Toyota's VVT-i System Explained

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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.

There are a number of different components in the motor which can affect air flow, but the main ones are the valves in the cylinder head. These control the amount of air entering the cylinder, and the volume of exhaust air which leaves it. Intake valves open just before combustion, allowing air to flow in and mix with fuels. After this mixture has been ignited, the exhaust valves open and suck the resultant gases out of the engine. The valves are timed by a rotating shaft called the camshaft, which has lobes that push up on the valves to open and close them.

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.

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 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 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.
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