Porsche Twin-Cam 6-Cylinder Engine

www.porscheroadandrace.com Porsche twin-cam 6-cylinder engine Published: 19th January 2018 By: Glen Smale Online version: https://www.porscheroadandrace.com/porsche-twin-cam-6-cylinder-engine/ Corsica Rally, 1967: Vic Elford and David Stone driving a Porsche 911 2.0 R Not a company to stand still for long, Porsche was constantly looking for ways to improve its engines in the ‘60s. Somehow the Type 916 twin-cam 6-cylinder engine always seemed to miss the limelight…not anymore! The promotional brochure gave the power output of the new Porsche 917 as 520bhp when it was launched at the Geneva Motor Show on 12 March 1969. The engine powering the revolutionary 917 was a 12-cylinder boxer unit of 4494cc capacity, which up until that point, www.porscheroadandrace.com was the largest capacity racing engine that Porsche had built. The Type 912 12-cylinder engine, correctly referred to as a 180 degree V12, was a large lump, and featured gear- driven cams instead of the double chain system that drove the cams on Porsche’s other engines. www.porscheroadandrace.com 12-cylinder Porsche Type 912 engine for the 917 race car ‘What’, one might ask, ‘is the link between Porsche’s mighty 12-cylinder 917 engine and their traditional 6-cylinder 911 engine’? The answer lies in a little known but technologically important experimental engine called the Type 916, a twin-cam 2-litre 6-cylinder engine. Very little is known about this unit outside of Porsche’s experimental racing department, and therefore not much has been written on this missing link over the years. Hans Mezger, Head of Engine Design at Porsche remembers, “It was Ferdinand Piëch’s idea, we worked together in Zuffenhausen at that time. He was in the engine experimental department and I was in the engine design department but we worked together on the original 6-cylinder engine, which by the way was also his idea. But then he wanted to improve the production 911 engine by using four camshafts instead of only two.” www.porscheroadandrace.com Hans Mezger leans against a Porsche 906 at the 1966 Spa race www.porscheroadandrace.com Porsche engineers had been experiencing some problems on both their racing as well as production engines with the cam followers on the two-camshaft version because the cam follower used a rocker arm and not a cup-shaped follower. Improvements to the 6-cylinder engine included trying a number of different surface coatings to address wear problems and to achieve higher engine revs in order to produce more power. The engineers at Porsche designed the boxer engine in such a way so that by adding cylinders engine capacity could be increased relatively easily. The engineers at Ferrari, Jaguar and others, who, when they wanted to develop a larger capacity engine, would have to basically create a whole new sub-assembly and top end which was a much bigger undertaking. With the cylinders and cylinder heads all being separate units, the Porsche engineers in the engine design department could increase the number of cylinders, and thereby the capacity, by building a longer crankcase (fabricated by either Honzel or Mahle) to accommodate two extra pots, which is how the 8-cylinder engine came about. www.porscheroadandrace.com First trial running of the 8-cylinder Type 753 engine: (from L-R) Hans Hönig (leaning forward), Hans Mezger, Eberhard Storz (Manager Testing and Engine Building) According to Hans Mezger, there had been a plan in existence from early on that allowed for a 4-cylinder, a 6-cylinder, and an 8-cylinder engine to be available in this manner. In this way, the 3-litre 908 engine was itself a development of the 2-litre 6-cylinder engine, as Hans Mezger points out, “We had a new project which was the 3-litre 8-cylinder, which we had to develop in a very short time, that’s why we didn’t change anything. Of course, we had to change the crankcase for the 8-cylinder, and some other castings like the camshaft housing, but not the cylinder and cylinder heads because we used separate cylinders. On all our engines, racing as well as the production, they all used singular cylinders and cylinder heads, so we could exchange these on all engines.” www.porscheroadandrace.com “By adding two additional cylinders to the Type 916 engine, we got the 908 engine, which was just a 3-litre engine,” Mezger explained. Actually, it was a little more complicated than that, because the engineers took the 2-litre engine cylinder dimensions with the standard 80x66mm bore and stroke and increased the bore to 85mm, which with the same stroke, gave the 8-cylinder 908 engine a capacity of 2997cc. However complex an operation that may