Honda's Hot-Rod Valvetrain

HONDA'S HOT-ROD VALVETRAIN

CRUISERS HAVE GOBS OF bottom-end torque. Sportbikes are strong on top. We want engines that are powerful everywhere. Honda's new Hyper VTEC variable valve-timing system is a step in that direction.

The cruiser's torque fades as you rev it up because its valve timing is too short for best high-rpm cylinder filling, and its port sizes are small, so peak flow comes at low revs. The sportbike is weak on the bottom because its large-area ports work best at high rpm. At low revs, its longer intake timing al lows the piston to stop the slow-moving flow and pump some of it back out of the cylinder.

In 1983, Honda pro duced a home-market CB400F equipped with a duplex-rockerarm system -that allowed the engine to operate on only two of its four valves per cylinder at lower revs. By keeping intake velocity higher at low speeds, some back-pumping was prevented and torque in creased. All four valves operat ed at higher revs.

---~-.~ Later, for its cars, Honda de veloped a multiple cam-lobe system to work with the duplex rockerarms. Here, each pair of valves is actuated by three cam lobes. The two outer lobes, with timing and lift optimized for lower revs, operate the valves most of the time. The center

lobe has both higher lift and longer timing for higher-rpm op eration. As the engine revs up, a hydraulically operated pin links the center rocker to the outer pair, so the valves are driven by the center cam lobe. With sporty timing and lift, the engine continues to generate good torque at higher revs.

The interesting thing about Hyper VTEC is that it does away with rockerarms and their weight and potential flexure. In their place are inverted bucket tappets-but with a twist: Two of the four tappets for each cylin der's four valves can be turned on and off. In the "off" mode, the tappet moves with the cam lobe, but the valve is disconnect ed from it and remains closed. In

the "on" mode, an electronically controlled spool valve sends hy draulic pressure to drive a pin that locks tappet to valve. Presto! What was a two-valve engine now has four.

In low-speed mode, the two operating valves move with cruiser-like timing, boost ing low-speed torque. In high-speed mode, another intake and exhaust valve, jf operated with sport tim ings, are added. The result is an engine that main tains high torque over an unusually wide range, and can safely reach high revs. Because there is less charge loss from intake back-pumping and over lap short-circuiting, fuel economy is also some what improved.

Although details are sketchy, it's implied that one spring controls the tap pet, and another controls the valve. There's an in crease in cambox height, approximately equal to the valve lift. Otherwise, the

vaivc IIIL. `..Ju1~1 wLs~, LH~ tappet couldn't be driven down ward over a stationary valve in low-speed mode. Honda has tried similar systems on its Formula One engines-both the turbo 1.5liter V-Six and the atmospheric 3.5/3.0-liter V-lOs.

Hyper VTEC's first applica tion will be on the Japanese-mar ket CB400 Super Four. No word as to if or when it will appear on U.S. models. Next-generation CBR600, anyone?

Kevin Cameran