Toyota's VVT-i System Explained | Cars and Trucks

By JasonLancaster
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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 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

Author Jason Lancaster administrates TundraHeadquarters.com, a web site with info, news, and reviews of Toyota Tundra accessories and Tundra parts.

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