Replica 917 & flat 12...etc....

[JohnC]

Thought this was interesting... two 2.7 to create a flat twelve.

http://www.cknet.org.uk/forum/topic.asp?TOPIC_ID=13

[emcon5]

Interesting.

[sammyg2]

That flat 12 is probably out of reach for most of us, but what is the story with the flat 8 in the second picture? Any details?

[jluetjen]

I was always under the impression that that was the concept of the 908 and 917 motor. Did you ever notice that those engines shared the same (or similar) bore and stroke dimension as the earlier 911 motors? The progression seemed to be...

Original 911 2 liter
906
908
917 Motor (AKA: 912)
Both 911 and 917 motors were bored and stroked
16 cylinder motor (Test driven by M. Donahue and Willi Kuelhus (I don't have WK's spelling in front of me)
Turbocharged 917 motor
Turbocharged 911 motor

I don't know the valve sizes and details of the 917 motors, but I was under the impression that the combustion chambers were similar. Given the 8100-8500 RPM rev limit of the NA 917 motors, I wouldn't be surprised if they weren't still using the 906 cam grind! Except that the 917's were all built out of "unobtanium", I wonder how many of their parts will bolt onto a 911?

[Early_S_Man]

John,

There is no doubt that the 908 and 917 were developments in the 911 family ... the 'test bed' for 908 cam drive parts was called a 916 ... and all were part of a Piech grand scheme for a family of lower-cost, production-based racing engines! Initially, the plan was to have 4/6/8 cylinder production engines based upon racing development and refinements! The 917 was just the ultimate extension of that plan based on rulebook loopholes ... something Porsche proved VERY GOOD at exploiting over the years!

Using 917 parts in a 911 engine??? Where do you think the Nikasil 2.7 cylinders came from?

A bit of details yet to be worked out on the flat-twelve ... only six cylinders would run off that single distributor ... whether it would be a left bank (sitting in its' mid-engine location) as in the original ... or something else ... is a curiosity at this point!

[ErVikingo]

I want one!!!!!!!!!

[jluetjen]

A bit of details yet to be worked out on the flat-twelve ... only six cylinders would run off that single distributor ... whether it would be a left bank (sitting in its' mid-engine location) as in the original ... or something else ... is a curiosity at this point!

If my memory is correct, the 917 motor had one 12 lead distributors at either end. Mirror images of each other. I guess if they wanted to "single plug" a 917, they could have run with 2 6-lead distributors at either end just like they have picture above. (Correction -- I see that they have a 12 lead distributor. And if you look at the opposite corner, you can just make out the 2nd distributor. I think that they got that part right!)

The tough part will be getting drive out of the engine. Porsche found that they had a lot of vibration on the long crank, so they ended up taking the power off of the center of the crank. If you were going to join 2 six cylinders, you'd most likely come to the same conclusions. The same goes with the firing order. Just putting to 2.7's together will give you a "boxer 12". The problem is that he space involved in running the crank back and forth between each opposing pair of cylinders makes the already long crank longer. I believe that this is why Porsche went to the 180 degree firing pattern where opposing connecting rods share the same crank throw. Doing this makes the crank lighter, simpler and shorter -- all good things if you are trying to make a light race car! Anyhow, I suspect that as a result the bore centers on the 917 motor are actually closer then on a 911 motor.

I've been dreaming about a 906 replica that I would think should be a lot easier. A stretch would be a 910. At least in those cases you could honestly cheat and use a 911 engine and gear box. By the time you get to a 908 and 917, pretty much everything is different (read custom made!). It sounds like those folks are jumping in on the deep end. I did glance at their 6 cylinder 917 pictures, but that just seems to be a bit hollow to me.

[JohnC]

Hey Warren

Did you happen to see the ghost view of the 917 motor?

There appears to be two oil pumps (similar to the oil pump used to scavenge oil from the turbo on the 930)located on the front of each cam cover.What are these pumps used for? To move oil out of the cam housings?

There's also what looks to be a distributor drive gear located on top and towards the front of the motor? Whats that for?

Point of interest: The spark plugs looks centrally located in the combustion chamber! A TRUE hemi!?

[JohnC]

the image is in here:

http://project917xxxxxxxxxxxxxx.homestead.com/917v20.html

[JmuRiz]

JohnC,
That is exactly what I was thinking. I want one now!, in Gulf livery, thank you

Nathan

[jhugh]

Those are definitely cool. But THIS is out of hand... an excerpt from Mark Donohue's book Unfair Advantage, describing when he's testing 917s for Porsche in Germany...

