Automobile buffs will be familiar with the VTEC-Variable Valve Timing and Lift Electronic Control-acronym, as the concept is widely used in Honda's car lineup in various forms. And while the Interceptor shares the VTEC name with cars such as the Acura NSX and Honda Civic, the system itself is quite different in detail. In general, VTEC enhances an engine's performance, fuel economy and emissions by controlling the valves differently at low and high rpm.
Honda's double overhead cam VTEC for automobiles (used on the S2000 and Acura NSX) utilizes six camshaft lobes for each cylinder; three each for the two intake and two exhaust valves. At lower rpm, the outer two (of each set of three) cam lobes operate their respective valves through a standard rocker arrangement. At higher revs, those rockers are pinned to a follower operating off the center lobe-which has more aggressive lift and duration specs than the outer lobes. Using this setup, lift and duration for low rpm can be different from that for high rpm, and the engine can benefit in both ranges. On models with single overhead cams, only the intake valves have the three-lobe setup, and the exhaust valves are operated in the normal manner.
An enhancement of VTEC used on the Civic HX and the hybrid Insight (dubbed VTEC-E for efficiency, economy and ecology) is closer to the Interceptor's arrangement, but still utilizes rockers. At lower RPM, only one of each cylinder's two intake valves operates completely-the other works from a second, much smaller lobe, which barely opens the valve. At high rpm, the rockers are pinned together so that both intake valves work from the normal-sized lobe. With this setup, the exhaust valves operate normally. When the engine operates as a two-valver, intake charge velocity is increased (because of the decreased area it flows through) and torque benefits at low rpm. And without having to worry about charge velocity at low rpm, a more aggressive cam profile that is better suited to high rpm operation can be used, boosting top end power.
At low rpm, a pin between...
At low rpm, a pin between the valve and bucket is extracted, leaving the valve closed at all times. At 7000 rpm, an electric solenoid opens the cavity containing the pin to oil pressure, forcing the pin between bucket and valve. No shim is used (where would it go?) which reduces noise and reciprocating mass.
In the mid-1980s, the Honda CBR400F (not sold in America) utilized a DOHC-VTEC system similar to that used on the cars. Back then, it was called REV (Revolution Modulated Valve Control) and the setup incorporated split rocker arms to work eight of the engine's 16 valves below 8500 rpm, and all of them above that. The trouble with grafting these automotive-style VTECs on a bike like the VFR is that bikes have long since outgrown rockers, and the size penalty for extra lobes would be too great given Honda's goals for the 2002 Interceptor. Hence a further enhancement called Hyper-VTEC. First utilized on the Japan-market CB400 Super Four, and now the Interceptor, Hyper-VTEC retains the top end's basic layout of DOHC with direct-actuation (no rockers or lifters), and two of each cylinder's four valves operate as before. The other two, a cater-cornered intake and exhaust pair, incorporate a small pin inside the bucket which operates under hydraulic pressure. At high rpm, the pin acts as an extension of the valve stem itself and the valve works exactly as normal. At lower rpm, the pin extracts, and the bucket pushes on the empty space-leaving the valve closed.
Similar to the automotive VTEC, the VFR's system is controlled by electronics and hydraulics. At the crossover point of 7000 rpm, an electric solenoid activates, allowing oil pressure to build in the cavity housing the pin. The pin is then pressed into place (in 15-20 milliseconds) between the bucket and valve. Honda anticipates that, because the VTEC valves are not under constant operation, they require adjustment less often than the non-VTEC valves. As a result, no shims are incorporated, but rather buckets with varying thickness, and this reduces reciprocating mass as well as noise. As you would expect, Hyper-VTEC also allows for more aggressive cam timing, and the VFR's cams benefit from more duration (7 degrees intake, 3 degrees exhaust) and overlap (12 degrees) compared to the previous model.
We're still lagging behind the automotive world as far as VTEC is concerned, however. The Acura RSX incorporates DOHC i-VTEC-"intelligent" VTEC-which adds variable intake cam timing on top of the DOHC system.
Honda VFR800 Interceptor: Full test