The main principle of this system is to be able to route engine exhaust through one or both turbo(s) in the most efficient manner possible, in order to achieve useful boost at both low and high loads.
At low engine RPM and load, all engine exhaust is routed through the #1 turbine in order to have boost at low rpms; this is called single mode. As engine load and RPM increase, the system will bring the #2 turbo online in a controlled manner, so there is no shock to the turbo or the engine, nor (in theory) any spikes or flat spots in the engine torque curve.
This is known as the transition mode. Finally when both turbos are online and boosting in parallel, this is called true twin turbo mode.
There are 4 sets of VSV’s (Vacuum Solenoid Valve), actuators, and control valves for the STTS. Each VSV allows stored air from a small accumulator (pressure tank) to pass to, or bleed from the actuators.
Each of the 4 control valves has an air operated actuator, and an ECU controlled VSV. Two of the control valves, the EGCV and IACV, have only two positions, either open or shut as their VSVs are controlled by On/Off signals from the ECU, while the other two, the EBV and WG, can be controlled to any position between 0 and 100%, as their VSVs are operated by PWM duty cycle signals from the ECU.
Following is a definition and description for each control valve:
Wastegate (WG):
This normally closed valve is located in a Tee of the engine exhaust, before the turbocharger turbines. When the wastegate is closed, all engine exhaust must pass through the turbo(s) to exit. When the wastegate is open, some exhaust can escape before the turbo(s) and will exit directly into the downpipe, cats, etc.
There is a control line connected directly from the turbochargers compressor discharge to the wastegate actuator, so whatever boost pressure is being made by the turbo(s) will be sent to the wastegate actuator. As boost pressure rises, so will the pressure in this line, which causes the wastegate to open.
As exhaust energy escapes by the opening of the WG, the turbos will slow down, and boost pressure will fall, which will cause the control line pressure to fall, and the wastegate to begin closing. By this inverse proportional action, the wastegate can control boost mechanically.
Mounted in the control line between the turbo and the WG, is the wastegate VSV. When this VSV is duty cycled by the ECU, it acts as a bleeder and reduces the pressure in the control line, which keeps the wastegate closed longer. The higher the duty cycle of the wastegate VSV, the more delayed the opening of the wastegate and the higher the boost.
If the wastegate VSV should fail, it will close and the wastegate will receive full manifold pressure, which will open it much sooner, reducing boost pressure. The configuration of this VSV in bleeder mode is also known as a fail-safe configuration.
Exhaust gas Bypass Valve (EBV):
This normally closed valve is located downstream of both the #1 and #2 turbines, but before the EGCV. At about 3500 rpm, the ECU duty-cycles the VSV for this valve causing it to open gradually. Normally, the exhaust from the #2 turbine is blocked by the closed EGCV, so when the EBV opens, there is now a path for a small amount of exhaust gas to flow through the #2 turbine and exit, this allows the #2 turbine to pre-spool.
This pre-spool smoothes the transition from 1 to 2 turbos, and cushions the shock of the EGCV opening. The EBV valve is sometimes confused for a wastegate, but it is located po the turbine wheels instead of in front of them, so it is ne a 2 nd wastegate.
Exhaust Gas Control Valve (EGCV):
This normally closed valve is located in the #2 turbine exhaust discharge piping, and it serves to block the exhaust flow through the #2 turbine. When this valve is closed, all exhaust flow must pass through the #1 turbine. At about 4000 rpm, and after the EBV has opened, the ECU energises the EGCV VSV to open the EGCV. This unblocks the discharge from the #2 turbine so exhaust gas can now flow unrestricted through the #2 turbine and out the exhaust system. This brings the #2 turbo up to full operating speed.
Intake Air Control Valve (IACV):
This normally closed valve is located in the intake system, just after the #2 compressor discharge. When this valve is in the closed position, boost is blocked from the #2 compressor, but more importantly, no backflow from #1 is possible which might cause #2 to spin backwards. Just after the EGCV is opened, the ECU energizes the IACV VSV to open the IACV. This allows the full boost pressure from #2 compressor to join with boost coming from #1 compressor and the system is now operating in true twin turbo mode.
There is also a mechanical 1 way reed valve within the same housing of the IACV, and in parallel with it, which allows boost from #2 to enter the common manifold if its pressure is equal to, or greater than the #1 boost during pre-spool and the initial opening of the EGCV.
Here is the sequence of events from single to true twin turbo operation.
- At idle the WG, EBV, EGCV and IACV valves are all closed.
- From 1500 to 3500 RPM, and low loads, the above valves remain closed, and the system operates only on the #1 turbocharger. (Single mode)
- Around 3500 RPM, the ECU will duty cycle the EBV open, and allow the #2 turbocharger to prespool. If/when #2 boost pressure is high enough, the mechanical reed valve will open, allowing #2 boost to join #1 boost in the common system. (Transition mode).
