Lead Vehicle Artist Casey Ringley on how the 917K was built for Project CASRS 2 …
I’ve never seen so much detailed documentation on a 45-year-old car before and it made putting this car together in-game very easy. Nice dyno plots, full gearbox/differential specs for various circuits, multiple wind tunnel and track aero tests in various configurations, an array of ride heights, complete chassis blueprints for suspension geometry, corner weights, fuel usage, and so much more―amazing stuff in those old Paul Frère books.
Engine for these 1971 cars was the 4.9L version of Porsche’s flat-12 making an even 600hp@8,400rpm and 415lb-ft@6,500rpm. Unusual engine as it was essentially two regular 911 boxer 6-cylinders joined together in the middle; the length of such an arrangement meant that power had to be taken from the middle of the crankshaft to help control vibrations, and the gears used to achieve this essentially made for a 32:31 overdrive.
Also note that while these are traditional “air-cooled” Porsches (legend has it VW helped finance the development cost of the engine, keen to prove the reliability of air-cooled engines which was a staple of all VW engines back then), there is a radiator at the front that you can damage―avoid this because it will cause you a lot of trouble.
They ran an 80cm-wide oil cooler there, and 917K variants even went so far as to carry a 55L (no, that’s not a typo) oil tank to help regulate engine temperature over the endurance races.
That engine drives the rear through the same type-917 gearbox we also find in the 936/77 (minus the power take-off overdrive), and was run as either a 4- or 5-speed unit depending on the track and model type (K models usually going 4-speed, LH 5-speed). Same internals, but they would just block off 5th gear and select ratios to suit a given track with only four gears; an easy thing to do as the power band is quite wide, and the engine doesn’t need to be up at the top of its range for peak performance.
Standard differential was a ZF with 75 percent locking (8 clutches, 40° ramps) and preload. Track tests were done with 25 percent locking and steady-state handling improved slightly, but sudden throttle lifting would then cause the car to instantly spin out.
K models tended to be lighter, even needing ballast to get up to the 800kg minimum, and roughly a 63.5 percent rear weight bias.
Suspension design is nothing too special and typical of the time: double wishbones up front, radius rods plus inverted lower wishbone at the rear. This is another area with great documentation for camber and toe changes with regards to suspension motion, and there is a fair amount of both.
Matching that in our model and using real setup values from the period all works out to make good sense; they often ran zero, or even positive, camber at the rear and quite a lot of toe-in at both ends. The camber helps agility at low-speed while the toe-in stabilizes, and it all cancels out at high-speed under aero load such that the rear tyres stand perfectly upright when you are over 200mph, which is good for endurance of the tyre carcass.
Complicating matters, progressive springs/geometry were used in the design, but some track aero tests provided spring compression for a known aero load, so working out typical average rates for the suspension stiffness was easy and drove well from the start on those numbers.
The blueprints plus old setup sheets also helped to work out wheel rate for typical anti-roll bar sizes used. Easy stuff and all just worked when we put it on the car.
Like the 911 Carrera RSR, this was the very beginning of the wind tunnel era, and it helped them make one slippery car. Downforce levels are nothing astounding, but useful and well-balanced for high speed stability. Drag levels are astonishingly low at around 400lb for the 917K, making it good for over 350km/h while feeling perfectly comfortable through the Mulsanne kink flat out. Overall a very easy drive at just about any track with great stability and tons of grip from the giant 14.5″ wide rear tyres.
Feels like they would be good for cruising around at race pace all day long, which is what they were designed to do.
The Porsche 917K comes with the Porsche Legends Pack, available now for purchase, or as part of the Season Pass.