While we were at the track a truck showed up with another 917. It looked like the Spyder we had seen before at Can-Am races -- an ugly, dirty, beat-up old thing -- except that it was longer. It turned out to have a sixteen-cylinder motor that they wanted me to try. I was told that it was built by special order from the Piechs, and it was quite expensive. The chassis was easily stretched the extra two cylinders' length, but the motor required a new case, a new crank, new manifolds and fan housings, and lots of other pieces. They didn't tell me much about it, but I clearly got the impression that it was a possible alternative to the turbocharged twelve, or that it would even be turbocharged itself -- for something like 2000 horsepower. So I drove the thing for them, and it was a real monster. The motor was so long that you could hear one end start up before the other. There were exhaust pipes sticking out everywhere. Although it wasn't set up properly to try for any lap records, it was truly impressive in a straight line. They just wanted me to feel the motor, anyhow, and when we were though they stuck it back in a barn somewhere. I was honored that they went to all the time and trouble to get it out and running just to show to me. It gave me something to think about.

YIKES!!!

[Early_S_Man]

Donohue was, of course, exaggerating about the flat-sixteen ... but it seems probable that it was the inspiration for his famous quote about 'enough power' and black streaks from one corner to the next, rather than coming from his 917-30 experiences! I expect that the lightning-quick response of a normally-aspirated MFI flat-sixteen could be terrifying with an unsorted chassis in some corners at Weissach! The gentler turbo-engine response was probably a welcomed change!

There was, back in those days, a new phenomenon turning up among the 700 hp Can-Am cars, and I believe it was reported first in Autoweek in 1971 about the McLaren M-20's of Revson and Hulme. Because of the massive 20"+ rear racing slicks and the Weisman locked differentials in use ... there was a fine line between traction and power causing the chassis to twist enough that the 'springback' (or untwisting) between shifts was causing difficulty shifting the Hewland transaxles ... or, if traction was marginal, a kind of high-frequency vibration associated with the tail skipping, or skittering sideways on the asphalt for a second or so ... and it was called Powerslide! Chatter marks were found ... rather than continuous black marks on the pavement! Powerslide! may well have been the reason Donohue had the ZF differential replaced with a solid Titanium 'spool' in the 917 transaxles!

No, I didn't see the engine drawing ... the page wouldn't fully load, it kept stalling, and now is closed because bandwidth limitations for the day have been exceeded! However, Frere's The Racing Porsches has more technical drawings than any other source I know of, and I have examined them many times! There are a total of seven oil pumps ... two scavenge and one main pressure pump in the traditional 911/908 location! There are two small 11 mm wide scavenge pumps at the ends of both exhaust cam boxes.

There are two spark plugs percylinder in heads that look just like 906E, 908 or RSR heads! The 2.7 blocks are oriented in a flywheel-to-flywheel configuration! After I enlarged and enhanced the pic ... I noticed the second distributor!

[sammyg2]

Hmmm, a flat 16. Talk about the ultimate "found in barn" story.

At least I have something to dream about tonight.

[jluetjen]

Powerslide! may well have been the reason Donohue had the ZF differential replaced with a solid Titanium 'spool' in the 917 transaxles!

Actually, I just got the "Unfair Advantage" for my B-Day and just finished reading it. (Highly Recommended!). Mark started to use spools long before he started work on the 917's. They suited his trail-braking style since they settle the rear end under braking. I'm shocked that I actually read that before you did! You always seem to remember the details!

I expect that the lightning-quick response of a normally-aspirated MFI flat-sixteen could be terrifying with an unsorted chassis in some corners at Weissach! The gentler turbo-engine response was probably a welcomed change!

Actually, I think the opposite was the case, especially since Donahue and Porsche didn't get the fuel map figured out for the turbo motor until just before the season started. Up to that time Willi was doing the test driving and the car apparently had an "awful" (as in to inspire religious awe!!!) case of turbo lag. Basically all 900-1000 HP would come on like a lightswitch after an indeterminite period of time. The trick was to time the trottle application during a corner. Early on, the throttle log also affected over-run, so just because you closed the throttle didn't mean that the HP went away!

Compared to that the 16 would have been pleasent. Apparently the primary problem with the 16 was twofold:

- Complexity: 4 more sets of parts to break!
- Packaging: The engine was BIG. Especially long, so they had to stretch the test hack to make it fit. There is a picture of the car in the book "917- The winning formula". Given the geometry issues that the pre-917/30 cars had, the big engine and long wheelbase made the car even more unwieldy. Mark describes how they fixed the 917's handling in his book. In addition to the longer wheel base, they had to change the front geometry to take into account the wider track and correct the track dimension issues caused by the wider can-am tires. They then had to narrow the rear track. This was all compounded by the decreasing rate spring geometry which was designed into the suspension (for packaging reasons?).

BTW - Mark also mentions the base chassis flex measurement for a 911 in his book, anyone care to guess? I'll give you a clue, it is in lb/ft per degree.

[jluetjen]

BTW - Mark also mentions the base chassis flex measurement for a 911 in his book, anyone care to guess? I'll give you a clue, it is in lb/ft per degree.

Answer: 2000 lb/ft per degree. Kind a neat number to consider if you considering adding a cage or strut brace to the front end.