GT86 press kit: Naturally aspirated sports engine developing 100 din hp per litre

During development of the GT86, Chief Engineer Tetsuya Tada established a unique combination of performance characteristics for the vehicle's powertrain. Those characteristics embodied a low centre of gravity, naturally aspirated, high-revving sports engine capable of developing 100 DIN hp per litre.

The solution to these potentially disparate requirements lay in combining Subaru's newly-developed boxer engine with Toyota's latest direct fuel injection system to create the world's first horizontally opposed engine with D-4S (Direct injection 4-stroke gasoline engine) technology.

However, Tada's reluctance to incorporate turbocharging still presented a considerable engineering challenge, there being no naturally aspirated, direct fuel injection engine revving to 7,400 rpm in existence before the development of the GT86…

World's First Horizontally Opposed Engine with D-4S Technology

To Subaru's newly developed, horizontally opposed, 1,998 cc, four-cylinder boxer engine, Toyota has added its unique, next-generation D-4S technology for the first time.

With separate twin injectors for both direct and port injection, D-4S performs both high-pressure direct injection into the cylinder and conventional intake port injection, or direct cylinder injection only, in accordance with engine speed. Hence, intake air and fuel are mixed evenly at all engine speed ranges, increasing throttle response, power and torque over a wide range of engine speeds without sacrificing fuel efficiency and environmental performance.

With a namesake 86 mm x 86 mm bore and stroke -a 'square' engine combining fuel economy with well balanced rotation, the GT86's engine block and 16 valve, DOHC cylinder heads have been newly developed to achieve both high engine speed and a high compression ratio of 12.5:1.

The piston mass has been minimised and its length optimised in the interests of engine speed. The crest shape has been optimised to facilitate direct injection and a high compression ratio. A crank pin with a 50 mm diameter has been adopted to increase rigidity at high engine speeds. And the connecting rod shape has also been optimised to help realise the high engine speeds required.

The GT86's boxer unit employs a front-mounted air intake layout to lower the engine position, with a low intake manifold to further lower the centre of gravity. In the interests of lowering the engine position as far as possible within the GT86, it was also decided to completely revise both the exhaust system and the oil pan design.

The vertical measurements of the exhaust system have been minimised to lower the engine position, and the middle pipe layout optimised to allow for the lowest possible vehicle height.

The system employs a dual pipe and muffler layout for optimum efficiency, with the manifold layout and the adoption of a large diameter pipe designed to reduce exhaust pressure losses. The system further features a front hexagonal cell catalyst and rear low pressure loss catalyst to enhance emissions performance.

Toyota engineers designed the oil pan to be as compact as a dry sump. The final design made it possible to lower the engine centre of gravity simply by making the oil pan thinner. A unique internal fin shape was also developed for the pan, allowing them to scoop up oil to promote thorough lubrication.

The new flat four revs freely to 7,400 rpm, at which point a gear shift prompt light flashes in the driver's instrument binnacle. Over-revving the engine introduces a well-judged, soft rev limiter rather than a sudden engine cut out.

Equalising the intake and exhaust timings has given the engine an extremely pleasant, smooth-revving sound. But Toyota engineers were concerned that various countries' strict noise regulations would make it difficult to increase the exhaust note volume as much as might be desirable for the driver to hear in a sports car.

In response, the engineers decided to bring the sound directly into the car the first time a system of this type has been used in a Toyota vehicle. When intake pulses hit the sound creator, a damper resonates at certain frequencies to optimise the intake sound. The optimised sound is then channelled directly into the cabin via a rubber hose. A stress-free, soft intake sound is produced under slow acceleration, while a true sports car intake sound is generated under full throttle acceleration.

6-speed Manual or Automatic Transmissions

Being the part of the car -along with the steering- most directly linked to the driver, a meticulous focus on the transmission has been fundamental to the successful development of the GT86. The flat-four engine may be mated to either a six-speed manual or a six-speed automatic gearbox.

An unprecedented number of engineers from Toyota, Subaru and Aisin AI worked with a researcher specialising in gear shift feel to significantly upgrade an existing Toyota 6-speed manual transmission.

It was unanimously agreed at the outset that, in a car created for manual transmission purists, the gearbox should feature a short shift, 'flick of the wrist' lever travel. The final transmission required the creation of five separate prototypes and the use of some 85% of newly designed components.

It offers quick, precise shifting through closely stacked ratios via the highly engaging action of a tactile, short-throw lever. Triple cone synchromesh has been adopted for 1st, 2nd and 3rd gears, and a highly rigid-feeling, floating construction offers a smooth yet firm operation without unpleasant stiffness when the gears engage, not only when the vehicle is being driven, but also when parked.

A new, Toyota designed, 6-speed automatic transmission combines direct response and steering wheel-mounted shift paddles with the world's fastest torque-converter AT shifting speed of only 0.2 seconds.

It incorporates high response gear change control, supporting sporting driving in M (manual) range and Sport modes by prioritising responsiveness during upshifts for extremely fast gear changes. Adding sensory appeal to sporting driving, blipping downshift control automatically revs the engine on downshifts, matching engine speed to the mesh speed of the next gear for extremely smooth and rapid downshifts.

Even when driving in D (drive) range, the transmission allows for the temporary manual override of the gearbox via paddle shift control. The Sport mode offers faster shift response times and activates lock-up control from low speeds providing a more direct gear shift feeling. Snow mode automatically adjusts throttle response to control torque delivery characteristics on snow and other slippery surfaces, maximising vehicle grip and traction.

Power is distributed to the rear wheels via a Torsen limited slip differential with a final drive ratio of 4.1, optimising traction and grip under all driving conditions.