Guide: Porsche 917 / 70 Spyder 16C
Background
Having made a solid debut in Can-Am with the 1969 Group 7 917 Spyder, Porsche had nevertheless been soundly beaten by the works McLaren M8Bs with their seven-litre 635bhp Chevrolet engines. In comparison, the Flat 12 engine used by Porsche in 1969 had displaced just 4.5-litres and produced 580bhp.
If they wanted to catch up, Porsche had three options: add displacement, add cylinders or add turbochargers. To get it right, they would sit out the 1970 Can-Am season and focus on developing a suitable engine.
It was thought Hans Mezger’s existing Flat 12 could be taken out to around 5.5-litres but an additional 1000cc would not be enough to close the performance gap.
Both remaining options had their drawbacks: taming turbocharger delivery was still a black art while adding cylinders would inevitably increase the size and weight of an engine. With neither route seeming the obvious solution, Porsche decided to explore both avenues in 1970.
Initially, a Flat 16 engine seemed the most likely to succeed. Porsche also thought a five-litre version of the 16 cylinder engine could be used to stay ahead of Ferrari in the World Sportscar Championship. Furthermore, the Flat 16 would have applications of up to 7.2-litres - perfect for Group 7 where there were no restrictions on engine size.
Engine
Ferdinand Piech was the inspiration for the Flat 16. His grandfather, Ferdinand Porsche, had created the supercharged V16 used by Auto Union to great effect between 1934 and 1937.
However, whereas the Auto Union approach was extremely complex, Porsche decided on a simpler route; they took the existing 180° Flat 12 motor and added a cylinder at each corner thus retaining the original pistons, rods and valves.
The original 4.5-litre Flat 12 would therefore become a 6-litre Flat 16 with an estimated output of 750bhp.
Canted inlet valves required the inlet trumpets to sit at an angle which happily allowed room for a larger Bosch mechanical fuel-injection system that employed a pair of 908 pumps.
Dyno tested with 690bhp at 9200rpm, such engine speeds initially resulted in overheating and damage to the outlet valves. The problem was solved by Acheson Colloids who supplied Porsche with a dry-film molybdenum disulphide lubricant.
Otherwise, the Flat 16 engine used a similar mix of exotic alloys to the Flat 12.
Compression was kept at 10.5:1 and dyno tests eventually got up to 755bhp with 880bhp predicted from the 7.2-litre version.
All this was promising enough to warrant installation to a car.
Porsche opted to try a 6.7-litre configuration that produced 840bhp at 8300rpm. So secret it never received a type number, the Flat 16 engine was fitted to Porsche’s original 917 Group 7 Spyder mule from 1969 (chassis 027).
To accommodate the massive engine, Porsche had to extend the original 2300mm wheelbase by 270mm. This inevitably reduced torsional rigidity and, with the Flat 16 engine weighing 80kg more than the twelve cylinder version, the 917 Spyder was unable to cope.
Testing
Jo Siffert and Mark Donohue both tested the car at Weissach, but neither was impressed with the handling.
The Flat 16 programme was quickly set aside in favour of pursuing the turbocharged route.
Subsequent History
After shelving the 16 cylinder engine, rumours of its existence began to circulate.
To confuse Ferrari, Porsche admitted they had been working on the engine and released a photograph of the Flat 16 next to their Flat 12. Cunningly, they shrunk the Flat 16 image so the two motors appeared dimensionally similar.
This led Ferrari to conclude Porsche had a five-litre Flat 16 ready for the 1971 World Sportscar Championship. It is understood to have influenced the Italian’s decision to drop the 512 M project despite its promising victory at the Kyalami 9 Hours in November 1970.
Sadly for race fans, Ferrari switched focus to their 312 P for 1971.
Four complete Flat 16 engines were made by Porsche and two survive, one of which remains in chassis 027.
Text copyright: Supercar Nostalgia
Photo copyright: Porsche - https://www.porsche.com
