Four-wheel drive on 993 TT, 4 and 4S

[matthewb]

Hi all

I've got a friend who claims that the viscous coupling to the front wheels on TT, 4 and 4S models isn't normally active, i.e. the car runs in rear-wheel-only mode most of the time.

I, however, disagree. The manual says on p31 (and p125) that it is "... equipped with permanent four-whel drive with variable power distribution on front and rear wheels."

Elsewhere in the manual it specifies (if memory serves correctly) that this distribution varies from 5% to 40% dependent on conditions. But I can't find this entry despite reading the manual from cover to cover. Does anyone know on which page I can find this information?

Thanks
M

 

[gzhpcu]

Your friend is correct. The 964 4WD version was permanent. The 993's isn't. Under normal driving conditions and as long as the viscous coupling temperature is below 320 degrees Fareinheit all drive occurs on the rear wheels.

 

[matthewb]

Hi Philip Thanks for your input. I have now found another 'reference'. It's an A5-sized hardback book - an official Porsche marketing piece called "The 911 idea". It covers the 993 Gen-2 (i.e. the Varioram version) up to the Turbo and the GT2 but not the Turbo S. It says, "At least 5% of engine power is permanently applied to the front wheels, this ratio increasing to 35% in normal driving conditions. At most, the front wheels receive approximately 40% of the power." I would have thought that under 'normal driving conditions' - i.e. straight-ish line, constant-ish speed, zero-ish acceleration, even-ish traction, the percentage would have decreased to its lowest, i.e. 5%, not increased to 35%. So confused! Any thoughts? Thanks M

 

[Dave Wilkinson]

The following is copied from [link=http://www.autozine.org/911/911_9.htm]http://www.autozine.org/911/911_9.htm[/link] and might explain things a little more clearly. In contrast to the 964, the 993’s 4-wheel drive system was rather conventional and simple, but it actually performed far superior in real world. Instead of epicyclic differential and mult-plate clutch, it used a simple viscous-coupling LSD as centre differential. To most FF car, viscous-coupling means understeer, but for the rear-wheel-drive-based 911, it means very much loyal to the Carrera 2’s character yet provided superior grip when needed. To make the viscous-coupling always engaged the front wheels, the rear tyres were made marginally smaller in diameter, enhance established a small speed difference between the drive shafts to front and rear. With the speed difference, the viscous liquid normally transferred 5-15% torque to the front axle, which was much less than the 964’s system. In abnormal conditions, that is, whenever one axle lost grip, the viscous-coupling LSD may send up to almost 100% torque to the other axle. Both the center LSD and rear LSD were now pure mechanical, but clever electronics was used in the newly-added ABD (Automatic Brake Differential). Again, ABD was simple yet effective. It was just a program, sharing all the hardware with ABS. Whenever rear wheels spin, it braked the spinning wheel thus the rear differential would send more torque to the other wheel. It was particularly useful for extreme conditions such as on snow, while LSD covered most normal conditions. The 993's system weighed only 50 kg, that’s just half of its predecessor. Energy loss was also halved. It made the 993 Carrera 4 nearly as quick as the RWD version. Production cost was reduced as well. Regards Dave

 

[matthewb]

How interesting. Many thanks for this, Dave. I know you were only quoting, but am I to infer from this that the rear wheels (wheel + tyre combo) are not quite as big in terms of overall circumference/diameter as the front wheels? It would be a neat trick to fool the system in this way but are we really saying that the rear tyres have a slightly smaller profile than the fronts? Have all tyre manufacturers agreed to produce so-called 285/30's that are really 285/29.5's just so Porsche can fool their 4WD system into providing permanent four-wheel drive? Your quote said, "It made the 993 Carrera 4 nearly as quick as the RWD version." Do you know how much faster (acceleration & top-speed) the 964 RWD was compared to the 4WD version? And finally, how much percentage-wise did the 964 4WD system give to the front axle compared to the rear? Cheers M

 

[Dave Wilkinson]

Matthew, Your inference is exactly right about the tyre sizes. However, there is no conspiracy with the tyre manufacturers as you will see from the attached comparison, a 285/30 IS actually considerably smaller than a 225/40 in terms of overall circumference, doing 841 revs per mile on the rear compared to 829.3 revs per mile on the front. This difference is obviously enough to heat the viscous fluid sufficiently to give the 5%-15% torque feed to the front axle. As far as the 964 is concerned the performance difference was small - but it was there. Because of the different gearing and, I suspect, the added sure-footedness of the C4, the C4 was actually quicker off the mark than the C2 up to 50mph where they appear to match each other. Beyond that, the C2 creeps ahead beating the C4 to 60 by 0.1 sec (5.1 compared to 5.2). By 130mph the margin has grown to 3.8 seconds (23.7 compared to 27.5). Top speed is only 1 or 2mph different around 160mph (+ or -). The torque split on the purely mechanical centre diff on the 964 C4 is 31% to the front,, 69% to the rear. With the hydraulic actuated locking mechanism under the control of the PDAS this could be infinitely varied from 0% to 100%. This answer has considerably stretched my research at 7 in the morning!!! Regards Dave

 

[gzhpcu]

Hmmm... pretty confusing... Now I really don't know what the answer is anymore. Adrian Streather's book on the 993 says that Porsche does not employ any deliberate methods to ensure some torque (drive) is permanently sent to the front wheels. But does mention the wheel diameter differences cause the viscous coupling to be activated... Regards, Phil