sound to the average car enthusiast, to a racing engineer it was a fairly straightforward concept. The 12-cylinder Type 912 engine in the Porsche 917 was again a derivative of the same basic 6-cylinder engine, and had the same 85x66mm cylinder dimensions as the 908 8- cylinder engine, but with two extra pots added at each end of the crankcase. The workshop was absolutely buzzing with technicians in 1969 as they worked frantically to assemble the 917 engines before the homologation inspection deadline The Type 916 project gathered speed when Ferdinand Piëch approached Valentin Schaeffer, www.porscheroadandrace.com Chief Race Engineer (Engines), for an estimation on how long it would take him to build a 4- cam engine. Schaeffer, who had the privilege of working on every Porsche race engine between 1955 and 1989, recalls, “At first Mr. Piëch asked another engineer from the production side, how long it would take and he said he would need six or seven months. Then he came to me and asked the same question and I said five weeks, so he said ‘make it.’” www.porscheroadandrace.com Valentin Schaeffer (left) works on a 917 engine Schaeffer remembers that the Type 916 study, to see what could be done with a 4-cam 6- cylinder engine, started in about 1966 or 1967. Actual work, though, only began on this engine in late ’67 and this was then developed at the same time as the 4-cam 908 engine, www.porscheroadandrace.com the latter engine only running on the dyno for the first time in December 1967. A list of what was needed in order to build the Type 916 engine was drawn up and it took some time for Schaeffer to assemble all the components, including the new camshaft housings, before work could begin. Schaeffer, “We used components from the 906 engine including the valves, and I only ordered a few small things but it took about two months before I had all the parts, and then I started. I needed four or five weeks, no more, and we built just one engine in our race shop.” This first engine was a 1991cc 4-cam engine that utilised the standard 911 or 906 crankshaft (made by Alfing of Wasseralfingen), with a 66mm stroke. At this stage, the 2-litre production engine was the largest engine that Porsche could practically use as a base unit for this development. The resultant Type 916 engine, regarded as very much an experimental engine, was then dyno tested and just a few small changes were necessary according to Schaeffer. www.porscheroadandrace.com Engineering drawing of the Type 916 6-cylinder twin-cam engine The 916 engine differed from the 2-litre 906 engine only insofar as it had 4-cams, but it was otherwise the same, a full race, open engine with no filters on the intake system. However, the intakes and mufflers were different from the 906 because the engine was located at the rear and not in a mid-engine position. About the 906 camshaft Schaeffer recalls, “It was very heavy in the first 916 engine, I think it was about 3kg [per cam], and so we made different camshafts, and the inside was hole-bored to make it lighter.” The motivation behind this project, however, was not to develop a twin-cam six, but to explore Piëch’s dream of having bigger and more powerful engines to power his crop of revolutionary ‘plastic’ race cars. Schaeffer recalls, “Mr. Piëch said to me, ‘how long do you think you will take to find out how much horsepower you can get?’” The power developed by the Type 916 engine was little more than the 906 unit, as Jo Siffert commented. Valentin Schaeffer again, “One time I remember in Mugello, Italy, Siffert said at this time that it was not much better than a normal [906] engine. But of course, this engine was not tested or developed and I didn’t make a proper injection cam for it.” www.porscheroadandrace.com Porsche Type 916 engine in situ Driven by Ferdinand Piëch’s ambition to rise to the top of the world of endurance motor racing pyramid, he pushed the race engine development team to come up with the horsepower numbers on the 916 engine. The idea of the Porsche 917 was already developing in his mind, and he knew that he wanted a twelve-cylinder engine, and what better way to get an accurate idea of power potential than to extrapolate the output achieved by a 4-cam engine of half the desired 4.5-litre capacity. Schaeffer then proceeded to increase the capacity of the Type 916 from 2-litre to 2.25-litre, exactly half the capacity of the 917’s Type 912 engine. In order to cut down on time, the development team used the standard stroke 66mm crankshaft and so the increase in the capacity of the Type 916 engine came by way of an increase in the bore.

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