A Guide to the Toyota VVT-i System | Cars and Trucks

By JasonLancaster
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Any mechanic or automotive enthusiast can tell you that an engine is essentially a large air pump. The more an engine can suck in air to mix with fuel, the more it can create power through combustion. Thus, the more efficiently an engine removes exhaust gases from the cylinders, the better it can manage that power. The key to a strong, healthy engine is adequate air from one end to another.

Air flow is affected by many different components in the motor, but the valves in the cylinder head are what directly control the amount of air entering a cylinder, and the volume of exhaust gases leaving it. The intake valves open up just prior to combustion in order to allow air to flow in and mix with fuel, and the exhaust valves open after the ignition of this mixture in order to suck out the resulting gases. The timing of the valves is controlled by a rotating shaft called the camshaft. The camshaft has lobes which push up on the valves in order to open them and drop them back closed again.

The amount of time these valves stay open, and at which point in the combustion cycle they are open, can strongly impact the drivability and power of an engine. If you want a really fast car, like a race car, you'll need to adjust the camshaft to perform well at the high RPMs needed for a powerful engine. This will mean poor performance at low RPMs, which isn't a problem for race cars. If you're looking for something with a lot of low-end torque, such as for towing, then you should adjust the camshaft accordingly to perform at low RPMs. Of course, this will also mean sacrificing high RPM performance.

Unfortunately, street vehicles are a compromise between reliability, fuel efficiency and power. While race vehicles have engines with camshaft designs that generate large amounts of power while being used only at specific, high revolutions, your daily driver sees a wide range of RPMs that make a broader power band necessary. While it is ok for a race car to have a lumpy idle that barely runs below 1000 rpm, it would do you no good if your street car stalled out at every stoplight. Regular vehicles usually have to make do with a camshaft that provides a good amount of power in the most often used range of engine RPMs, but runs out of steam at high speeds.

The problem with compromise camshafts is that they're not all that efficient. Since everyday vehicles operate at a variety of different RPMs, the engine needs to be just as capable of accelerating from a dead stop as it is of zooming along at highway speeds, and everything in between. The result is that your engine often ends up burning too much fuel while underperforming.

Automakers know about this problem, and have created something called "variable valve timing" (VVT) in response. The Toyota Tundra's i-Force 5.7L V8, Toyota's newest VVT-i engine, can use engine oil pressure to move the camshaft slightly, so that the timing of the valves can be adjusted in relation to engine speed. This way, more aggressive lobe designs can be used when the engine is working at a higher RPM. The VVT system allows the i-Force V8 to run a camshaft profile that gives good fuel efficiency in regular driving, but that can also crank up the power when it's called for.

The Tundra's dual VVT-i goes one step further by allowing the exhaust and intake valves to open simultaneously at extremely high RPMs, to scavenge airflow as much as possible. This results in a V8 engine that can produce 381 horsepower at 5600 rpm, and 401 lb-ft of torque at as low as 3600 rpm - while still getting a respectable 20 miles per gallon on the highway with the 2 wheel drive models. And what's best is, the Toyota variable valve timing system gives you killer horsepower without killing you at the gas pump.

About the Author

Author Jason Lancaster administrates TundraHeadquarters.com, a web site with information, news, and reviews of Toyota Tundra parts and Tundra accessories.

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