Originally posted from ChrisD from celicatech.com Some additional information on TVIS: ------------------------------------ Info brought to you by: http://turbomr2.com/ Direct link to article: http://turbomr2.com/MR2/Reference/TVIS/TVIS.htm ------------------------------------ Toyota T-VIS System Variable Air Induction Systems Purpose of Variable Induction Systems Toyota engines have taken advantage of four valves per cylinder technology throughout the later half of the 1980s. This cylinder head and valve arrangement allows better engine performance at high rpm by improving the engine's volumetric efficiency. Enlarging the port area of the intake valves does little for engine breathing unless the intake manifold is enlarged as well By enlarging the intake manifold runners and plenum area, a greater volume of air is available to the intake valves at high engine rpm. At lower engine rpm, however, an enlarged intake runner has a negative effect on volumetric efficiency due to reduced air velocity at the intake valve. This characteristic causes a four valve engine to have a very healthy torque curve at high engine speeds but a comparatively weak one at lower engine speeds. The variable induction system is designed to give a four valve engine the best torque characteristics at both low and high engine speeds. This is accomplished by changing either the effective length or diameter of the intake runner through the use of an intake air control valve. This valve is activated by an ECU controlled Vacuum Switching Valve (VSV) and vacuum actuator. Toyota Variable Induction System (T-VIS) T-VIS System Components The T-VIS system is the fast variable induction system used on Toyota engines built for use in the U.S.A. The system is used on the 3S-GE, 3S GTE, and 4A-GE engines. The induction system consists of the following components: * Four individual intake air control valves supported on a common shaft * Vacuum actuator which rotates the shaft * ECU controlled Vacuum Switching Valve (VSV) * Vacuum storage tank T-VIS System Operation The intake manifold feed for each cylinder is divided into two separate runners. The main runner is provided for low speed operation while the other is provided as the variable induction runner. Each intake runner is purposefully designed to flow approximately half of the air volume required by the engine at full power. The variable induction runner is equipped with an intake air control valve. The main intake runner supplies air to the intake valves at low speeds. Intake air flows at high velocity due to the long and narrow runner design. The intake air control valve opens the variable induction runner when adequate engine rpm is reached, thereby providing sufficient air volume for high speed operation. This design makes it possible to maintain strong engine torque at both low and high engine rpm. The intake air control valves, installed between the cylinder head and intake manifold, are all closed simultaneously by the vacuum actuator when vacuum is applied. When vacuum is relieved from the vacuum actuator, the air control valves return to their fully open position. When the engine is running below the 4000 to 5500 rpm threshold, manifold vacuum from the vacuum storage tank is supplied to the actuator through the ECU controlled VSV. The vacuum storage tank is required as a vacuum reservoir to hold the intake as control valves open whenever the engine rpm is below the operating threshold but manifold vacuum is too low to hold the intake air control valves closed. When the pre-programmed rpm is reached, the ECU signals the VSV to switch vacuum away from the actuator and open an atmospheric bleed. This action causes the actuator to release the air control valves to their open position, allowing maximum air volume to enter the cylinders. T-VIS Vacuum Switching Valves (VSV) and Operating Strategies There are two different VSVs used for T-VIS control depending on application. The 3S-GE engine uses a VSV with a normally open valve. This VSV passes vacuum to the actuator when de-energized by the ECU. During low speed operation, the ECU keeps the VSV de-energized to keep the actuator applied, closing the air control valves. At high speed operation, the VSV is energized, blocking the vacuum supply to the actuator and bleeding any trapped vacuum. This allows the air control valves to open. The 3S-GTE, 4A-GZE, and 4A-GE engines use a VSV with a normally-closed valve. This VSV passes vacuum to the actuator when energized. When the engine is operating at low speed, the VSV is energized, allowing vacuum to apply the actuator and hold the air control valves closed. When the ECU parameters are met to open the air control valves, the ECU de-energizes the VSV to block the vacuum signal to the actuator and bleed trapped vacuum from the actuator diaphragm. 3S-GTE Fuel Judgment Strategy Using signals from the knock sensor, the 3S-GTE engine incorporates a fuel judgment strategy to control maximum turbo charger boost pressure and T-VIS operation. When fuel is judged to be of premium grade, the T-VIS system functions as outlined above. The ECU will de-energize the VSV when engine speed reaches approximately 4200 rpm. When fuel is judged to be of regular grade, however, the ECU operates the air control valves based on throttle position. When the IDL contact is closed (closed throttle), the VSV will be energized, allowing vacuum to pass to the actuator, closing the air control valves. When the IDL contact opens, the ECU de-energizes the VSV, blocking the vacuum supply to the actuator, causing the air control valves to open.