open and one full-open or both full-open, purely for fuel economy purposes. This system was first applied to the F23A series engine.

During the K20A era on the Integra Type-R (DC5), the VTEC system underwent deep enhancement. Honda added a VTC system capable of changing camshaft timing on the K-series engine ⚙️, thus achieving variable valve timing functionality 🔧. However, the so-called black-top i-VTEC and red-top versions had completely different valvetrain layouts. The former had VTEC only on the intake camshaft, while the red-top version had variable lift on both intake and exhaust valves.

After reviewing the development of the VTEC system, we now briefly introduce the concept of AVTEC. As mentioned earlier, the past VTEC systems worked via rocker arms between camshaft and valves, using hydraulic pins to switch high or low cam lobe engagement. AVTEC, however, was completely different. Its complex rocker arm structure was significantly simplified. In the new system, the rocker arms had relatively single functions, and there were no pins inside them.

As shown in the figure, Honda engineers abandoned the complex rocker arm mechanism and instead created a camshaft sleeve-like structure around the camshaft ⚙️. This mechanism could rotate independently from the camshaft. It used a slider with varying thickness to achieve engagement with cam lobes at different RPMs, transmitting the thickness variation directly to the rocker arm, thus changing valve lift.

The figure below shows valve operation at low and high RPMs. At low RPM, the thinnest part of the slider contacts the rocker arm. When the camshaft lifts the slider, the lift effect is reduced, resulting in smaller valve lift. As engine speed increases and more air intake is needed 🌬️, the outer sleeve of the camshaft drives the slider to rotate (clockwise in the figure). During the rotation, the slider remains in contact with the rocker arm, and due to the irregular thickness of the slider, the valve li...

Using many layman terms, we roughly described the mechanical layout of AVTEC ⚙️. Of course, the official introduction was much more detailed and complex. The new AVTEC was released in 2006. In the official data, no exact engine power output was provided, but it did show environmental benefits 🌿—for example, fuel consumption was reduced by 13% compared to i-VTEC on the same K24A engine, and emissions were 25% lower than Japan’s 2005 emission standards.

Interestingly, Honda’s promotional image featured a red-letter engine cover, reminiscent of the red-letter K24A on Accord 24S, which originally had 200 horsepower 💥. If AVTEC were applied, would the output increase further? Would the continuously variable VTEC system make torque output more linear?

However, to this day, such an advanced AVTEC system still has not been implemented in production cars 🕰️. It is assumed that practical issues forced it to be scrapped. But whether as a technology reserve or practical function, we still hope it will reappear one day ✨.