Mating two 911 engines for the 917, HOW?

[algs911]

Hello all,

I just got my latest issue of "Pano" and there's a great article in there about the 917. I'll have to pick up a book some time, however I have a question I was hoping someone could answer.

The article states that making a flat 12 cylinder engine would have caused too much torsional rigidity and vibration. So, "to cope with (this) I (Mezger) came up with the idea of a centralized power pick up, essentially mating two six cylinder (911) crankshafts together and thereby overcoming the problems we almost certainly would have otherwise had."

How was this accomplished? There are no schematics in the article.

How was the flat 12 mated to the tranny? Was it on top of the transmission? This doesn't seem logical as it would place the motor high up in the chassis, thus raising the center of gravity.

Please help me understand. I'm currently unemployed and need to use my brain (ha ha).

Thanks,

Al

[slodave]

Try this link.

http://www.pbase.com/917carl/917_12_cyl_engine

[Flieger]

The 917's 912 engine was a flat 12. It used two connecting rods per crank throw to minimize crankshaft length. Because of the vibrational dynamics of 6 and 12 cylinder crankshafts, there was a vibrational node in the middle (what would be the end of the crank for a 911 6 cylinder). Therefore, Mezger put a big gear in the center of the crank in order to drive the layshaft on top and the crankshaft on bottom. The crankshaft ran under the rear half of the engine and the clutch had to be small otherwise the engine would need to be mounted way too high for a good center of gravity. That small clutch could be a weak spot but the multi-plate units held up remarkably well.

[javadog]

So, you can see that the 917 engine (type 912, by the way) wasn't really two 911 sixes bolted together. That's just the writer being a little lazy. There were some things carried over from the 908 engines and some broad similarities between that engine and the 4 cam version of the 911 engine, but really, it was all new.

JR

[psalt]

essentially mating two six cylinder (911) crankshafts together and thereby overcoming the problems we almost certainly would have otherwise had."

Hello Al,

This is misleading, the 912 engine had a single solid crankshaft, not two cranks connected at the middle. The central power take off and the central cam drive were not new ideas. The problems of long shafts has been known since the straight eights of the 1920's. The valve timing events at the end of the camshaft are measurably different from those next to the drive. Emil Petit solved this problem in 1928 in the Salmson straight eight and it was copied by Jano for his Alfa 6C and 8C engines and by Honda in their early racing bike engines. The Alfa 8C did use a two piece crank with a gear in the middle. BRM combined the central cam drive and the central power take off shaft in the 1949 1.5 liter V-16. The BRM V-16 and the later 1965 Conventry Climax Flat-16 used a two piece crank, the CC Flat engine had an unusual SKF oil injection coupling at the middle. Mercedes Benz used the central gear drive and power take off in the 1955 W196 engine.

There was nothing really new in the 912, just Porsche's typical use of some of the better ideas lying around. Like the horizontal engine, forced fan air cooling, and chassis of the Tatra for the VW.

[Wil Ferch]

Pairing up 2 cylinders on a single crank "throw".....effectively makes the 912/917 engine a "horizontally opposed, 180 degree V-12".....if you catch the nuance difference. A regular 911 engine had the individual pistons on individual crank throws....120 degrees apart...with a main crank bearing in-between each. Essentially a "boxer". Adjoining cylinders on a 917 did not "box".

BTW....the 912/917 engine had an unusual "hunting" gear set between the power take-off at the crank center gear... and the mating gear on the layshaft below... that transmitted the engine torque and revolution to the clutch. So the engine rpm was not exactly the "clutch" rpm. For those not familiar with a "hunting" gear set....one gear may have ( say) 100 teeth and the mating gear would have ( say) 101 teeth...so you get close to the desired 1:1 ratio...but the same 2 gear teeth would never come together until 100 revolutions were made....not on EVERY revolution. This helps spread the gear wear pattern over a greater number of teeth. Unfortunately, there ends up a skewing of the 1:1 ratio. Interesting...the 917 has a gearbox inside the engine !!

So.....trivia question.....when a 917 engine says 8500 rpm on the tach.....is it "crank" rpm or "clutch" rpm?.....

I love these cars !!!!