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.
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.
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.
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.
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