By Frank Balogh
The MR2 ‘s high-revving 1 .6-liter, 16-valve, twin-cam EFI engine (4AGE) adopted a Roots-type - supercharger (4A-GZE) with an air/air intercooler to boost the vehicle’s horsepower rating by roughly 30%, from 112 hp at 6,600 rpm to 145 hp at 6,400 rpm. Redline tops at a whining 7,500 rpm.
The supercharger complements the engines inherent strength at the top of the rpm band with a substantial improvement in low-end performance. Torque is also increased almost 45% (!) from 97 ft-lbs. at 4,800 rpm to a healthy 140 at 4,000 rpm. in addition to the supercharger, valve timing, new two-aperture fuel injectors, and revisions to the manifold and camshaft design are also responsible for additional power output. Add it all up and what you get is a slick little performance that yields 0-60 times of about 7 seconds, while braking from 60-0 mph stops the clock at about 3 seconds. These times were run under ideal conditions. Under normal conditions you can expect 0-60 mph times clocked in the low 7’s.
A Toyota-designed Roots-type supercharger, like a turbocharger, forces air into the engine to increase combustion pressure and power. But unlike the exhaust-driven turbocharger, the supercharger is belt driven off the crankshaft via a V-ribbed belt and crankshaft pulley supplying boost in direct relationship to crankshaft rotation resulting in almost instant throttle response. In fact, when comparing 0 to 60 and quarter-mile times between the normally aspirated and blown version, times drop well over a full second for both runs.
To eliminate the supercharger’s inherent drag on the engine at low speed and to maintain good fuel economy, Toyota has developed an electromagnetic clutch that is engaged and disengaged by a signal from the ECU and working in conjunction with the air-bypass system (ABV-Air Bypass Valve) power is provided as demands vary.
During low and mid-range engine operation, when power is not required, a computer-controlled clutch on the supercharger’s input shaft is disengaged, idling the supercharger unit. Meanwhile, the ABV opens to redirect intake air around the supercharger.
Now during acceleration and high engine load, the computer closes the air-bypass valve and engages the electromagnetic clutch for full supercharger benefits. Power is transmitted from the engine crankshaft pulley to the V-ribbed belt and the magnetic clutch, and finally to the lower rotor shaft. The upper and lower rotor shafts are geared together. The two rotors turn in opposite directions and move air between the housing and rotors as they rotate. Air is pumped out four times per rotor revolution.
The ABV is fitted to the number 1 air inlet duct (VSV). This opens when the ECU turns the VSV off, allowing engine vacuum into chamber “A.” When vacuum in the VSV is cut off by the ECU, atmospheric pressure closes the valve, When the VSV valve is open, most of the inlet air bypasses the supercharger and is sucked into the intake manifold through the ABV. When the supercharger pressure rises too high, the pressure opens the ABV valve and allows part of the supercharging air to bypass to the air inlet, effectively controlling maximum boost of the supercharger.
At steady engine operation over the 4,000 rpm mark, the air-bypass valve varies the volume of intake air into the supercharger to regulate overall boost pressure. According to our sources, the supercharger can provide up to 10.2 psi of boost, depending on load. I’m sure that with a little “tinkering” you could achieve the same or even more boost under less load.
During deceleration, the electromagnetic clutch disengages below the 4,000 rpm line, which discontinues supercharging. A light on the dash panel illuminates when the supercharger clutch is engaged and active.
The TCCS (Toyota Computer Controlled System) for the supercharged MR2 4A-GZE engine is based on the normally aspirated 4AGE engine, but the system components have been modified for the supercharger. Among the main components, the following items are different to those installed in the 4A-GE engine: Air flow meter, distributor, fuel injector, cold start injector, knock sensor, and oxygen sensor.
There are two injection holes for each injector that match two intake ports and are located at the end of the injector. The fuel injector has a ball type needle valve to reduce the possibility of injector clogging. Injection volume is 223 cu.in./min (365cc/mm) per injector.
