However, a breakthrough was made in the í73 Carrera RS 2.7. It introduced Nikasil technology to get rid of the original Biral cylinder, hence allowing the bore to be grown from the original 2.0-litre unitís 80 mm to as much as 95 mm while still had a sufficiently thick cylinder wall. To understand that, we must have a little bit explanation to the original cylinder design.
Since the first 911, it used so-called "Biral" cylinders, which is basically a cast iron cylinder liner with aluminium air-cooling fins casting around. Why not all-aluminium? because the pistons were also cast aluminium. It is commonly known that the contact between two aluminium surfaces always result in higher friction and wear than the contact between aluminium and iron. Therefore an all-aluminium engine without special treatment is always infeasible. Biral cylinders were employed to solve this problem.
As the Biral cylinder has two layers of different materials made in casting, the cylinder wall is inherently thicker than a pure aluminium cylinder yet doesnít provide superior mechanical strength. Instead of cast iron liner, Nikasil treatment coats a layer of Nickel-silicon carbide, usually by electrolytic deposition, to the inner surface of aluminium cylinders. Since Nikasil layer generates even less friction than cast iron liner, revability and power are both enhanced. Moreover, it is only a few hundreds of a millimetre thick, therefore the bore can be enlarged significantly. Porsche had already tried this technology in the 917 racing car successfully before applying to the 911 RS 2.7.
This was only the beginning. In fact, the Nikasil gave the engine a second phase of life, enabling the bore to be increased to 102 mm (thus displaced 3746 c.c.) eventually. Of course, the stroke was also increased from the 2.7 RSís 70.4 mm to the 3.8 RSís 76.4 mm, thus involving some revisions to crankshaft and con-rods. The magnesium crankcase used since the 2.2-litre had to be changed back to the heavier aluminium one for the advantage of strength.
The production 2.7-litre unit once discarded the Nikasil treatment and in favour of a cheaper arrangement - pure aluminium cylinders matched with iron-coated aluminium pistons, which was just a reversal pair of the original Biral cylinder / aluminium piston. However, as Nikasil had superior power advantage, it was adopted again since the 3-litre engine appeared.
The advantage of turbocharging is obvious - instead of wasting thermal energy through exhaust, we can make use of such energy to increase engine power. By directing exhaust gas to rotate a turbine, which drives another turbine to pump air into the combustion chambers at a pressure higher than normal atmosphere, a small capacity engine can deliver power comparable with much bigger opponents. As a result, engine size and weight can be much reduced, thus leads to better acceleration, handling and braking, though fuel consumption is not necessarily better.
Turbo lag can cause trouble in daily driving. Before the turbo intervenes, the car performs like an ordinary sedan. Open full throttle and raise the engine speed, suddenly the power surge at 3,500 rpm and the car becomes a wild beast. On wet surfaces or tight bends this might result in wheel spin or even lost of control.
Besides, turbo lag ruins the refinement of a car very much. Floor the throttle cannot result in instant power rise expected by the driver - all reaction will appear several seconds later, no matter acceleration or releasing throttle. You can imagine how difficult to drive fast in city or twisted roads.
Next step was to transform turbocharging for road use. As we have learned, turbo lag was the biggest difficulty preventing turbocharging technology from being practical. To solve this, Porsche's engineers designed a mechanism allowing the turbine to "pre-spin" before boosting. The secret was a recirculating pipe and valve: before the exhaust gas attains enough pressure for driving the turbine, a recirculating path is established between the fresh-air-charging turbine's inlet and outlet, thus the turbine can spin freely without slow down by boost pressure. When the exhaust gas becomes sufficient to turbocharge, a valve will close the recirculating path, then the already-spinning turbine will be able to charge fresh air into the engine quickly. Therefore turbo lag is greatly reduced while power transition becomes smoother.
In 1983, the 3.3 Turbo was upgraded to Type 930/66, which employed a more sophisticated KE-Jectronic electronic injection and improved ignition. The result was increased torque to 318 lbft although peak power remained unchanged.
The introduction of turbocharger lifted the 911 into the league of supercars. Between 1978 and 85, the 3.3 Turbo was the fastest accelerating production car in the world, beating all expensive opponents from Ferrari and Lamborghini.
Apart from the increase of capacity, most power came from the higher compression ratio, which was 11.3:1 compared with the previous 10.3:1. This was made possible by the introduction of twin-spark per cylinder and knock sensor. The former alone contributed to 10 hp and 3% reduction in consumption, thanks to more efficient burning. The latter was attached to each bank of cylinder and detected the shock wave resulting from knock. From the crankshaft angle, the advanced Motronic engine management system calculated in which cylinder the knock took place, and then retarded ignition in that cylinder. Therefore, the increase of compression was achieved without requiring higher Octane fuel.
The M64/01 engine also introduced
a new "resonance" variable intake system which boost mid to high rev efficiency.
Each bank of cylinders was fed by a common plenum chambers through separate
pipes. The two plenum chambers were interconnected by two pipes of different
diameters. One of the pipes can be closed by a valve controlled by engine
management system. The firing order was arranged such that the cylinders
breathed alternately from each chamber, creating pressure wave between
them. If the frequency of pressure wave matched the rev, it could help
filling the cylinders, thus improved breathing efficiency. As the frequency
depended on the cross-sectional area of the interconnecting pipes, by closing
one of them at below 5,400 rpm, the area as well as frequency reduced,
thus enhanced mid-rev output. At above 5,500 rpm, the valve opened and
increased high-speed efficiency.
| Other changes included:
The system added six long pipes for low-speed breathing, as longer intake manifolds always lead to lower frequency of air mass thus serve better for low rev cylinder filling. Below 5,000 rpm, only the long intake manifolds were used for breathing, thus resulted in higher torque at such rev. Between 5,000 and 5,800 rpm, the original resonance intake system with short pipes also intervened, but one of the interconnected pipes was closed so to provide better mid-range output. At above 5,800 rpm, both interconnected pipes of the resonance system were opened thus resulted in higher resonance frequency, and of course better filling at such rev.
Besides, the M64/21 engine
also employed slightly larger valves. The output was raised to 285 hp and
rpm (left A and top right) : long pipes; resonance intake disabled.
5,000-5,800 rpm (left B and middle right) : long pipes plus short-pipe resonance intake, with one of the interconnected pipes of the resonance intake closed.
Above 5,800 rpm (left C and bottom right): long pipes plus short-pipe resonance intake, with both interconnected pipes of the resonance intake opened.
The advanced engine management enabled a rather high 8.0:1 compression ratio to be realised. Unlike Porsche 959, the twin-turbo of the 911 was arranged operated in parallel rather than sequentially. More accurately speaking, each turbocharger was driven by exhaust gas from one bank of cylinders, with individual waste gate. The pressurised fresh air from the two turbines combined together and served all six cylinders. Due to the advanced boost control and 750 c.c. more displacement, the 911 engine actually felt more responsive and linear than the sequential-turbo 959. In torque, the 911 also beat the 7 years older 959: 398 lbft of torque versus 369 lbft, no wonder 4-wheel-drive was compulsory in this Turbo. Ultimate power, however, was less impressive. It was not until the final form, Turbo S, that the 911 can level the 959ís 450 hp output.
To cope with the new found output, the twin-turbo got reinforced con-rods and hollow valves cooled by natrium. Like the 3.6 single-turbo, it had single-spark instead of the naturally aspirated engineís twin-spark for simplicity.