If you do not have much experience with systems that alter valve timing, lift, and/or valve duration, and have wondered why some Honda engines have rocker arms that seemingly do nothing but take up space, this article will answer all the questions about Honda’s VTEC system you have ever wanted to ask, starting with this question-
“VTEC” stands for Variable Valve Timing & Lift Electronic Control, and in its most basic form the system switches between cam profiles to produce more power at low engine speeds, while reducing fuel consumption at engine speeds above about 4 000 RPM.
Unlike other variable valve timing systems that alter the phasing of the camshafts relative to base settings or the position of the crankshaft without altering valve lift or valve duration, VTEC uses different cam profiles, and by switching between profiles, VTEC can alter valve timing, lift, and duration at the same time. In fact, VTEC was the first commercially viable (and successful) variable valve timing system that could alter cam profiles in real time, and it would be fair to say that all other variable valve/cam timing systems flowed from Honda’s VTEC system, which brings us to-
Like other designs that use one rocker arm per valve, VTEC also uses one rocker arm per valve, but unlike other designs, VTEC uses an additional, high-lift cam profile and an additional rocker arm that can be hydraulically locked to the rocker arms that follow the “normal” cam profiles.
While the flow dynamics of the air/fuel mixture allows stable combustion at low engine speeds, the demand for fuel and air at higher engine speeds requires more valve lift and longer valve duration times in order to prevent a drop-off in engine power. In an ideal [perfect] engine, the valve timing, lift, and duration would always be optimised to ensure optimum performance at all engine speeds and/or loads, but since perfect engines do not exist, Honda engineers did the next best thing. They designed a system that could switch between cam profiles to improve a small- capacity engine’s volumetric efficiency at high engine speeds.
The switching between cam profiles is controlled by the ECU that monitors parameters such as the engine speed, throttle position, the rate of throttle movement, oil pressure, and vehicle speed. When all required conditions are met, the ECU activates an oil control solenoid that locks the rocker arm that follows the high-lift cam profile to the rocker arms that follow the “normal” cam profiles.
In practice, the high-lift cam profile and its rocker arm is located between the two normal cam profiles and their associated rocker arms, whose movement is independent of the rocker arm between them. When conditions are met, an oil control solenoid allows pressurised oil to act on a sliding pin in the normal rocker arms to engage with the central rocker arm, which locks the three control arms together. Since the central rocker arm follows the high-lift cam profile, its movement is now transferred to the two adjacent rocker arms, and valve lift and duration is therefore dictated by the high-lift cam lobe’s profile.
When the ECU determines that operating conditions no longer requires high-lift valve operation, the ECU-controlled oil control solenoid opens and relieves the pressure on the sliding pin, which disengages the high-lift rocker arm from the normal ones, and normal valve lift and duration resumes.
However, it must be noted that to avoid a situation in which the engine may be expected to operate continuously at, or close to the switchover point, the switchover point to hi-lift operation and reversion to normal operation is not the same. The ‘switch down” point is therefore variable between a maximum and minimum value based on engine load (as opposed to engine speed) but is generally below the engine speed at which hi-lift operation is initiated.
As a practical matter, the advantages of VTEC are obvious, since it offers stable low RPM performance in the urban cycle, while providing increased performance and power at highway speeds without a corresponding fuel consumption penalty. While the foregoing described the basics of VTEC, the system exists in various forms, some of which include the following-
VTEC on SOHC engines
Unlike VTEC on DOHC engines that use two valves for intake and two valves for exhaust, and can therefore accommodate a third rocker arm between each set of normal rocker arms, the design of SOHC engines does not make this possible.
Therefore, on engines like the D-, and J-series engines, VTEC works only on the inlet valves since on these engines the spark plug is located between the two exhaust rocker arms, leaving no room for the third rocker arm on the exhaust side.
However, Honda engineers overcame this problem in the J37A4 3.7L SOHC V6 engine that was first used on 2009 Acura AWD models. This engine uses six cam lobes and rocker arms per cylinder, with primary and secondary rocker arms on each rocker shaft. In this version, the primary rocker arm contains the VTEC sliding pin, and the secondary contains the return spring, with “primary” referring to the rocker arm that contains the sliding pin, and not to the rocker arm that acts on a valve.
