T-VIS - The real explanation
T-VIS stands for Toyota Variable Induction System. It came on two of Toyota's Sports-type Twin Cam engines, the 4A-GE (Corolla GT-S, MR2) and the 3S-GE (Celica). Here's how it works as described by Toyota*...
>> This system's intake passages are divided in two and a valve is placed in one side. Then, depending on engine rpm speed, the valve is opened or closed, which changes intake efficiency and improves performance across the rpm range. At low-to-mid rpm, the valve is closed. This effectively narrows the intake passage, which increases the speed of the air flow. Greater air flow speed results in greater volumetric efficiency, which boosts torque output at low-to-mid revs considerably. At high rpm, the valve is open, which reduces resistance within the intake passage and allows full development of the high-rev power potential inherent in the 4-valve configuration. <<
*info gathered from "Toyota Engine Technology"
Toyota part# PR-E-8905
printed in Japan 1989
T-VIS was used only on the first model of the 4A-GE (bluetop), being in all Corolla GT-S' (AE86's) and MR2's (from 85-87), as well as the FWD Corolla GT-S up until 1990.
For the 4A-GE's which are T-VIS equipped, the system will open the intake valve at approx. 4400 rpm. A noticeable <glitch> is usually heard when the T-VIS kicks in. It's more noticeable when a free-flowing aftermarket intake is installed.