- As RPM and load increases, the EGCV will open, allowing more flow of exhaust gas from the #2 turbine. At about the same time, the IACV will also open, allowing the #2 compressor to flow into the intake system. Once the EGCV and IACV are both open, the two turbochargers are operating in True Twin mode, with (in theory again) equal exhaust and intake flows.
Boost pressure of the overall system is always controlled by the duty cycled VSV for the WG
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The Mark IV Toyota Supra was offered with two different engine options; a naturally aspirated 2JZ-GE, 3.0 litre, straight 6 with 220 horsepower and 210 ft/lbs of torque, and a 2JZ-GTE 3.0 litre twin turbocharged straight 6 with 320 horsepower and 315 ft/lbs of torque.
For many, part of the twin turbo Toyota Supra’s attraction comes from the easily upgraded power output. 450 — 500 horsepower can be had out of the stock turbos with a free flowing intake and exhaust system coupled with an aftermarket boost controller.
The MKIV Supra’s turbochargers operate in a sequential format where, at first, all of the exhaust gasses are routed to the first turbine for reduced lag. This results in increased boost and enhanced torque as low as 1800 rpm. Approaching 4000 rpm, the exhaust gasses are routed to the second turbine for a «pre-boost» mode, although none of the compressor output is used by the engine at this point. Around 4500 rpm, the second turbo’s output is added to the intake air, and both turbos operate in parallel. The sequential operation of the Toyota Supra’s turbos allow for improved low-end response.
The Mark IV Toyota Supra, although no featherweight, was extensively lightened compared to the previous generation. The use of aluminium for the hood, targa top, front crossmember, oil pan, and upper A-arms helped. As did dished out head bolts, a magnesium steering wheel, plastic petrol cover, and a gas injected rear spoiler. Despite having more features such as dual airbags, traction control, larger brakes, larger wheels, and larger tires, the MK IV Supra was at least 45 kgs (100 lbs) lighter than the outgoing model.
External review by: AJI
The MKiv Toyota Supra saw an extensive weight reduction diet over the previous version of this model. At a time when Supercars were being produced left right and centre, Toyota decided to get in on the act with the MKiv. Honda had already realeased the NSX and Nissan with the Skyline and the 300ZX, and with Europe and its choices of British, Italian and German high performance cars the Supra was fitted with a twin turbo setup and the export version hit the market with 326bhp as standard.
The export version of the Supra was the highest specification version of the Supra, this is strange as the Japanese home market versions are usually this way. So therefore this saw the export versions fitted with stronger steel turbos, bigger brakes, bigger fuel injectors, glass headlights, full leather interior, and generally more ‘toys’. They were all twin turbos and came in either 6-speed manual or 4-speed automatic. The export MKiv was generally over engineered, it therefore produces a car that can easily be tuned to the 450-500bhp area without the need to upgrade vital internal parts. The getrag 6-speed gearbox is a good strong unit and the 6-cylinder engine is a racey unit with plenty of torque.
The export Supras were tested and produced a 4.9sec time to get to 60mph. The top speed of the export versions is 180mph, and 6th gear if it were allowed would result in a top speed of 196mph if the car had the power to get there.
The Japanese home market versions of the Supra came with a large tick-box on the specification lists. But the two main models were the RZ (the twin turbo) or the SZ (the normal aspirated version). Later in the production line the GZ was released which was basically an RZ with all the extras ticked. And later still the Japanese home market version went through a facelift giving it the bigger brakes and other details which included a modification to the front bumper.
The engines between the J-spec and the export version has some considerably differences. The turbos were ceramic as opposed to steel, the fuel injectors were smaller and although the torque figure was still very coparable to the export version the bhp figure was slightly down.
The J-specs had a top speed of 170mph due to the slightly lower bhp figure. And, they got to 60mph in 5.1sec. The automatic versions always had slightly worse performance figures due to the power loss through the transmission, but the auto box is still considered one of the best on the market.
The Euro-spec version of the export model saw the inclusion of a bonnet vent to aid cooling to cylinders 3 and 6, the Japanese home market and US export version never had a bonnet vent. Some home market Supras appeared with no rear spoiler, this is down to the tick-boxes on the spec. list.
YEAR OF SPECIFICATIONS: 1993 – 2002
MOTOR: 3.0 litre in-line 6-cylinder twin turbocharged
PŘENOS: 6 speed manual or 4 spped auto, RWD
MAXIMÁLNÍ RYCHLOST: 160 h.
0-60 MPH: 5.1 sekund
KOŇSKÁ SÍLA: 326 k při 5,600 ot./min
HMOTNOST: 1,565 kg / 3,450 liber
