Though it's been called many things, the VTEC acronym stands for Variable Valve Timing and Lift Electronic Control. VVTLEC doesn't quite roll off the lips, but the adjectives one tends to spew out after experiencing it for the first time do. They run the gamut but almost always include the "F" word. Ol' Soichiro-san would've been pleased. April 2009 marks the 20th anniversary of VTEC in the automotive world. The 1989.5 Integra XSi was the first production Honda that featured the technology. Shortly after, Civics and CRXs received the same B16A engine.
But all of this began much earlier-sometime during the early 1980s, in fact-and none of it had anything to do with automobile engines. The technology behind VTEC originated from Honda's motorcycle side. Honda's engineers knew that four-valve engines made great top-end power, but that two-valve ones were competent down low and even idled nice. The quest soon became one in which a 500cc engine would spin to 11,000 rpm yet idle patiently at a mere 1,000 rpm. The result was what Honda internally labeled as its "REV" mechanism, or "HYPER VTEC" to the rest of us. The technology allowed for only one intake and one exhaust valve per cylinder to operate below certain engine speeds but for two intake and two exhaust valves per cylinder to function above that threshold. It allowed for the best of both worlds.
How i-VTEC Works
The "i" in i-VTEC stands for "intelligent." The system features variable camshaft timing but the "VTEC" part doesn't exactly stand for VTEC as we know it. But let's start with the variable valve timing part. Nearly everyone's seen aftermarket adjustable cam gears. Honda's i-VTEC system dynamically adjusts camshaft timing using an oil-driven system that positions the camshaft within a 50-degree timing range depending on the circumstances. It's a heck of a lot easier than loosening and tightening a bunch of cam gear bolts just to pick up a few horsepower. The technology is only featured on the intake camshaft, and is almost exclusive to the K-series. Alone, Honda refers to it as VTC (Variable Timing Control). The obvious benefit is improved emissions. At startup and while idling, the camshaft can be retarded, reducing valve overlap for cleaner combustion. It also works well for mid-range power. Advancing the intake cam in the low- and mid-range areas provides a noticeable bump in torque. To date, Honda has not used this technology on the exhaust side.
Honda's also been known to match i-VTEC on engines for use as a valve-idling mechanism, not a timing and lift one. This system operates a single intake valve per cylinder until the VTEC mechanism locks the two intake rocker arms together, causing both intake valves to open. It's not the three-lobe mechanism we're accustomed to by any means, but it's still referred to as VTEC. K-series engines like the Accord's are different. Here, i-VTEC allows for variable intake camshaft timing as well as the valve-idling mechanism. Most SOHC engines like the L-, R-, and J-Series engines simply take advantage of the valve idling, where as few as 12 out of the 16 valves can operate at a time. The "intelligent" camshaft timing changes won't work with SOHC engines like these since both the intake and the exhaust share the same camshaft gear.
What A-VTEC Is
The future of VTEC, A-VTEC, is the pinnacle of camshaft timing. It'll allow for continuously variable camshaft timing and lift. Much like a pneumatic or an electronic valvetrain promises, the valves would be able to open at any time, for any amount of time, and at any lift. Of course, this is all pending on whether or not the technology goes into production, as it hasn't made its way onto any Honda yet. At this point, it's little more than a patent application.
As you might have guessed, the "A" in A-VTEC stands for "advanced." In A-VTEC systems, the camshaft remains enclosed within a sleeve with openings between 270 and 360 degrees of camshaft rotation. Also occupying that space is a semi-rocker arm that features a roller, spanning the same 270-360 degrees, which rides along the camshaft lobe. The system also features a "shoe" that's positioned on top of the roller for the conventional rocker arm (the one that actually activates the valves) to ride on. The previously mentioned sleeve also traps the semi-rocker arm. The semi-rocker arm features a curved profile-it's thick on one end, thin on the other. The sleeve rides along a gear positioned on its bottom side that controls the orientation of the semi-rocker arm. From here, it's relatively easy to see how the variable lift part happens. As the gear that's attached to the camshaft sleeve rotates, the thicker portion of the semi-rocker arm transitions over the camshaft lobe creating maximum lift. As it rotates the other way and the rocker arm's thinner portion transitions over the camshaft lobe, lift decreases. It's all kind of like BMW's Valvetronic system but far less complex and with less reciprocating mass. By adding i-VTEC's VTC system, camshaft timing becomes variable. With both systems in place, there's potential for both economy and efficiency to make significant leaps.