Technical Highlight of 911

The long-live 901 engine

From 2.0 to 3.8: the power of Nikasil

It is hard to believe an engine could be enlarged so much without even altering the distance between bore centres. The biggest difficulty faced by engineers was how to squeeze more capacity out of the unchanged dimensions. According to the original calculation - although Porsche had already built in considerable potential for development into the original design - it was expected to be stretched to maximum 2.7 litres only. Anything larger than 2.7 litres required a bore so large that the cylinder wall would have become too thin to be reliable. As the 911 was designed with endurance GT racing very much in mind, and admitted, strong reliability was always one of the core valves of Porsche’s philosophy, it seemed that the Type 901 engine would have never grown to more than 2.7 litres.

However, a breakthrough was made in the ’73 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 unit’s 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 doesn’t 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 RS’s 70.4 mm to the 3.8 RS’s 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.

Pioneering Turbocharging

Although turbocharing had been appeared in Chevrolet Corvair and BMW 2002 turbo in the late 60s, Porsche 911 Turbo was unquestionably the first to cure the turbo lag problem and made turbocharging practical for road use.

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.

Problems

However, no matter the Corvair or the 2002, they failed to make turbocharging practical for road use. The main obstacle was turbo lag. Before low inertia turbine appeared, turbines were very heavy, thus could not start spinning until about 3,500 rpm crank speed. As a result, low-speed output remained weak. Besides, since the contemporary turbocharging required compression ratio to be decreased to about 6.5:1 in order to avoid overheat to cylinder head, the pre-charged output would be even weaker than a normally-aspirated engine of the same capacity !

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.

Porsche's Breakthrough

Like BMW, Porsche started developing turbo for the purpose of motor racing. In the early 70s, in order to fight with the 8-litre Chevy in Can-Am, Porsche created the mighty 1000 hp turbocharged, flat-12 engined 917 racer, which soon dominated the whole world. Successful experience led to the application of turbocharging to 911 Carrera RSR Turbo GT racer, which finished 2nd in LeMans. So far, Porsche made turbocharging became the dominating force in GT racing.

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.

Turbo 3.0

This technology was first introduced to the Turbo 3.0 of 1975. The Type 930/50 engine was derived from the RS 3.0, with compression ratio reduced to 6.5:1 rather than 8.5:1, a KKK turbocharger generated boost pressure up to 0.8 bar (governed by a mechanical waste gate). Like the RS, it employed forged pistons, but the fuel supply was changed to cleaner (if less powerful) Bosch K-Jetronic mechanical injection while electronic ignition was first introduced. Power and torque jumped to 260 hp at 5500 rpm and 254 lbft at 4000 rpm respectively, compare with the RS's 230 hp at 6200rpm and 203 lbft at 5000 rpm. The turbo engine was lazier to rev but performed a lot stronger since middle rev, hence providing superior performance in an effortless way.

Introduction of intercooler

The 3.3-litre version of the turbocharged engine, Type 930/66, superseded the Turbo 3.0 in 1978. It raised output to 300 hp and 303 lbft. The increased power thanks to the use of intercooler between the compressor and the engine, which was located under the rear spoiler. It reduced the air temperature for 50-60°C, thus not only improved the volumetric efficiency (in other words, the intake air became of higher density) but also allowed the compression ratio to be raised to 7.0:1.

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.

The M64 series: pushing to the limit

When the M64/01 engine appeared in the 964 Carrera 4 in 1989, its origin was already 25 years old. The distance between bore centres was never changed, but Porsche managed to increase the bore to 100 mm and the stroke to 76.4 mm (once again employed Nikasil treatment). That resulted in a displacement of exactly 3600 c.c.. Power and torque increased from 231 hp / 210 lbft to 250 hp / 228 lbft, even though now the catalytic converter was standard and emission regulations had been tightened.

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:

2.2 kg lighter crankshaft;

Plastic intake manifold;

increase valve overlapping, higher lift;

quieter, all-new timing chain tensioners;

drilled and sodium-filled intake valves, which were lighter and increase rev by 200 rpm;

ceramic exhaust port liners, which reduce head temperature by 40°C and made sodium-cooled exhaust valves unnecessary.

M64/05 engine for 993

Modified from the M64/01 engine, with the following changes:

Lightened con-rods from 632 g to 520 g per piece;

Lightened pistons from 657 g to 602 g per piece;

10 g lighter valves;

Redesigned cam box;

Enlarged intake port;

Freer exhaust system by means of larger silencer and catalyst;

98 RON fuel instead of 95 RON;

Reinforced crankshaft thus made vibration damper unnecessary;

Overall speaking, the engine was 6 kg lighter than the predecessor and rev higher, thus generated more horsepower and torque - 272 hp and 243 lbft.

M64/21 engine with Varioram

The 993 engine was updated in 1995, mainly with the introduction of Varioram. It was a 3-stage variable intake system based on the existing 2-stage resonance intake. The space-engaging system can be seen easily above the engine (see picture).

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 251 lbft.

Illustration to Varioram

Below 5,000 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.

M64/60 - welcome twin-turbo

Ignoring minor revisions, the last new engine for the 911 was the M64/60 twin-turbo engine used in the 911 Turbo of 1994. This is not the first turbo based on the 3.6-litre M64 engine - it was the 3.6 single turbo which served the 964 Turbo of 1993. However, unlike its predecessor, it was an advanced design (if not ground-breaking) rather than a stop-gap design, employing a sophisticated engine management system including electronic boost control. In other words, the waste gate was governed by computer, allowing different max. boost pressure for different rev. For instance, a maximum 0.94 bar was available for 3,500 rpm, 0.6 bar for 5,200 rpm and 0.75 bar for 6,500 rpm. This made the engine extremely responsive and linear.

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 959’s 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 engine’s twin-spark for simplicity.

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