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Toyota's VVT-i System Explained

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.

An engine's drivability and power can really be affected by length of time and the point in the combustion cycle at which the valves are open. A really fast car, such as a race car, will need an engine that produces lots of power at high RPMs. To get this, the camshaft can be adjusted to perform well at higher RPMs, but the trade-off will be poor performance at low RPMs. Following the same principle, adjusting the camshaft to perform best at low RPMs will give you lots of low-end torque, which is great for jobs like towing, but your high RPM performance will suffer.

Street vehicles are a compromise between reliability, fuel efficiency and power, which can RPM performance, but street vehicles need a broader power band because they function at a wide range of RPMs. A race car can get away with an idle that barely runs below 1000 RPM, but you can't have a street car that stalls at every stoplight. Everyday vehicles usually compromise with a camshaft that works well at the most often used range of engine RPMs, but don't deliver at high speeds.

These compromise camshafts aren't terribly efficient. Because they try to do so many things - from accelerating your car from a dead stop to providing performance at highway speeds, and everything in between - they don't do any one of them very well. This means that your engine burns too much fuel most of the time, while also 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 dual VVT-i in the Tundra takes things a step further by allowing the exhaust and intake valves to open at the same time at very high RPMs in order to scavenge the airflow as much as possible. This all adds up to a V8 engine that produces 381 horsepower at 5600 rpm while still generating 401 lb-ft of torque at as low as 3600 rpm. Not only that, but in the 2 wheel drive models, the Tundra gets a respectable 20 miles per gallon on the highway. Perhaps most importantly, Toyota's variable valve timing system lets you have killer horsepower without getting killed at the gas pump.

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About the Author

Author Jason Lancaster is the editor of TundraHeadquarters.com, a website with information, news, and reviews of Toyota Tundra parts and Tundra accessories.  

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