The VTEC Funda

VTEC stands for Variable Timing and Lift Electronic Control System. It was designed to resolve once and for all the usual compromise engine designers have traditionally been forced to make between low-speed torque and high-end horsepower. It lets the engine breathe according to its needs.
Engine breathing is analogous to the breathing of any living organism. At rest, the lungs take in the necessary amount of air for normal function. When running, the lungs and heart work faster to supply more oxygen to the system. Engines can't do that because their breathing apparatus (comprised intake maifolds, intake runners, valves, valve lift and throttle bores) is fixed. Not so with VTEC.
Using an innovative arrangement of three intake and exhaust lobes per cylinder, VTEC increases both valve lift and valve opening duration as the engine climbs through the rpm band. By varying valve lift and timing, the engine can aspirate more air when it needs it at high speed operation. It boosts power without resorting to the complex and sometimes overstressed system of turbocharging or supercharging.It's an elegant and organic solution to a problem that has plagued engine designers for generations, and exemplifies the Acura approach to engineering. Simplicity, elegance and function.
[DK] VTEC is Honda's trademarked acronym for Variable valve Timing and lift Electronic Control. Put simply, it's a method of directly altering the cam profile that valves "see," so that the optimum grind can be utilized at either high or low RPM. Honda currently has three different VTEC systems for sale in the U.S., but the primary differences are level of complexity and design purpose.
The high performance VTEC system, which made it's debut in the Acura NSX, is also available on the Integra GS-R, Prelude VTEC and del Sol VTEC. Using radical cam grinds to improve engine horsepower is certainly nothing new, but the problem lies in driveability. The very aspects of a cam grind that work so well for horsepower (high lift, long overlap, etc.) do so by creating an ideal situation for a high air flow at high engine speeds. Unfortunately, what works well for that situation has the inverse affect on low speed torque and driveability. Perhaps at one time or another we've all heard the V-8 hot rod that has a cam grind so radical that it can't even maintain an idle, and the driver must constantly goose the accelerator to keep the engine running.
What to do? How about two different cam grinds, each optimized for a different half of the rpm range. Honda achieves this with a rather simple method.
Picture, if you will, one cylinder of a DOHC, 4-valve per cylinder engine. There are 4 cam lobes, each directly operating a valve (two intake, two exhaust). The VTEC system has two more cam lobes, in between each pair of the other respective sets. These two can then be our high-rpm lobes, while the other four are the low-rpm lobes.
The low-rpm lobes in this case then actuate the valves through a set of rocker arms, so that the mechanical connection can be broken if desired. The third, high-rpm lobe also has it's own follower, but it is in a freewheeling state, flopping around and not contributing anything. As our engine accelerates through it's rev range, it passes through the power peak of the low-rpm lobes. Then, at the engine speed and throttle position programmed into the computer's memory map a signal is sent which electronically opens a spool valve, which then directs oil pressure to a mechanical sliding pin. This pin locks the rocker arms actuating the valves to the follower on the high-rpm cam lobe. As this grind is steeper and higher then the other four cams it will supersede them. In a few milliseconds you have completely altered the valve timing and the engine's power band begins anew.
The obvious benefits to this are the high-rpm power associated with a radical cam grind, but with little or no negative affects on low speed idle, driveability or torque. Just changing a fixed timing engine to a cam grind equal to the high-rpm one used in the VTEC would produce an engine which is utterly gutless below 5000rpm.
Clearly, this system is intended to improve performance first, with little effect elsewhere. But such a system can be used differently, as that in the Civic VX. In this instance, the low-rpm lobes give a staggered timing, where one valve opens fully but the second cracks just a bit. This is to induce a high swirl rate into the chamber to promote better combustion, which, when combined with a computer-actuated lean burn helps to achieve high mileage. The high-rpm lobes in this engine are a more conventional grind associated with a sixteen valve 4-cylinder, to provide extra power in cases of passing or merging. The VTEC system used in most Civics (EX, Si and the del Sol Si but NOT del Sol VTEC) is a little closer to that in the NSX, etc. The difference being that this system is vastly simplified and operates on the intake valves ONLY. The exhaust valves are actuated conventionally, which reduces the effect somewhat from the full VTEC system. This is partly to reduce costs, and partly because this engine is a SOHC, and the complex system of rocker arms to actuate sixteen valves is prohibitive to the full VTEC system. The VTEC on the Accord is close in design to that on the Civics as well, optimized more for a smooth power delivery then high horsepower.
The negative effects? Very few, really. Obviously it's very expensive, with many complex parts involved. The biggest drawback is the limitation to only two "modes" of valve timing. Most engineers are still seeking ways to obtain unlimited variance of the valve timing, so that it can be optimized to any engine speed, not just high or low rpm. BMW's system approaches this method with a completely different method of varying the valve timing. It is almost infinitely adjustable *within it's range*, but alas it has a much smaller envelope between the two extremes of it's variability than is possible with the Honda system.
[RT] The Honda Civic with VTEC was first introduced in 1992.(USA & Canada). There are two different engines. The VTEC(D16Z6) and VTEC-E(D15Z1). The VTEC is equipped with multiple cam lobes per cylinder, providing one valve timing and valve lift profile at low speed and a different profile at high speed. Switch-over from one profile to the other is controlled electronically, and is selected by monitoring current engine speed and load. In general, it would be ideal if the high rpm performance of a racing engine and the low rpm performance of a standard passenger car engine could be combined in a single engine. This would result in maximum performance engine with a wide power band. Two of the major differences between racing engines and standard engines are the timing of the intake/exhaust valves and the degree of valve lift. Racing engines have longer intake/exhaust timing and a higher valve lift than standard engines. The Honda Variable Valve Timing and Valve Lift Electronic Control System takes this into account. When valve actuation is set for low rpm timing and lift, low rpm torque is better than in a standard engine. When valve actuation is then switched for high rpm timing and lift, output improves to the level given by a racing engine can offer. Until now, few variable valve timing systems have been commercialized. In those that have, only the time that both valves are open (intake/exhaust overlap) could be changed. Honda's system is the first in the world in which the intake valve timing and the degree of valve lift can be changed as needed, making it the most advanced valve train mechanism available.
The VTEC-E has a normal 4 valve per cylinder valve arrangement. At low rpm, the primary intake valve operates at normal lift while the secondary intake valve opens only slightly to prevent fuel accumulation in the intake port. At high rpm, the secondary intake valve rocker arm is connected to the primary intake valve rocker arm to allow normal valve lift, A synchronizing piston connects/disconnects the two intake valve rocker arms. Hydraulic pressure against a timing piston moves the synchronizing piston one direction, while a stopper piston and return spring moves the synchronizing piston back when hydraulic pressure is reduced. A variable valve timing and valve lift mechanism is used so the engine achieves both low fuel consumption and high output. With this system, a very lean fuel/air is efficiently burned to achieve high torque characteristics and low fuel consumption in the low rpm range, while in the high rpm range, high output, equivalent to that of conventional 4-valve engine, is achieved. The VTEC-E engine has two intake valves per cylinder. In the high rpm area, both intake valves are activated while only one valve operates in the low rpm area. Switching between one valve operation and two valve operation is controlled be hydraulic pressure.