TVIS - Toyota Variable Induction System
In this article we address the specifics of the TVIS system, we must cover a few engine design and performance basics. There are two main design concepts behind a variable induction system.
1) Runner Length Variation: The design calls for two (or more) unequal paths of airflow that could be actuated depending on engine RPMs. This design will often benefit the air/fuel admixture or 'swirl' patterns in the intake manifold and combustion chamber. When at a low RPM, the speed of airflow is increased by routing the air through a longer path with great restriction. Whilst at high RPM the shorter and less restrictive route opens so that a greater load of air/fuel can enter the chamber. Many new engines are designed so the manifolds air passages are connected to separate intake valves, allowing the shorter path to be excluded by disabling the intake valve itself.
2) Pressurization: Intake paths can be designed to have a pressurizing effect. A variable intake can be created by implementing a restriction or diverting device. With two or more pre-designed pressurized "hot spots", a switchable device can allow both 'hot spots' to be utilized.
The 4AG engine in the AE86 came with a system called T-VIS, you may notice the badge on the top of your intake manifold. T-VIS stands for Toyota Variable Induction System; the essence of the TVIS system is to change the rate of airflow depending on manifold pressure. The TVIS design falls under the pressurization design of variable induction systems. The TVIS system runs a sandwich plate between the intake manifold and engine's cylinder head. The plate itself is matched to the intake manifold runner design the TVIS plate includes four butterflies that open and close, thus restricting airflow out of the manifolds runners when closed. The butterflies are managed by an electrical solenoid and a vacuum diaphragm that is controlled by the ECU. The ECU sends a signal at 4,400rpm to allow the manifold pressure to engage the diaphragm opening all 8 runners. While an increase in audible induction noise will be noted, a major power gain will not when the butterflies disengage. The TVIS system maintains a strong low RPM power output and does little to aid in the increase in power at high RPM. With a working TVIS system the lower RPM band will have created a much more power than a 4AG with TVIS removed or disabled.
Will engine modification effect TVIS? Should it be removed?
The most common question in relation to TVIS: If/when is it worth removing?
The modifications and changes that you make to your motor will all have some effect on TVIS, at what point will it become useless or even detrimental to have TVIS installed varies. The general rule of thumb with regard to TVIS that its worth keeping around until you have installed a major power adding component such as; high duration/lift cams, turbo, supercharger, ITBs and others.