The knock sensor is mounted on the left side of the cylinder block and incorporates a piezoelectric ceramic element. If knocking is present in the engine, the piezoelectric element resonates and generates a voltage in proportion to the knocking strength. This signal is sent to the ECU for further analysis and then the ECU judges whether the knocking strength is at one of three levels, strong, medium, or weak, according to the strength of the knock signals. It then changes the supplementary ignition advance angle accordingly. In other words, if knocking is strong, the ignition timing is retarded a lot, and if it is weak, it is retarded a little. When engine knocking is not present, the ECU stops retarding the ignition timing and then advances the timing by fixed angles a little at a time.
The supercharged MR2 has a selective operation mode (Fuel Control System, or FCS) that maintains high estimated fuel-economy ratings of 23 mpg/city and 28 mpg/hwy and also helps provide quiet engine operation at cruising speeds. Premium gasoline is recommended for best performance, but if it’s unavailable, a fuel control switch on the instrument panel can adjust engine timing to compensate for lower octane ratings.
The FCS control system is designed to switch the ignition advance angle according to the gasoline octane- rating (premium or regular). Switching is accomplished by the driver by operating the fuel control switch located on the instrument panel. The ECU stores two types of data for the basic ignition
Also contributing to quieter supercharger operation is a specially designed tapered air inlet that gradually compresses air from the supercharger to the engine. This design makes this particular supercharger less noisy than conventional superchargers which “squeeze” air into the engine. Toyota’s choice of a Roots-type blower was due to the design’s reputation for reliability and long life.
An air/air intercooler is utilized to cool the charge air by up to 140 degrees F for increased charge density, as well as to reduce the chances of detonation. The intercooler is mounted above the engine and directly beneath a new louvered engine cover that maximizes air flow to the engine.
Other modifications unique to the supercharged MR2 engine to compensate for the additional power that supercharging provides include: • Forged aluminum pistons that replace the die-cast aluminum originals. The forged pistons are lighter, stronger and more
heat resistant. • The compression ratio has been reduced from 9.4:1 to 8.0:1. • The cylinder-head gasket incorporates a steel wire around each cylinder bore to provide additional sealing against increased pressures.
Increased torque from the supercharger necessitated the introduction of a new wide-ratio 5 speed manual transmission, while strengthening of the current 4 speed automatic filled the bill. The new manual transaxle also is accompanied by a larger clutch, up from 8.5 inches to 9 inches. In addition, for improved braking performance, supercharged models adopted a new dual-diaphragm tandem brake booster to accommodate more severe braking conditions and situations.
New to all MR2 models is an engine-mounted device that aids hot restarting. The high-temperature fuel-pressure control sensor boosts fuel pressure during hot restarts to compress residual vapor in the fuel lines or injectors.
The supercharged MR2 is available only with the T-bar roof, which has about the best designed components that I’ve had to deal with. Unlike the Toyota Supra Sport Roof, the MR2 T-bar roof release system is a breeze to operate. Removal and installation of the roof halves is very straightforward, with storage provided behind the driver and passenger seats. The roof halves can also be locked by the ignition key to prevent theft.
Cockpit accommodations are visually pleasing, as well as being “user friendly.” When one is in the driving position, the interior seems to be much more spacious than it can possibly be. Leg-room and headroom is more than adequate for a six-footer, with no problems at all. Seating is 7-way adjustable with an optional leather interior that has to be seen to be appreciated. All controls fall within easy reach due to the ergonomic interior design and layout, thus reducing driver fatigue.
The only interior component that was missing was a boost gauge for accurate monitoring of boost pressure. About the only other appointment that would be perfect for touring would be the installation of a CD player and lots of amplification. Comfort. That about best describes the interior in one word.
One thing that deems a mention here is the ultra-wide, 309-degree field of vision from inside the cockpit. For a small car of this type, a wide field of view is a must.
The supercharged MR2’s mid-engine design yields a 42%158% front-to-rear weight distribution for
The full potential of the MR2 will be realized for the serious enthusiast soon by the likely release of aftermarket upgrade components ranging from larger intercoolers, exhaust systems, suspension refinements and as soon as the system is interpreted proficiently, a way to turn the boost up a little. Any combination of this variety of bolt-ons should result in an even stronger performer for your driving enjoyment. From what we understand, in regards to boost, Toyota went on the conservative side of high-performance in order to deal with different grades of fuel availability across the nation.
Again, driving the supercharged MR2 is an honest pleasure. With its strong and wide power range, you will find that even in stock trim, performance levels are quite admirable. •