Last Updated: 09/21/96

From: Daryl Krzewinski

VTEC is Honda's trademarked acronym for Variable valve Timing and Electronic lift 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. Since the high performance version is the most oft discussed, I'll describe it:

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.

From: Shawn Church

VTEC is actually a generic name for several different systems within the Honda engine lineup. You have the SOHC VTEC which operates only on the intake valves, the DOHC VTEC which operates on both, and in other countries there is a three stage VTEC system which I believe only comes on DOHC cars (130 hp from 1.5 liters and excellent mileage is the example I'm familiar with).

The principle of all the VTEC systems is the ability to change cam profiles at a given point within the rpm range. This allows a cam which will easily idle and pass emissions for everyday driving, but provides an extra kick on the top end (it can also be used to improve economy or low end torque).

This change in cam profiles is accomplished through the use of a solenoid and oil pressure. At a given rpm, oil pressure is allowed to lock together the normal cam followers and a third follower. This third follower is actuated (normally) by a higher lift and duration cam lobe and is supported be a lost motion assembly. When the followers lock together, the two valves being controlled are following the exact same profile (most DOHC motors have a slightly different profile for each valve to improve swirl and combustion).

The implementation is much more complex in the SOHC system because of the need for a very complex actuator system to get around the limitations of a single cam to actuate 4 valves per cylinder.

The actuation of the system does indeed follow a hysteresis loop and the actuation point is variable based upon load and throttle position. Without the hysteresis loop, you would have a certain point where the system would rapidly cycle in and out, causing problems.

The three stage system actually actuates only one valve at low rpm, both valves at mid rpms and then switches to a high lift lobe on the top end. Sounds complex, but the implementation is actually elegantly simple.

To sum up, there are no additional valves in VTEC systems, just a different cam lobe with which to actuate your valves. The only necessary additional parts are the solenoid, different cams, lost motion assemblies and the ECU to run the system. In general, you won't hear the system activate on a stock car. On DOHC VTEC systems, what most people hear activating is the secondaries on the variable path intake manifold. To take advantage of the system, keep the revs up, hopefully high enough so that when you shift, you won't drop out of the VTEC range. If you had a DOHC car, you'd also want to try and stay in the secondary range of the intake manifold.

From: LEE Kok-Bin

Racing Engine      Conventional Engine       Fuel-Efficient Engine
-------------      -------------------       ---------------------
High lift valve	   Average power at low      Average power at low
timing             speed                     speed

High power at      Average power at high     Poor performance at
high speed         speed                     high speed

Undriveable at     Average fuel              Excellent fuel
low speed          consumption               consumption

High fuel          A "COMPROMISE"
consumption
                       |                   /
                       |                  /
                       |                 /
                       |                /
                       |               /
	 	VTEC                   /
		----
	Variable valve timing & lift
	Good power & economy at all speed.

So if you've read this far and you still don't understand what VTEC is, watch this Quicktime movie which shows the inside of a VTEC engine as it goes through the gears.

Copyright 2002, Temple of VTEC