Rice4Brains
New Member
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.
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.