Cupcar

Had an opportunity to do a photo op on a C4 Lightweight today.

This would be a good project for Garrett and his C4, here are the steps to the project:

1. Remove the heavy C4 accessory pumps the ABS and power brake system and all the computers from the car. The trunk should look like this when done.



2. Install dual master cylinders and an adjustable balance bar to replace the power system.



3. Install an adjustable proporting valve between the front and rear brakes to touch up the final brake balance.



4. Now for the trick part, install a couple of master cylinders with hand operated screw jacks to adjust the pressure in the tranverse and longitudinal differentials along with pressure gages to monitor the pressure while driving. This gives the car a constant front to rear and rear side to side torque split all controllable by the driver. No fancy computer required other than the driver's brain. Totally cool.

Chad T

Very Cool! Neat to see the trick details. What were those car built for anyway? Rallying?

Chris M.

What's the final weight?

c

Cupcar

I dunno what they were built for, I suspect Rally work or perhaps just to be cool.

It is basically a 964 Carrera Cup with the simplified 4 WD which I suspect adds much less than the normal 110 pounds a C4 adds. My guess is it weighs in at 2600 - 2650 pounds.

pete000

Who's car was that? Very rare

garrett376

Did you see the exhaust on that car!!? It has some sweet headers and individual mufflers! Bob, I bet you and me confused those guys at Symbolic - as my wife said, this 964 is probably the plainest looking, least viewed car in that entire place!!!!

The differential locking system is interesting - a constant lock vs. the continually varied lock system in the "regular" C4. I still think the electronic version would be better for overall consistent handling - when driving there are so many varied situations that having constant lock would not seem to be of any advantage. For certain situations (corners of different speeds, angulation, incline - all things that vary the slip angles of the tires), a continuous lock would have less stable handling. The electronic version prevents preloading of the driveshaft at lower speeds, in tight corners, or under braking - if it were preloaded as in a continuous lock that was "over-locked", it would make the car understeer significantly - not so good! It would be an interesting project to tackle, though! Most of all, with this non-electronic system I would sure miss the ABS - that's crucial to the deep braking with no flat-spots!

garrett376

The cage attachment points were also very interesting... I think I'll add attachment points to the sills to stiffen things up... very cool indeed

The car was owned by a Japanese man - the sales guy told me his name but I don't remember it... started with an "M"

garrett376

The Brochure says it weighs 1100kg = 2425.1 lbs

Cupcar

I bet it was Matsuda who owned it. I may have spelled it wrong.

Cupcar

I think the rear differential would behave the same as a normal limited slip but with a driver adjustable locking percentage. The torque split to the front being constant would maintain a non variant understeer balance that could be varied from 0 to whatever. The car would handle like anything from a normal C2 with an open differential to a C2 with a tight differential to a fully locked up C4- all at the drivers whim. I bet on a track the car could be adjusted to be more pleasant to drive and faster than with the electronically managed system.

Theroretically one could use the ABS from a C2 but I bet there would be some issues with calibration of the ABS to the driver selectable front to rear torque split. I am not sure, but I think the electronics opens the center differential when the ABS is working.

garrett376

Except that it would not have any decrease in the locking percentage for acceleration vs. braking - so it's not as good as a 20/100, right? Hmm..

But at lower speed you'd have problematic handling if too much lock was present - that's why the current system also doesn't engage up to a certain speed (can't remember). I'd rather have the electronic version - I've already got too much to think about on the track!

You're right, that's why Porsche solved this issue of braking/ABS and driveline preload altering tire slip angles by having the center differential release on braking to remove any preload of the driveshaft...

Man, I love this stuff - so interesting!

Cupcar

The normal C2 was optionally delivered with a 40% lock LSD and handles very pleasantly. Presumably the C4 Lightweight- if the longitudinal differential was set at Zero pressure-could be adjusted to handle essentially identically.

The Turbo and RS were delivered with a mechanical variable lock 20% under acceleration and 100% under deceleration differerential- which actually turned out to be not so pleasant to drive. I found that for autocross there is more understeer and the car still spins the inside rear wheel on corner exit.

I would be very interested to know how Porsche did the algorithm for the electronic variable lock rear differential in the C4. I have a book on the Ferrari F1 car which in 2000 at least used an electronically controlled clutch system almost identical to the C4's and Ross Brawn said it is the hardest part of the car to set up for a track- so I wonder how Porsche did it.

garrett376

I just finished (last weekend) reading the book Porsche Carrera 4 Allrad 1900-1990 - which covers their process of determination and testing for torque split, driveline preloading, types of 4wd systems, and all factors that I had never considered before that go into making a "non-problematic all-wheel drive system" (as Porsche calls it) for road and race cars - fabulous reading!!!

garrett376

Yes - you are right that it's hard to have a solution for every situation! That setting with zero lock for the longitudinal differential would cause the inside front wheel to break free and spin on tight corner acceleration which unloads or increases that tire's slip angle, making the entire torque delivery of the all-wheel-drive system equal to that spinning tire (=very low!). Which is why I would think a continuously variable system would be better than a set one (or one needing adjustment while driving) - lockup (or a high percentage of it) of the center driveline is needed for situations like that.

Cupcar

I just looked at a diagram of the C4 drive line and I note there is no differential between the front and rear but just a clutch pack that, through variable friction based on the loading pressure of the hydraulic ram, changes the torque delivery to the front wheels. I thought there was a differential of some sort.

Wouldn't the car just behave like a normal C2 with the pressure in the "center clutch pack" at Zero?

This is interesting to think about.

Edit: I looked again and see there is a planetary differential....very interesting there is a hollow ouput shaft from the transmission to a pinion shaft that runs in the hollow shaft...this C4 stuff is cool actually.