VTEC is an acronym for Variable valve Timing and lift Electronic
Control. It is a mechanism for optimizing air/fuel mixture flow
through the engine.
An internal combustion engine converts the chemical energy stored
in fuel into thermal energy. The increased thermal energy within a
cylinder causes the pressure to build. This pressure acts on the
pistons and the result is a mechanical force rotating the
crankshaft. This mechanical force is measured as crank torque. The
ability for the engine to sustain a certain level of crank torque
at a certain RPM is measured as Power. Power is the rate at which
the engine can do work. This conversion process is not 100%
efficient. In fact, only about 30% of the energy stored in the fuel
is actually converted into mechanical energy.
Physics says that for a given efficiency level, a higher rate of
fuel consumption is needed for the engine to generate power. So it
becomes obvious that if you want more power, you need to increase
the rate of fuel combustion. One way to achive this goal is to have
a bigger engine. A bigger engine with larger cylinders will be able
to combust more fuel per rotation than a smaller engine. Another
method is to pre-presurize the fuel/air mixture and cram it into an
existing engine size. Thus even though the cylinder size stays the
same, more fuel is combusted per rotation. This second method is
referred to as forced induction.
Honda chose to explore another method: keep the engine size the
same, but turn the engine faster to consume more fuel. Here is an
analogy: You want to move foam peanuts from one bucket to another
with a cup. You can increase the size of your cup, compress/cram as
much peanuts as possible into the cup each time, or you can just
move the cup faster. All three methods moves more peanuts. Honda
uses the last method. And again, more fuel combusted equals more
power generated by the engine.
As the engine speed is increased, more air/fuel mixture needs to be
"inhaled" and "exhaled" by the engine. Thus to sustain high engine
speeds, the intake and exhaust valves needs to open nice and wide.
Otherwise you have what is akin to athsma: can't get enough
air/fuel due to restrictions.
If high speed operation is all we have to worry about, Honda
wouldn't need to implement VTEC. Indeed, race engines that operate
mostly at high rpms do not utilize any mechanism like VTEC. But
street cars used for daily driving spend most of their time with
the engine at low RPMs. Valves that open wide for high RPM
operation contributes to rough operation and poor fuel economy at
low RPMs. These undesirable traits are directly against Honda's
design goals.
The solution that Honda came up with is the VTEC mechanism: open
the valves nice and wide at high RPMs, but open them not as much at
low RPMs. So now you have a engine with smooth operation at low
RPMs, and high power output at high RPMs.
And that is basically what VTEC is. It's nothing magical. The idea
has been around for a long time. Honda's VTEC is just a very
simple, elegant and efficient implementation that is extremely
effective at achiving its design goal. Honda automobiles are the
first among modern automobiles to utilize this mechanism in such a
large scale of distribution.
Pros
The main benefit of VTEC is that the resulting engine is very
versatile. The torque curve is very flat: among the flatest of all
the engines on the market. Thus where other engines are running out
of breath, a VTEC engine maintains a nice and steady output of
torque, making the whole RPM range usable for acceleration. So when
you are just driving around at a reasonable pace, the car is very
smooth and fuel consumption is similar to other engines of the same
displacement. When you need more power for passing, all you have to
do is down shift and take advantage of the extra power available at
the higher RPMs. So you get the smoothness and fuel efficiency of a
small economical engine when you drive a low RPMs, and the power
output of a much larger engine at high RPMs.
Due to the greater range of usable RPMs, shorter gears can be used.
Thus for any given speed or engine RPM, a VTEC engine will allow
for a larger ratio multiplier, resulting in more wheel torque. Thus
the benefit of the VTEC technology in terms of acceleration
improvement also affects low RPM operation.
Since VTEC creates more power without increasing displacement, the
engine is likely to be smaller and lighter.
Cons
A vehicle achives its greatest acceleration by keeping the engine
RPM as close to the HP peak as possible. And for DOHC VTEC engines,
this means keeping the needle at some rather lofty RPMs, and more
frequent shifts to keep the RPMs up. To some people, including
yours truely, this is a desirable trait: lots of driver involvement
in the process of extracting excellent performance. To others,
especially those accustomed to the Kansas-flat HP curves of muscle
cars, the high RPM and frequent shifts become bothersome.
For a good launch off the line, such as at the start of a drag
race, a certain amount of tire spin is desired. Muscle cars have
torque peaks at low RPMs, and then taper off as the RPM builds.
This is perfect for drag racing as the initial torque peak
generates the desired tire spin, and then the lower torque at
higher RPMs allow the tire to find and maintain grip. But DOHC
VTEC's torque curve is very flat, so the initial tire slip is much
harder to generate. And once the tire looses traction, the flat
torque curve makes it hard for the spinning wheels to find
traction. So to properly launch a DOHC VTEC car, the driver must
slip the clutch at high RPMs to generate the initial tire spin, and
then carefully modulate the clutch and gas to regain drive wheel
traction while maintaining maximum acceleration.
Even though Honda's VTEC engines has lived up to the legendary
reliability of Honda products, the fact remains that having the
VTEC mechanism adds complexity and cost.
