Understanding Toyota's VVT-i System

By now, you have probably heard a mechanic or automotive enthusiast tell you that an engine is essentially a large air pump. Basically, the more air an engine can suck in to combine with fuel, the more power it can create through combustion. It also follows that the more efficiently an engine can remove exhaust gases from the cylinders, the better it can manage that power. Air flow from one end to the other is the key to a healthy, strong engine.

Many different things can affect air flow in a motor, but the primary control over the volumes of air entering a cylinder, and exhaust leaving it, comes from the valves in the cylinder head. The intake valves open immediately before combustion to allow air to flow in and combine with the fuel. After this mixture has been ignited, the exhaust valves open and suck out the resulting gases. Valve timing is controlled by the camshaft, which is a rotating shaft with lobes pushing up on the valves to open them, then drop them 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 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.

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 info, news, and reviews of Toyota Tundra parts and Tundra accessories.

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