Innovations
Valve Timing Is Everything
By Karl Brauer
By Edmunds.com Editors
Date Posted 11-09-2000
If you've been paying attention to the acronyms used by Honda, Toyota and BMW over the last few years, you've probably noticed that the letter "V" has become quite popular lately. You've got "VTEC" appearing in all the Acura and Honda brochures and "VVTi" showing up in the majority of Lexus and Toyota literature. Finally, beginning this year with select BMW 3 Series engines, a new Double VANOS system will be advertised.
What do all these Vs have in common? Well, in case you don't already know (or haven't yet guessed despite the monster hint in this column's title), the V stands for valves or, more specifically, variable valve timing.
Before you can appreciate how important valve timing is, you have to understand how it relates to engine operation. Remember that an engine is basically a glorified air pump and, as such, the most effective way to increase horsepower and/or efficiency is to increase an engine's ability to process air. There are a number of ways to do this that range from altering the exhaust system to upgrading the fuel system to installing a less-restrictive K&N air filter. Since an engine's valves play a major role in how air gets in and out of the combustion chamber, it makes sense to focus on them when looking to increase horsepower and efficiency.
This is exactly what Honda, Toyota and BMW have done in recent years. By using advanced systems to alter the opening and closing of engine valves, they have created more powerful and cleaning burning engines that require less fuel and are relatively small in displacement.
Before we take a look at each of these variable valve-timing systems, let's rehash how valve timing normally works. Until recently, a manufacturer used one or more camshafts (plus some pushrods, lifters and rocker arms) to open and close an engine's valves. The camshaft(s) was turned by a timing chain that connected to the crankshaft. As engine rpms rose and fell, the crankshaft and camshaft would turn faster or slower to keep valve timing relatively close to what was needed for engine operation.
Unfortunately, the dynamics of airflow through a combustion chamber change radically between 2,000 rpm and 6,000 rpm. Despite the manufacturer's best efforts, there was just no way to maximize valve timing for high and low rpm with a simple crankshaft-driven valve train. Instead, engineers had to develop a "compromise" system that would allow an engine to start and run when pulling out of the driveway but also allow for strong acceleration and highway cruising at 70+ mph. Obviously, they were successful. However, because of the "compromise" nature of standard valve train systems, few engines were ever in their "sweet zone," which resulted in wasted fuel and reduced performance.
Variable valve timing has changed all that. By coming up with a way to alter valve timing between high and low rpms, Honda, Toyota and BMW can now tune valve operation for optimum performance and efficiency throughout the entire rev range.
Honda was the first to offer what it called VTEC in its Acura-badged performance models like the Integra GS-R and NSX (it has since worked its way into the Prelude and even the lowly Civic). VTEC stands for Variable Valve Timing and Lift Electronic Control. It basically uses two sets of camshaft profiles-one for low and mid-range rpm and one for high rpm operation. An electronic switch shifts between the two profiles at a specific rpm to increase peak horsepower and improve torque. As a VTEC driver, you can both hear and feel the change when the VTEC "kicks in" at higher rpm levels to improve performance. While this system does not offer continuously variable valve timing, it can make the most of high rpm operation while still providing solid driveability at lower rpm levels. Honda is already working on a three-step VTEC system that will further improve performance and efficiency across the engine rpm range.
Toyota saw the success Honda was having with VTEC (from both a functional and marketing standpoint) but decided to go a different route. Instead of the on/off system that VTEC employs, Toyota decided it wanted a continuously variable system that would maximize valve timing throughout the rpm range. Dubbed VVTi for Variable Valve Timing with intelligence (Is this a dig at Honda, suggesting their system isn't intelligent?), Toyota uses a hydraulic rather than mechanical system to alter the intake cam's phasing. The main difference from VTEC is that VVTi maintains the same cam profile and alters only when the valves open and close in relation to engine speed. Also, this system works only on the intake valve while VTEC has two settings for the intake and the exhaust valves, which makes for a more dramatic gain in peak power than VVTi can claim.
Several other manufacturers, including Ford, Lamborghini and Porsche have jumped on the cam phasing bandwagon because it is a relatively cheap method of increasing horsepower, torque and efficiency. BMW has also used a cam phasing system, called VANOS (Variable Onckenwellen Steuerung) for several years. Like the other manufacturers, this system only affected the intake cams. But, as of 1999, BMW is offering its Double VANOS system on the new 3 Series. As you might have guessed, Double VANOS manipulates both the intake and exhaust camshafts to provide efficient operation at all rpms. This helps the new 328i, equipped with a 2.8-liter inline six, develop 193 peak horsepower and 206 pound-feet of torque. More impressive than the peak numbers, however, is the broad range of useable power that goes along with this system. Take it from someone who's driven the new 3 Series and who loves torquey engines-it works!
As the benefits of variable valve timing become more apparent to both consumers and manufacturers, you can expect to see it on just about every vehicle sold in America. I suspect that in five years, variable valve timing will be like ABS or side-impact beams: only really cheap cars won't have it.[/b]
