# T-VIS
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'''Toyota Variable Induction System''', or '''T-VIS''', is a [variable intake](/source/variable_intake) system designed by [Toyota](/source/Toyota) to improve the low-end performance of multi-valve engines.[{{citation | title = All About the Toyota Twin Cam, 2nd ed. | url = http://www.3sgte.com/page_12.htm | page = 10 | publisher = Toyota Motor Company | year = 1984 | location = Tokyo, Japan }}]
T-VIS is intended to improve the low-end [torque](/source/torque) of high-performance, four-stroke [internal combustion engine](/source/internal_combustion_engine)s - by changing the [geometry](/source/geometry) of the [intake manifold](/source/intake_manifold) according to the engine rotation speed. The system uses two separate intake runners per [cylinder](/source/cylinder_(engine)), one being equipped with a [butterfly valve](/source/butterfly_valve) that can either open or close the runner. All valves are attached to a common shaft which is rotated by a [vacuum](/source/vacuum) [actuator](/source/actuator) outside the manifold. T-VIS does not actually keep one of the intake valves from opening or seal off the port for one valve.[[http://www.3sgte.com/page_13.htm ''Toyota Twin Cam''], p. 11]
The [engine control unit](/source/engine_control_unit) (ECU) allows vacuum into the actuator by powering a [solenoid valve](/source/solenoid_valve) when the engine rotation speed is low. At higher engine speeds (e.g. 4,200 rpm), vacuum is cut off and a spring inside the actuator causes the butterfly valve to fully open. The idea behind the system is that in the lower engine speed band, the speed of the intake air will be increased because the intake runner [cross section](/source/cross_section_(geometry)) per cylinder is smaller. However, when engine speed increases, the second runner is opened, decreasing airflow speed, but increasing the airflow volume, better matching the engine's airflow needs at higher revolutions and improving top end power. With modified engines it may be desirable to have the T-VIS open earlier than stock, because modifications that improve an engine's power output may do so by increasing airflow per engine revolution, resulting in a high airflow at a lower rpm.[{{cite web|title=What is TVIS?|url=http://www.gtfours.co.uk/what/tvis/tvis.htm|publisher=www.gtfours.co.uk|accessdate=19 August 2015}}]
In addition to providing more air at higher engine speeds, the system also creates a swirl in the combustion chamber at lower engine speeds. The swirl makes for more efficient combustion, and is due to the asymmetric nature of the airflow with one intake runner closed.