steve_eo

After reading the article in 911 & Porsche World (March 2002) about the 964 RS Lightweight, I'm really tempted to build a replica. I think the most difficult part will be locating (and possibly affording) the transmission. The tranny for this car was a rather special. It allowed for adjustable torque split front to back. I'm not sure I'd ever have a need to adjust the split, but I love the idea...and the car.
So can anyone suggest where I might find one of these trannies? I've done the typical look around on the web, but only finding references to the car as a whole. There is a reasonable chance that the parts for the 953 were one offs and I'll never find the tranny....but I'm going to try at least.
Thanks for any help!
-Steve

Cupcar#12

they were very special tranny's - that being said jeff zwart built a C4 for SCCA pro-rally competition which used one of these paris-dakar transmisions - the car was built by rod millen (of rally fame) and as i recall was very expensive (think 25K+) for a replica i would go with a 993 C4 setup and shorten the gears - you won't have adjustable torque split of course.
hope that helps - but if you need one i have a light pod for a 964 w/ lights and relays - it cost a fortune to make

steve_eo

Rob, thanks for the reply.
What is a light pod?

garrett376

Theoretically speaking (dreaming?), you should be able to construct a mechanical device to control the hydraulic locks on the lateral and longitudinal locks on a normal C4. The standard C4 system has an electrical solenoid that controls the locks - find a way to make a "pump" that is controlled with some dials that look like the C4RS's lock controller and you could swap out the electrical controlled solenoid valve block, and use your "home-made" pressure controller. That would be cool because you can set the differentials how you want by increasing the system pressure on the stock differential locks and they'll stick that way, unless you crank on the dials to change the static pressure. That would be a fun project!!

Now if you needed a different set of ratios, that isn't difficult. It would just be hard to fabricate the pressure valve to each lock. No hard for R. Linton...

NotTwinTurboYet

Beautiful picture garett, do you have any others of that car?
NTTY

garrett376

Got it from this RennList post:
https://rennlist.com/forums/showthre...6&page=3&pp=40

steve_eo

Hmmm interesting idea about building a variable system. But I thought the front/back torque split on the C4 was fixed at 31/69?
Obvioulsy I know nothing about how it works. What is the function of the clutches in the C4? If the 31/69 split is there in every day driving, how does the car put more up front if the back end looses traction? Or visa-versa? Is it possible to change the 31/69 split? It's not mechanical?
-Steve

garrett376

The torque split is set - no changing this unless you change the front and rear drive shafts...

My understanding is that if there is a speed differential between front and rear axles, the longitudinal differential (center one) will approach 100% lock over a period of time as the wheel speed sensors are monitored and showing a difference of greater than 6% wheel speed. From looking at the diagram of how the front and rear wheels are linked, the torque split comes from the rear wheels having a completely rigid drive line. The front drive line, on the other hand, is slightly "flexible" compared to that rear drive shaft (one that goes through the hollow shaft) - it is similar to having a 30mm torsion bar as the rear drive line, and a 20.7mm drive line heading to the front differential. So, when the engine is transferring torque to the front and rear - because the front drive shaft is slightly "less torsionally stiff" than the rear, it will get a lower percentage of the torque when all wheels have traction - as in the example of the torsion bar sizes, a 31%/69%. That split is always active, because it is just how the front and rear are hooked together - to change this split would require changing the drive shaft thicknesses to obtain a different ratio of torsional resistance.

The center differential then controls the slippage between the front and rear - if the rear is on ice, and slipping, then differential will lock making all power routed to the front wheels which will cause the overall driveline speed to slow, allowing the speed sensors to see that there is no longer a greater than 6% difference present (which is when it starts to activate the differential locks). It will then release the center differential and by now all 4 wheels should be at the same speed - it has done its job!

Oh, and the "lateral" differential is similar to the center differential - if there is a wheel speed difference between the right rear and left rear, the differential lock will increase in force in a step-wise fashion until both wheels are turning the same speed (within 6% again, I believe). The difference here, is that the right and left driveshafts are the SAME thickness, so the right and left wheels get a 50/50 torque split -equal. Unless you were running in circles all day, that is good.

The right and left is NOT monitored at the front differential - this is a standard open differential unlike the rear lateral differential. I guess if you wanted you could put a limited slip differential up there, then you'd have a very expensive driveline - but well controlled in slippery conditions.

Ok, that is my understanding on how it works based on looking at the technical drawings and such. Someone please correct me or I'll be running around telling people false information!!!

NotTwinTurboYet

Cool, thank you!!!

NTTY

steve_eo

What an awesom car!
I'd love to build one and then have ninemeister (http://www.9mracing.co.uk/html/index.php go to work and see her making >340HP!
2400lbs/340=7.05lbs/HP.......very sick. Faster than the GT3s I'm trying to catch up to!
-Steve