Hey guy I have an idea to handle torque to the 901s from a V8. It involves a rubber guibo or flex disc. My E30 M3 has one that fits between the drive shaft and transmission flange. Now, I was looking through a book I recently bought entitled "917 the winning formula" all about the great Porsche race car and others like the 907, etc. A great book with lots of pictures and the inside scoop on the early racing successes - I highly recommend. In one of the shots you can see very distinctly 2 rubber flex discs (guibos)between the half shafts and tranny on the 917. I swear from the picture they looked just like the one on my M3. Of course major mods to the shafts and tranny flanges would have to be developed for our application but do ya'll think this would help torque issues. Maybe the BMW parts could be incorporated with larger CV joints? What would be some of the most obvious hurdles? TIA.

Doug C

Most of the issues with guibos are durability and balance issues. Designing in the right amount of flex to allow the coupling to move as it should, yet still be strong enough not to be shredded quickly by torque, is not a task for amateurs. The only real place to use them on the 914 would be on the output flanges of the transaxle, replacing the inner CV joints. This would help with shock loads, but the relatively large angle they'd have to operate through to allow the suspension to move wouldn't suggest a long life for these parts.

Lotus and others used these in exactly this fashion on a number of race cars, but I don't know of any street cars using guibos in exactly this way. Street cars that have used them typically mount them between the gearbox and the driveshaft, where the angular changes are fairly small, so the rubber can be much stiffer and stronger. I wasn't aware BMW used them, but Alfa used them for decades as driveshaft couplings. Life in the Alfa application was typically 50-80K miles, and they're a serious pain in the ass to replace. Used as an inner CV replacement, I'd not be surprised to see a lifespan of no more than 10% of this, which I'd think would be unacceptable to most street car owners.

You might want to check out your BMW again. I'll stand corrected, but I think they use them on the output flanges on the diff to the axles.

Andy

Well, I have had a few BMWs and had to replace those stupid things on all of them. In fact, I have a brand new one for a 325I as a spare, they only cost about 40 bucks or so and a friend of mine is the parts manager for a local BMW dealer. I got it at cost. It goes between the transmission and the drive shaft that goes to the back of the car and is called a transmission coupler. If neglected, I have heard they will break, and could rip the car apart from the underside including breaking off the back of the transmission and even coming up through the floor board. Not something I would want to happen to me. They look like this.

So you may stand corrected.

DougC,
I heard the CV isn't the primary weak point in the drive train so it may not be the area to improve. You may also prefer a failed CV before a failed gear. Many run stock 914 CV unless higher hp is used. There are 911 or bus upgrades already or why re-engineer another part?

BTW, they are used on rear drive 80-90's Benz's too.
Felix

There are bigger, beafier flex discs than the one used on my 4cyl M3, actually we replace them with one from an E34 M5 (IIRC). You wouldn't be shreading two of those quickly I don't think. I replaced my original one after like 120K+ miles on the car and it wasn't shreaded yet but of course we're not talking much torque at all with the E30 M3. I think if you let it get to a point of shreaded and doing damage to the car then you're probably not paying much attention - but I guess it has happened. I see the point though of angles of operation and like someone mentiond, there's more a problem with tranny internals and it would add unsprung weight I guess. Just an idea after seeing those 917s which would run 24hs sometimes. BTW, they are a pain to change on the BMW..

Doug C

Felix, the point isn't so much to replace a potentially weak inner CV, it's to provide some softening of the shock loads caused by the V8 torque. The hope would be to extend the life of the entire gearbox. They may very well provide some help in that area, but I don't think they'd last long used that way. 24hrs in a 917 is still only 3500 miles or thereabouts, and road racing will actually produce much lower shock loads than street driving would (far fewer starts from a dead stop).

Isn't this the job that the springs in the clutch disk are supposed to do? If you added one of these to each driveshaft I think they'd see about double the peak torque that they were intended to (~4x because they're after instead of before the final drive, divided by two because there's two of them)

my old MB 350 SE ('76) had one of those, after the tranny, sandwiched into the driveshaft.

if one of those can handle the torque of a 350/8 MB, 2 should be plenty for a V8 914 ...
Andy