1969 Porsche 912 4.5-litre Flat-12
1969 Porsche 912 4.5-litre Flat-12: The car was the 917 and its engine designation was type 912. Several aspects of the engine were ‘givens’ from the start. One was that it would be air-cooled as “we have never lost the air”. With air cooling went the opposed horizontal layout, which deployed the cylinders receptively to the air stream. Equally straightforward was the decision to make the engine a 12, composed of the same cylinder units used in the successful 908 flat-8, keeping the same bore and stroke of 85*65mm giving the 912 4494cc.
Its crank was like that of an in-line 6, with each throw rotated at 120 degree to its neighbor and each rod-journal carrying 2-con-rod big ends. The center distance between cylinders was 118mm except for the centre pair of cylinders on each bank, the same spacing that had been used for both the 901 and 911- 6 and the 908-8.
Steel backed multi-layer bearing shells were used for both the mains and the rod bearings. The 912’s forged titanium con-rods were similar to those used successfully in other Porsche racing engines and were the same length, 130mm, as those of the 901 and 911 and 908. Rod-bearing journals were52mm. This was the longest crank Porsche had made for a car, in its final form measuring 31.3 inches end to end. Analytical studies forecast large amplitudes of vibration at both ends of the shaft, but at its centre vibration was a node, a point that remained at rest.
Porsche decided to power not only the camshafts but also all the drives, from the engine’s output as well as for its accessories, from the centre of the crank, a straight-cut drive take-of gear with 32 teeth was formed at the centre of the crank. It was flanked by 2-larger 66mm main bearings, the remaining main bearings, 3-on each end, were 57mm diameter, bringing the total main bearing count to 8.
So that its central gear could be hardened to the degree necessary, the crank was forged of a chrome-nickel-molybdenum alloy steel. It was carried at the centre of a box-shaped cast magnesium crankcase that was split vertically from front to back. Studs and bolts around its periphery were chiefly of titanium. The halves were principally bound together by 16-bolts that went all the way through from one side to the other, one above and one below each main bearing. these bolts were made of a steel alloy called Dilavar, which had a coefficient of expansion only slightly lower than that of magnesium.
Set in a ball and needle bearings adjoining the centre split of the crankcase were shafts above and below the crank, both driven by its central gear. The lower shaft took drive to the clutch at the rear of the engine. Its driven gear had only 31 teeth, one less than the gear on the crank. A small step-down gear set at the front face of the output-shaft gear powered a pack of oil pumps at 0.54 times crankshaft speed. One was a pressure pump, with gears 64mm wide, and the other were separate 42mm scavenge pumps, one drew oil from the front of the crankcase while the other had a pickup about two-thirds of the way to the rear, at the front and rear ends of each exhaust camshaft more pumps were provided to scavenge spent oil from the cam-boxes.
Like the 912 engine’s other subassemblies, the oil pumps were mounted on the right-hand half of the crankcase, while the oil galleries were in the left-hand half. The pressure pump output was fed to a filter at the front of the engine that also contained a relief valve set for 71psi for the oil to the main and rod bearings. Because the crank had no power take-off at either end, it was easy matter to arrange a direct delivery of oil to the rod bearings through drillings in each end of the shaft.
Each drilling supplied oil to the 6-rods on each side of the central drive gear. Each of the 4-hardened-steel camshafts was carried in 8-plain bearings 30mm diameter. These were directly oiled as were the cup-type tappets that slid in aluminum inserts in the magnesium cam carrier housing running the full length of each cylinder bank. The fact that the delivery port did not open until the tappet had been depressed 2mm reduced the volume of flow by 60 percent from what it would otherwise have been.
The 912 was the first Porsche overhead-cam engine to drive its camshafts by trains of gears. four shafts carried steel gears on needle bearing between the crank and each pair of cams. All 4-shafts were supported by a magnesium housing that contained the gears and was principally bolted to the crankcase and also attached to the camshaft housing.
The 912’s valve timing pattern was the same as the 90’s as follows: Inlet opens 104 degrees BTDC. Inlet closes 104 degrees ABDC. Exhaust opens 105 degrees BBDC. Exhaust closes 75 degrees ATDC. Lift was 12.1mm for the inlet valve and 10.5mm for the exhausts. Both valves were hollow, with sodium-filled stems. 2-coil valve springs per valve were made from vacuum-melted alloy steel wire. Valve head diameters were the same as those of the 908, 47.5mm for the inlets and 40.5mm for the exhaust valve.
Cast of aluminum in a permanent mould, the individual head for each cylinder also resembled that of the 908, in fact it bore a 908 part number. Its valve angles were 30 degrees for the inlets and 35 degrees for the exhausts. Individual cylinders were forged of Mahle’s high-silicon aluminum alloy number 124, deeply finned by individual machining of each cylinder head, and given chrome-plated walls.
They bore forged fully skirted aluminum pistons of the same alloy with 2-compression rings above the gudgeon pin and a single oil ring bellow it. Cooling-oil jets 1-mm in diameter were fitted to the main bearing webs and aimed at the underside of the piston crowns.
Each head and cylinder was attached to the crankcase by 4-long cap screws. To minimize stress changes with heat, these were made of the Dilavar steel alloy that was also used for the crankcase bolts. Because these head screws were cooled by the fan, each was given an insulating jacket so it would be warm enough to maintain the proper expansion rate. Between the cylinder and the head. Porsche used a flat face-type joint that was sealed by a ring inserted in a groove machined in the top of the cylinder.
Like the 752-8, the 912’s cooling fan was placed flat and was mechanically driven by a bevel gear from the engine-speed shaft above the crank. From the front and back ends of the shaft 2-ignition distributors were turned by skew-gears. Each served 1-bank of 6-cylinders in the 912’s dual ignition system. The Bosch system used transistorized magnetic triggering and 4-ignition coils. From a pulley at the front end of the accessory drive shaft a v-belt drove an 860-watt alternator.
A double row Bosch fuel injection pump was mounted atop the front of the left inlet cam housing and driven by a short cogged belt from the end of the camshaft. Space-cam metering was used. Set for delivery pressure around 250psi, the injection nozzles were at the top of the plastic inlet ram pipes and were fed by nylon tubes. Slide throttles in 4-separate groups of 3 were joint by adjustable linkages.
Straight from the drawing board the engine delivered 542bhp. When it was ready to race it was producing 580bhp@8400rpmwith a compression ratio of 10.5:1. The flat-12 was enlarged in 1970 to 5.0-litres (86*70.4mm with a shortened 127.8mm con-rod.
Stroke/bore ratio 0.78:1.
Compression ratio 10.5:1.
Con-rod length 130mm.
Rod/crank radius ratio 3.9:1.
Main bearing journal 57mm.
Rod journal 52mm.
Inlet valve 47.5mm.
Exhaust valve 40.5mm.
Inlet pressure 1.0 Atm.
Engine weight 241kg.
Peak power 580bhp@8400rpm.
Piston speed corrected 20.6m/s.
Peak torque 510Nm@6800rpm.
Peak bmep = 207psi.
Engine bhp/litre 129.1bhp/litre.
Engine weight per bhp 0.41kg/bhp.