Honda VTEC: A Cam For All Reasons

The classic problem facing high-performance street cars has always been how to maximize performance without sacrificing driveability. Short of power-adders like turbos or nitrous oxide, significant power increases are only achieved by extending the engine's rpm operating range, usually with some combination of intake, exhaust, camshaft, and cylinder head changes. But any significant gains upstairs inevitably cause a loss of low-speed performance, decreased fuel efficiency, and increased emissions.


The '00 S2000's 2.0L, 11.0:1, DOHC VTEC four-banger (above) developed 240 net horsepower at 8,300 rpm, the highest output per liter of any normally aspirated production automobile engine ... yet the engine was one of the first to fully meet California's LEV (Low Emission Vehicle) smog standards. The two-seat pocket rocket had a 9,000-rpm rev limit, accelerating 0-60 in under 6 seconds.


The conundrum of camshaft selection is one of the key determinants in altering the powerband. Racing engines have longer intake/exhaust timing, a higher valve lift, and a narrower lobe displacement angle (LDA) than standard engines, which for low-speed response and power tend to have short durations, low valve-lift numbers, and a wide LDA. Beginning with the '89 Honda Integra in Japan and the '90 Acura NSX supercar in the U.S., Honda implemented a novel solution to the problem, effectively combining the attributes of both passenger-car and race-car cam timing with its unique VTEC system. The initials stand for Variable Valve Timing and Lift Electronic Control, and as the name implies, a VTEC engine can actually alter the cam timing while the engine is running.


Honda's VTEC system was more sophisticated than earlier variable-valve-timing systems developed by other manufacturers, which could only change the time both valves are open during the intake/exhaust overlap period on the transition between the exhaust and induction strokes. By contrast, the VTEC setup can alter both camshaft duration and valve lift. In its classic form as used on Honda's DOHC motors, each cam has two different sets of lobes: one used under low-speed conditions, and one that is only activated at high rpm (usually at points over 4,900 rpm). The high-speed lobe is almost a race-quality cam, with (on the popular Acura Integra B18C5 engine) 290 degrees advertised duration (about 242 degrees at 1mm [0.040-inch] lift).

VTEC technology has spread
throughout Honda's lineup--even
to its workhorse sedan
V-6 engines.


As first implemented on the high-performance DOHC motors, there are three cam lobes and three rocker arms for each pair of intake and exhaust valves. At low to midrange engine speeds, the rocker arms aligned with the two outboard low-speed cam lobes directly open and close the valves, but at high rpm, the outer rockers are joined to, and directed by, the middle, high-rpm lobe and rocker arm.

VTEC proved so successful that as it evolved, it eventually found its way, in one form or another, into every Honda model, even the gas/electric Civic hybrid vehicle. The VTEC system used on standard SOHC (single-overhead cam) engines is simpler--the three cam lobes/two-valve system is only for the intake valves; the exhaust valves operate like those on a non-VTEC engine. The VTEC-E was developed for high fuel-mileage VX and HX Civics. Like the SOHC VTEC, it uses two lobe profiles to actuate the intake valves, but to maximize efficiency and achieve a very lean burn, both lobes are extremely mild and are basically intended to induce a high mixture swirl. There are also D-series VTEC motors used in some Civics that broaden the powerband by simply advancing or retarding the cam under different conditions.

The key to VTEC is an extra "high-speed" cam lobe. Normally each pair of valves operates on the outer set of cam lobes and rocker arms. At high-speeds, the center lobe and rocker arms are actuated, effectively changing to a racing cam profile. The quick-burn, pent-roof, high-swirl chamber with a centrally located spark plug is icing on the cake!


The latest and most sophisticated VTEC development is i-VTEC ("intelligent" VTEC), which combines features of all the various previous VTEC systems for even greater powerband width and cleaner emissions. The i-VTEC is used in the Acura RSX Type S, the Civic Si, the '03 Honda Accord, the '04 Acura TSX, and other models. With the latest i-VTEC setup, at low rpm the timing of the intake valves is now staggered and their lift is asymmetric, which creates a swirl effect within the combustion chambers. At high rpm, the VTEC transitions as previously into a high-lift, long-duration cam profile.



On i-VTEC engines, the engine computer also monitors cam position, intake manifold pressure, and engine rpm, then commands the VTC (variable timing control) actuator to advance or retard the cam. At idle, the intake cam is almost fully retarded to deliver a stable idle and reduce oxides of nitrogen (NOX) emissions. The intake cam is progressively advanced as rpm builds, so the intake valves open sooner and valve overlap increases. This reduces pumping losses, increasing fuel economy while further reducing exhaust emissions due to the creation of an internal exhaust gas recirculation (EGR) effect.

When combined with advanced cylinder head and intake and exhaust manifold design, the VTEC concept permits some Honda engines to deliver over 1.5 hp/ci, pass future low-emissions standards, and meet all fuel economy targets. In it, we see perhaps the ultimate and final evolution in internal combustion engine technology. With such exceptional engineering, it's no wonder that Acura and Honda products account for nearly half of the customized cars in the sport-compact market segment.

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