VTEC is a technology invented and developed by Honda, for use in internal combustion engines. It can in theory be used in all engines that use camshafts, but has in practice only been used in cars, motorcycles, and 4x4 vehicles.
VTEC stands for Variable Timing Electronic Control, and is short for Variable valve Timing and lift Electronic Control. It is a technology which allows cars to behave differently at different engine speeds, by actuating different cam lobes at different speeds. (it's all explained more clearly later in the article, don't worry...)
Honda were the first to implement a VTEC technology into a production engine. The technology was invented by Honda's premier engine designer, Kenichi Nagahiro. Other manufacturers have followed suit - such as the VarioCam system from Porsche, VVT from Toyota, but arguably never managed to mimic the reliability and success of the VTEC system in use in Honda cars.
How it works
In a car engine, fuel and air is let in through a set of valves - known as intake valves - and exhaust is let out through exhaust valves. The opening and closing of these valves is done by lobes on a camshaft, which push the valves open. Springs make the valves close again.
The timing and speed of how fast these valves can open and close is controlled by the shape of the lobes on the camshaft. Different shapes of lobes are called different 'cam profiles'. This is mentioned in lift (how much a valve is opened) and duration (how long it is opened).
It's a very clever way of letting air and fuel into the engine, and letting the exhaust gasses back out. The only problem is that both the air/fuel mixture and the exhaust gasses behave quite differently at different engine speeds. As such, engine manufacturers have to make a compromise: An engine that runs very smoothly and quietly at low speeds loses top-end power on high revs, and an engine that has lots of power under high revs (such as race car engines) is nigh-on impossible to drive at slow engine speeds.
When modifying a car engine, changing a camshaft to a more 'aggressive' cam profile can unleash a lot more power out of an engine, but – as mentioned – can make the car difficult to drive and control at low engine speeds. Fast road cars will often opt for a 'fast road' cam, which will be better than the standard cam profile at higher engine speeds, and sacrifices low-end smoothness. Engines used in F1 cars are an excellent example – they regularly have idle speeds of 5-8,000 rpm.
Where does VTEC come in, then?
Well, the ideal car would not use a camshaft at all – it would use electric motors to open and close the valves at exactly the right time. However, with the abuse that an internal combustion engine gets, that's not really an option. The next best thing is to have one cam-shaft for low revs, and one for high revs, and that's exactly what VTEC does. A VTEC camshaft has two sets of lobes on the shaft.
At a pre-determined set of circumstances (a certain rev range, if the engine is at the right temperature, if there is enough oil pressure etc), a locking pin is pushed into the camshaft by the oil which is let into the shaft by a solenoid, and from that point onwards, the high-rpm cam profile is used.
VTEC makes an incredible difference in engine performance. In my 1995 Japanese import Honda Prelude 2.2 litre VTEC, the power curve rises evenly to about 120 hp @ 5,300 rpm. Then, over the next 300 rpm, the VTEC kicks in, and gives the car a 20 horsepower jump, before continuing to rise gradually to about 170 hp. In real-life terms, this means that you have good acceleration, but when the VTEC engages, you feel a significant (17%) increase in power, accompanied with the characteristic change in engine note – from an angry whine, the car goes to a throaty, wild roar.
From the factory, the 2.2 litre H22a VTEC engine found in my car offered 197 horsepower. The 2.3 litre non-VTEC engine, which was also used in Honda Preludes, only had 165 horsepower. Who said there is no replacement for displacement?
Nifty, when was this all invented?
The VTEC system has been around since 1989, and was first introduced on a 1.6 litre dual over-head cam (DOHC) engine in a Honda Integra. These engines have a separate camshaft for the intake and exhaust cam, and the VTEC changes the profile of both camshafts at the same time.
Later, Honda started introducing VTEC also in single cam (SOHC) engines (i.e with a single camshaft for both intake and exhaust). The downside of this is that the VTEC system on these engines only work on the intake valves.