Nonetheless, the “Y” shape of the secondary rocker arms makes it possible for the secondary rocker arms to act on two valves at once when the primary rocker arms (that follows the high-lift cam profile) are locked to the secondary rocker arms on both the exhaust and inlet sides.
VTEC-E
On the earliest iterations of this variation of VTEC, the system uses only two cam lobes per cylinder, as opposed to three lobes per cylinder. This version also uses roller rocker arms; one that follows a very “mild” lobe profile with very little lift and short duration, and another that follows a cam lobe with moderate lift and duration.
When the engine is in non-VTEC mode, the low-lift intake valve is opened partially, while the other is opened to a larger extent. In practice, this causes the intake charge to swirl, which in turn, improves combustion. However, when VTEC is engaged based on engine load, vehicle speed, and throttle position, an oil control solenoid acts on a sliding pin that that locks the two rocker arms together, which causes the moderate cam lobe to open both inlet valves by the same amount, and for the same duration.
Since the profile of the moderate cam lobe is identical to the normal profile on non-VTEC Honda engines of the same displacement, the power delivery characteristics of the two types of the engine are identical, assuming that everything else on the two engines is identical.
However, on later iterations of VTEC-E, the moderate cam lobe profile was replaced with a more aggressive profile, which delivered power and performance in VTEC mode that was comparable to that of the original VTEC design.
3-stage VTEC
Due to limited space on SOHC cylinder heads, 3-stage VTEC works only on the intake valves, but this version manages to combine the low fuel consumption advantages of VTEC-E and the performance improvements of conventional VTEC in a single system.
This version uses three cam lobes and three rocker arms per cylinder, and two oil control solenoids that depending on the engine speed and load, can lock either only two rocker together, or when conditions allow, lock all three rocker together.
In non-VTEC mode, the two different intake cam lobes open the two inlet valves by different amounts (just like in VTEC-E) to introduce swirling of the intake charge, which improves fuel economy at low engine speeds. When conditions demand VTEC operation, one oil control solenoid locks two rocker arms together, which opens both inlet valves by the same amount to improve torque in the 3 000 RPM to about 5 400 RPM range.
However, at engine speeds above about 5 500 RPM, the second VTEC oil control solenoid locks the third, high-lift rocker arm to the other two, which means that the high-lift profile now controls the lift and duration of the intake valves up to the red-line engine speed limit.
While the iterations of VTEC described above should suffice to describe VTEC technology as a whole, three more variants merit a special mention-
VTEC in the R-series engines
Also somewhat confusing designated as i-VTEC, this variation uses three cam lobes per cylinder on the intake camshaft, but unlike other versions, this iteration functions in a reverse fashion, and then only at low to mid-range engine speeds.
In practice, the R-series engines have camshafts with two high-lift cam lobes, and one lobe with very little lift. When VTEC mode is initiated, the rocker arm following the low-lift lobe is locked to a high-lift rocker arm, which keeps one of the intake valves partially open during the compression cycle. As a practical matter, this arrangement is analogous to the Atkinson Cycle and switching between conventional and the Atkinson Cycle when required results in excellent fuel economy over wide range of engine operating conditions without a significant performance penalty.
i-VTEC i
This version of VTEC uses both VTEC and VCT (Variable Timing Control) in conjunction with direct fuel injection to produce an ultra-lean combustion engine. In this case, the various technologies are combined to make a 2.0L DOCH engine run efficiently with air to fuel ratios as lean as 65 parts of air to one part of fuel, which is significantly leaner than “normal” direct injection engines that usually run on about 40 parts of fuel to one part of air.
VTEC TURBO
Honda introduced a range of forced induction VTEC engines as part of the Earth Dreams Technology initiative in 2013. The range included a 1.0L- 3 cylinder engine, and 2, 4-cylinder engines with displacements of 1.5L and 2.0L, respectively and all have turbo chargers, VTEC on both the intake and exhaust camshafts, and direct fuel injection. The first model to be released in Europe with this version of VTEC was the Honda Civic Type R with a Euro 6-compliant 2.0L engine.
VTEC in all its iterations and variations has proven itself to be a reliable and effective means to increase the volumetric efficiency of small capacity engines. In fact, VTEC is the least troublesome of all valve timing/lift altering systems, and it consistently performs at least as well, if not better than similar systems on high-end applications that cost several times as much as a humble Honda Civic or Accord.
