964 C4 awd/trac system self-contained?
 

I'm looking at doing an engine swap in a C4 and have a few questions.

1) Will I have trouble retaining the trac system? From what I've read, it has its own control unit, its own sensors, and seems pretty self contained. I'm just wondering if by removing the motor or changing the flywheel I will have trouble with it.

2) Does the high pressure pump rely on engine vacuum at all? The motor I want to run does not pull much of any vacuum.

3) If I disable the trac system, and there is no pressure running to the diff locks, the car will be a fully functional rear driver, correct?

If anyone has a good article or link to a thorough guide to the system, that would be great.

Thanks
Shiva

HI the diff locks are plumed into the ABS pump and runs of the ABS sensors, not engine sensors, why are you changing the engine??and to what!.

regards mike

Oh, good-- I wonder does the trac computer look at engine speed, too? Or just wheel speeds/accelerometers?

I want to put in a diesel engine, for fun, really. Plus I think it would be cool to have a Porsche that gets 40 or 50mpgs, can run on biodiesel, and is still reasonably quick, even if its demeanor has changed completely.

HI now i am getting interested, what sort of diesel?? what year C4 is it and i will look on the diag for the wiring to check engine/diff locks.

regards mike

HI just remembered we had a guy over hear put a BMW 2.5 TDI into a 928 and ran that for some time, when he sold the car he dropped the 8 back in, got a no'ther 928 and popped the 2.5 in that.

regards mike

The PDAS system uses the 4 individual wheel speed sensors to determine whether to actuate the solenoid valve, or disable the system entirely based on speed. The ABS valve block doesn't share the hydraulic circuit with the solenoid valves - they're separate just to clarify... :)

1990 C4

I want to use a VW 1.9L TDI (hopped up, of course-- a 200hp diesel C4 is what I have in mind).

So it looks like I won't have a problem!

Just in case it's disabled, does anyone know if there's an easy way to get the system to hold a steady pressure to get at least some front/rear split even if it won't change and will be engaged at all times? and this is, of course, without having pressure going to the rear diff lock...

Also... if I run no pressure and it's effectively RWD, I will have a rear diff that's effectively open, right? That might be ugly with 3-400 lbs-ft of torque...

As long as there are no wheels slipping, the torque split is fixed to all wheels. With no pressure, it is ALL WHEEL DRIVE with open differentials front and rear, meaning if ANY tire slips, you will ineffectively put the power to the ground. That is why you need to have a functional longitudinal differential - when you accelerate through a corner, the inside front tire is the least loaded and will spin, decreasing the amount of torque reaching the ground. Now with a 1.9L engine, it probably won't matter!!! But if one wheel gets loose, you are basically driving a one-wheel-drive car.

Why wouldn't you have pressure? The system is totally independent of the engine.

HI Garret would that be the open rear diff with the multiple-plate limited slip diff which is variable from 0-100 % locking, and with that in mind i would say it was a 3 wheel drive car as the front diff has no LSD?? and the front inside wheel can spin but the center transfer and rear diff will drive the rear wheels (69% power to rear, in a strait line) and the rest to the front (31%), the drive torque can be changed infinitely from 0-100% or 100-0% in fractions of a second in case of front or rear axle wheel slip.

regards mike

Hi Mike - if the center differential is "open" where the PDAS is not preventing its slip - with even one wheel loose (say the front inside tire) - even the rear wheels will go nowhere. You are right, that's how the system works when it's active - i was referring to his question about NOT having the PDAS active, resulting in an open rear and center (and front) differentials.

HI there is no center diff it is a epicyclic gear box.it will drive but not as efficiently as the 'center diff' and the rear diff will not lock to transfer the power/drive on wheel slip.

regards mike

That's fine; I am just reporting on how the Porsche engineers describe it and how it works when I race my C4.

The book "Carrera 4 Porsche Allrad 1900-1990" covers exactly how the mechanicals of the 964 C4 all-wheel-drive system works. It's a great read for anyone interested (it's in German and English)! ;)

HI what you require is the booklet '89 service information technik No'WKD495 120 from porsche for the info you require.

regards mike