The engine of the Porsche 917

One of the mightiest race engines of them all

by Julian Edgar

Click on pics to view larger images

The Porsche 917 gave the iconic company its first Le Mans win back in 1970 and 1971. Regarded by many as one of the prettiest race cars ever built (especially in its low drag long-tail version), the 917 is also famous because of its engine. With a power of up to 1100hp (some say 1200hp) from its approximately 5 litre capacity, the flat 12 was air-cooled and ran twin turbos.

So let’s take a look at it, relying on pics snapped in the Porsche museum in Stuttgart, and on one of the best technical descriptions you’ll ever find in The Porsche Book, by Lothar Boschen and Jurgen Bath (Patrick Stepehens, 1978).

The engine started life with a 4.5 litre capacity. Designated Type 912, it was an air-cooled, four stroke, flat-12 with twin overhead camshafts per bank.

Fascinatingly, the power take-off was in the centre of the crank, rather than the conventional position at the end of the crank. At the centre, a 32-tooth cog meshed with a 31-tooth cog in the power take-off shaft. This approach was adopted to overcome the torsional vibrations that would have been present if the crank power take-off had been in the conventional position at one end. Another advantage was that the centre position coincided with a node of fundamental vibration, meaning that at this point torsional vibration was effectively zero.

Accessory drives were also taken off from this point, reducing transmitted vibrations to other components.

Two different types of crank were used. One used a single forging, into which the central gear was cut. The other was made in two pieces, welded to the hardened power take-off gear in an electron ray welding machine. Each crankpin carried two con-rod big-ends, mounted side by side. Eight main bearings were used.

The 130mm long con-rods were made from titanium, with the same material also used for the big-end bolts. The magnesium alloy crankcase was split vertically. The cylinders were forged aluminium, with machined fins and chrome-plated (later Nikasil) bores.

The heads, one per cylinder, were aluminium sand castings. They used two valves (titanium intake valves on later engines) and the heads also featured twin sparkplugs.

The engine used a vertical axis, fibreglass cooling fan 330mm in diameter. At peak power revs of 8400 rpm, the fan moved 2400 litres of air per minute, with 65 per cent of this flow cooling the cylinder heads and the remainder cooling the cylinders. The fan absorbed 17hp.

Fuel injection was by a mechanical, 12-plunger injection pump developed by Bosch. Fuel passed through equal-length nylon tube to the injectors. Ball-mounted slide throttles were used in these naturally aspirated engines (this changed to butterflies when the engines were turbo’d).

Twin distributor ignition systems were used, with one distributor at the front and the other at the back of the engine. Equal firing intervals of any group of three cylinders occurred (engine firing order: 1-5-12-3-8-6-10-2-7-4-11).

No less than seven pumps were used in the dry sump oiling system. A thermostat (opening at 90 degrees C) controlled flow through an oil cooler. An 860W alternator was fitted – for night racing, a second was fitted.

In this form, the engine developed around 600hp – but a lot more was to come… turbos!

The first trials of the turbocharged version used a 4.5 litre capacity, from which 850hp was obtained. However, for Can-am racing, a 5.4 litre version was developed that produced 1100hp at 7800 rpm, and a peak torque of 820ft-lbs. Incredibly, the engine, complete with twin turbos, weighed only 285kg.

So what engineering differences were incorporated into the turbo version? The power take-off shaft was made from steel, 24mm in diameter. The cooling blower was geared-up to give a flow of 3100 litres/minute, which increased the power absorbed by the fan to 31hp. Compression ratio was lowered dramatically – from the naturally aspirated engine’s 10.4:1 to just 6.5:1.

The turbo engine had different intake cams, but the exhaust cams remained the same. The equal firing intervals were retained, allowing the engine to be treated as two separate six-cylinder engines, each fed by its own turbo. (However, the plenums of the ‘two engines’ were linked.)

The turbos were commercial diesel truck units, modified only in using ball rather than sleeve bearings. Boost varied from 20 to 22 psi, with the non-intercooled intake air having a temperature of up to 150 degrees C. Exhaust gas entry temperature was approximately 1000 degrees C.

The fuel injector pump had increased diameter plungers to pass more fuel, and the system incorporated boost pressure compensation. Incredibly, the ignition timing was fixed!

The Porsche 917’s turbo engine remains one of the most incredible race car engines ever built. But it’s also very much a creature of its time - mechanical fuel injection, fixed ignition timing, two-valve heads, 6.5:1 compression and truck turbos. And even air cooling…

But what an engine!

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