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Home VTEC - Home What's VTEC? Why do I want it? How does it work? DOHC VTEC SOHC VTEC SOHC VTEC-E 3-Stage VTEC VTEC Misconceptions |
Honda's use of the VTEC technology has lead
to some pretty impressive achivements. This results in
three camps of people: those that think VTEC is nothing but hype, those
that know what VTEC is in terms of its benefits and
limitations, and finally, those that think VTEC is the best thing to
happen to automobiles since round wheels. Inevitably,
misconceptions about VTEC are formed and thrown around. Much of the
arguments on such forums as rec.autos.makers.honda is caused
by such misconceptions. Here are some rather common ones:
DOHC VTEC engines have low crank torque compared to non VTEC engines of similar power output, and crank torque alone is an important indication of how well an engine will accelerate a car. Therefore the VTEC engine's power rating is not "real".An engine's crank torque is directly related to how much fuel/air is combusted per engine cycle. For normally aspirated engines, this means that increasing the displacement size will usually result in increased crank torque. For forced induction engines, the effective displacement is larger than the numerical displacement since the air is pre-compressed before it is forced into the engine. Unlike increased displacement or forced induction, the VTEC system optimizes engine breathing at high RPMs to increase power. Therefore, a VTEC engine's displacement is the smallest of the three methods of increasing power output. And since crank torque is limited by displacement, a VTEC engine's crank torque output is smaller compared to non-VTEC engines of similar power output level. But this doesn't mean that a VTEC engine's HP is somehow worth less. In fact, Honda automobiles equipped with VTEC engines have performance numbers that agrees with the tried and true power-to-weight-ratio method of estimating acceleration performance. People hold this misconception because they have a fundamental lack of understanding of the relationship between crank torque, horse power, and acceleration. Crank torque by itself is meaningless in determining the engine's ability to accelerate the car. This is because the crank torque is multiplied by the gearing and final drive ratio before it is converted to forward thrust. And physics dictates that an engine putting out 160HP absolutely will provide more forward thrust than a 150HP engine, regardless of what crank torque the two engines have, assuming similar transmission efficiency and optimal gearing for both cars. This is plain high school physics. Unless someone can prove that the laws of thermodynamics and Newtonian physics are false, there is no way around this fact. There is some significance to the shape of the crank torque curve, however. When drag racing a car, it is desired to have a bit of initial wheel spin, and then have the tire hook up with the ground. A torque curve with a peak early in the RPM range and then tapers off as RPMs rise is well suited to this purpose. This is why big displacement American muscle cars are so good at drag racing. VTEC engines, on the other hand, have very smooth gradually rising torque curves. The initial wheel spin, therefore, is harder to achive. And after the initial wheel spin gets going, the level torque curve means that very precise clutch and gas pedal control is needed to allow the drive wheels to regain traction while maintaining maximum acceleration. This is why VTEC engines are more difficult to launch off the line than large displacement muscle car engines. VTEC only works in high RPMs, therefore a VTEC car is just like a non-VTEC car at low RPMs. And since most people drive at low RPMs most of the time, VTEC is a waste of money. ---------------------------------------------------------------------------------------------------------------------- The info on this page is a copy of the webarchive.org pull of www.leecao.com, which at the time of this posting this (10/31/2007) was no longer a valid site. |