VTEC

(Some links are given at bottom of this page)

Honda's VTEC system depends for its operation on cleverly designed valve rockers. In an Integra GSR engine there are 24 of these, six for each cylinder, three on the intake side and three on the exhaust side. One such set of three is shown in an exploded view below. The blue colored rocker is the Primary Rocker Arm, the red one is the Secondary Rocker Arm, and the one in the middle is the mid Rocker Arm. Riding in holes at the ends of the rockers are two pins that either lock all three arms together or allow them to move independently, depending on the position of the pins. These pins can be moved back and forth in their holes by oil pressure coming through passages from the rocker shaft. This oil pressure is controlled by a solenoid that is operated by the car's ECU.

VTEC parts, exploded

Below is a picture of the assembled set of three rockers mounted on the rocker shaft. Unlike conventional rockers which have a pivot point in the middle of the rocker, these have their pivot point at one end. On the far end of the Primary and Secondary Rockers there are extensions with ajusting screws and locknuts. These rockers each operate a valve.The camshaft in this system is positioned on top of the rockers, and there are three lobes on the cam for each set of three rockers. Each cam lobe has a different contour and lift. The lobe of the secondary cam on a GSR is 1.27mm higher than the primary on the exhaust side for instance, and the mid cam is 2.609mm higher. The primary and secondary cams have a "mild" contour for smooth low speed operation. One operates its rocker and opens its valve slightly before the other on the intake side to impart a swirling action to the incoming fuel/air mixture, for combustion efficiency.

VTEC parts, assembled

When operating at low speeds all three rockers move independently, each under the impetus of its own cam. The Mid Rocker has no effect on the valves at all, for the latter are being opened and closed by the "mild" low speed cam lobes. When the engine revs get high enough however, a solenoid located at the rear of the head routes oil under pressure to the rocker shafts and through passages in the rocker arms to a pair of pins in each rocker triplet. The three rockers are locked together and now the Primary and Secondary rockers no longer come in contact with their cam lobes, but are lifted sooner, faster and higher by the agressively ground "hot" cam operating the Mid rocker.

The end result is not an engine with a torque curve that peaks at high RPM and "has to be revved to get any power", but rather an engine with a torque curve that is virtually flat from 3000 RPM all the way up to 8000 RPM. It is docile and managable in traffic, idling at a mere 750 RPM unlike most nervous four-bangers, but it will stay in a lower gear and give you torque longer than bigger engines that have to shift to a higher gear because they ran out of breath at 6000 or so.

More here:
http://www.leecao.com/honda/vtec/index.html (the best reference I've found so far)
http://asia.vtec.net/spfeature/vtecimpl/vtec1.html
http://www.teamdelsol.com/technical/techstuff/vtec/
http://vtec.net/articles/view-article?article_id=3261
http://honda_fans.tripod.com/vtec-info.html (has a cool animation)