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Understanding Toyota's VVT-i System

Author: JasonLancaster
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Word Count: 669
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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.

How long these valves remain open, and at what point in the combustion cycle, can have a big impact on the drivability and power generated by an engine. For instance, if you want to have a really fast car, like a race car, you'll want the engine to produce a lot of power at high RPMs. You can adjust the camshaft to perform well at higher RPMs. This will result in poor performance at low RPMs, but that's OK with a race car. Conversely, if you want a lot of low-end torque - which is great for towing - you need to adjust the camshaft to perform well at low RPMs. This, of course, will hurt high RPM performance.

Unfortunately for street vehicles, they need to be a compromise between reliability, fuel efficiency and power. Vehicles like race cars, which perform within a specific range of RPM, can afford to have poor performance outside their optimal RPM, in return for large amounts of power and high performance at their ideal RPM range. Street vehicles, however, need to function over a large range of RPMs. It would be no good if your street car stalled at every traffic light or ran out of steam whenever you tried to take the highway! Regular vehicles need to use a camshaft design that provides adequate power in the most often used range of RPMs.

These types of camshafts obviously aren't too efficient. Since they're trying to do everything adequately, they don't really do any one of them superiorly. Your engine needs to be able to perform just as well accelerating from a stop as it does speeding down the highway, which means that much of the time, it's burning too much fuel and also underperforming.

Automakers have developed something called "variable valve timing" (VVT) to address this problem. Toyota's newest VVT-i engine, the Toyota Tundra's i-Force 5.7L V8, can vary the timing of the valves to match engine speed. It uses engine oil pressure to make slight adjustments to the camshaft, so that more aggressive lobe designs are used when working at higher RPMs. This makes the i-Force capable of running a camshaft configuration which provides fuel efficiency for everyday driving, but that can still turn out lots of power when you press the pedal to the floor.

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.

About the Author

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