From the two original VTEC systems, Honda started doing all sorts of other interesting things. The most notable is the VTEC-E system. The E stands for Economy, and instead of offering a kick of extra performance at higher revs, it essentially the same system to make extremely frugal engines at lower revs – the rev range where cars are mostly used around town – yet have 'normal' performance for motorway use. This is achieved by only opening one of the inlet valves fully, allowing for a better fuel-air mixture and more complete burn of the fuel/air mixture, which again means that the engine can inject less fuel, and as such consume less
The next stage up is a 3-stage VTEC system, which combines the regular VTEC system for performance with the VTEC-E system for frugality at low RPM. It's the best of all worlds, but the cost and complexity of building these engines is prohibitive, and cars with 3-stage VTEC are not currently marketed outside of Japan.
From the VTEC-E came the i-VTEC system, the I standing for intelligent. Essentially, this system doesn't use two different cam profiles, but uses an adjustable cam gear, which means that the intake camshaft can advance steplessly. This only changes the timing, not the lift of the camshaft, but is a rather elegant solution nonetheless, and allows for performance and relatively low fuel consumption at the same time. There are also other i-VTEC systems out there, and it appears that i-VTEC is the working name at Honda for more advanced VTEC systems with limited roll-out, regardless of how the system differs from the original VTEC.
The Future of VTEC
The most current update of VTEC came in September 2006, with the launch of the Advanced VTEC engine, which achieves high performance along with outstanding fuel economy and lower emissions. The new engine combines continuously variable valve lift and timing control with the continuously variable phase control of VTC (Variable Timing Control). Honda plans to release a production vehicle equipped with the new engine within three years.
This new system permits optimum control over intake valve lift and phase in response to driving conditions, achieving improved charging efficiency for a significant increase in torque at all engine speeds. Under low to medium load levels, the valves are set for low lift and early closure to reduce pumping losses and improve fuel economy.
In combination with optimised intake components, these advances in control technology result in world-class dynamic performance along with approximately 13% improvement in fuel economy. The new engine is also exceptionally clean, with exhaust emissions that meet both U.S. Environmental Protection Agency LEV2-ULEV regulations and Japanese Ministry of Land, Infrastructure and Transport requirements for Low-Emission Vehicles, with emission levels 75% lower than those required by the 2005 standards (based on Honda calculations).
Advantages and disadvantages of VTEC
VTEC sounds like a rather advanced system, but it is quite a simple idea. The true strength of the system, however, is that it is extremely reliable - In fact, Honda have never had any failings of the VTEC system within the warranty time of any vehicle. Combine this with the general reliability of Honda engineering, and you get some truly outstanding engines.
Engines equipped with VTEC used to be significantly more expensive than non-VTEC engines, but the economy of scale involved means that the premium for VTEC vs non-vtec has lessened.
The largest disadvantage of the VTEC system is that the power is only available in high revs. In fact, the power is often only available in rev ranges that most people rarely ever use - from 5,000 rpm and upwards. The argument is that Honda only added VTEC to be able to claim higher horsepower figures
People who favour VTEC argue that it is better to have a car which behaves calmly and has a decent fuel consumption, and then has power on tap when it is needed, rather than to have to compromise on lack of power or better consumption.
It cannot be denied, however, that at the pinnacle of VTEC development a 2.0 litre naturally aspirated VTEC engine – the F20C engine used in the Honda S2000 sports roadster – is the most powerful naturally aspirated engine in the world, when considering engine size versus power: By revving to a mind-boggling 9,000 RPM, the engine delivers 247 horsepower in stock form straight from the factory, vastly surpassing the magical 100 horsepower per litre barrier.
Finally, just watch the eyes of any petrol-head, as their eyes roll back into their head and their butt-cheeks clench, the very second the VTEC roar fills the cabin of a well-tuned Honda. To me, that's argument enough to never have a non-VTEC car again.
(if you have problems visualising how vtec works, this film on Google VIdeo may help)