A Guide To The Toyota Vvt-i System

You probably know or have heard that engines are basically large air pumps, and the more air an engine sucks in to combine with fuel, the more power it will create through combustion. Also, an engine that can remove exhaust gases from the cylinders more efficiently will be better able to manage that power. Good air flow from one end of an engine to the other is the key to a strong, healthy 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.

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, 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 dual VVT-i in the Toyota Tundra goes even further - at high RPMs, it allows the exhaust and intake valves to open at the same time, which scavenges airflow as much as possible. The result? A V8 engine that can produce 381 horsepower at 5600 rpm, but also generate 401 lb-ft of torque at as little as 3600 rpm. And what's more, the 2 wheel drive Tundra can still get a respectable 20 miles per gallon on the highway. Possibly the best part about Toyota's variable valve timing system is getting killer horsepower without getting killed at the pump.

Article Source: http://www.search4allinfo.com

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

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