 

[Dave Wilkinson]

I think I would read Adrian's comment as "nothing deliberate other than the wheel size difference". Dave

 

[matthewb]

Hi Dave Wow - up and about at 7 o'clock - I wasn't sure there really were two 7 o'clocks per day but you've just proved it Thanks for the dedication to duty I guess if I'd done the arithmetic, i.e. 285mm*30%=85.5mm and 225mm*40%=90mm, I could have seen for myself that there would be a difference in profile. Duh! Just to confirm, regarding what power is transferred to the front axle under which conditions in a 993 - [ul][*]I believe that if you're driving on a flat, straight road with a consistent surface and at a constant speed, then there would only be a 5% -ish transfer of drive to the front axle. Yes? [*]And I believe that if you're really going for it on some bendy mountain road or you're driving on ice, the max transfer of power to the front axle is 40% -ish. Yes? I imagine the reason for this small amount (compared the 964's 100%) is that the limited-slip properties of the 993's front/rear differential prevents any more than 40% from being transferred to the front axle. Yes?[/ul] Thanks again M

 

[Dave Wilkinson]

Matthew, I'm not completely convinced that your synopsis is right. I agree that it would seem that under "normal" driving conditions you would expect a minimal torque transfer to the front (i.e. 5% or so). That's unlikely to change unless there is some condition that causes the rear wheels to change speed when compared to the front. Slower or faster - the result is the same - the VLSD will heat up and the diff will increasingly lock, up to 100% according to the bit I copied before. I'm not sure where the 40% figure came from. In the 993 the operation of the viscous-coupled diff is aided by the ABD which automatically applies the brake to a spinning wheel. Regards Dave

 

[JB 3.2]

When I have the annual MOT done on my 993 C4, they are unable to use the normal 'brake' rollers. As soon as any brake is applied to the rollers, the front wheels start to rotate forcing the car out of the rollers.

 

[matthewb]

Sorry Dave - I've read the piece from autozine again - got it now With regard to 'normal driving conditions' and '40% max transfer', I am referring to the official Porsche hardback promotional book, "The 911 idea" that I mentioned in my second post in this thread. [ul][*]On second thoughts, I decided that this book might have defined 'normal' as hanging the car round some corners rather than than my initial interpretation of a Sunday drive - so I edited my post a bit - sorry, I'm afraid your answer no longer applies to my post [*]I'm just wondering if perhaps the limited-slip properties of the 993's front/rear differential might prevent any more than 40% from being transferred to the front axle. Are you sure about 'almost 100%' possible to front axle? [/ul] Cheers M

 

[Dave Wilkinson]

ORIGINAL: JB 3.2 When I have the annual MOT done on my 993 C4, they are unable to use the normal 'brake' rollers. As soon as any brake is applied to the rollers, the front wheels start to rotate forcing the car out of the rollers.

John, I hear what you say, but I can't, for the life of me explain it!!! It will need someone with a better understanding of the system to explain why that should occur. As I understand it, the rollers are driven and the car should resist it with the brakes. Why that should cause the other axle to move just beats me. It's also only at less than walking pace too. FWIW my 964 C4 does the rollers with no problem - but then, it IS a different system so that doesn't really mean much. Regards Dave

 

[gzhpcu]

Just to add to the confusion, from Porsche advertising....

 

[matthewb]

Ha ha - LOL - thanks Philip. We seem agreed on around a minimum of 5% permanent drive to the front axle but we have a choice of 40%, 50% and nearly 100% for the maximum. So, gents, what's it to be? M

 

[pse_SC]

When I have the annual MOT done on my 993 C4, they are unable to use the normal 'brake' rollers. As soon as any brake is applied to the rollers, the front wheels start to rotate forcing the car out of the rollers.

At a rolling road (dynometer) day I attended a couple of years ago, the 4WD 993s had to have their front couplings disconnected temporarily, in order for them to get a proper reading (bhp). This gives some indication to the fact that the front wheels must be turning as you say, John. But, of course, this is when the car is under power as well, as opposed to just braking the rear wheels (at the MoT). Not sure of any of the other technicalities though[8|].

 

[JB 3.2]

Probably a bad description on my behalf, when the rollers start to turn the rear wheels (which the car has to brake) the fronts start to turn. I'm sure other C4 owners must have the same issues at MOT time.

 

[szklarek]

As a C4 owner I'm not aware of this being an issue when the car is MOT'd, in fact, I'm not aware of there being any 'rollers' at any of the MOT stations I've used! So either they test the brakes some other way or they've found a way round the problem - I'll look more closely next time(September)! Kind regards Anton

 

[totyfinalist]

mot manual states that all four wheel drive 911 models including 964 must not be tested in the rollers, they must use the tapley decellarometer(gmeter) out on the road because of their mechanical 4wd system,this also applies to lots of other older 4wd vehicles like sierra 4x4 ,cavalier 4x4 ,405 4.4 and of course the range rover(WHICH HAVE NO CENTER DIFF).All 911 models from the 993 forwards can use the roller brake tester to check the brake efficency as long as the roller speed is 6km/h or less, their are some testers that are slightly faster although i have never seen any.So a 993 should be no problem. Also when trying to get 993 and 996 vehicles in to the workshop in the recent snow ,they had very little if any tangible advantage over a 2wd model,I.e they were absolutely hopeless!! maybe we didnt keep it spinning for long enough to get the viscous fluid hot to transfer the torque even with the abd light going and the front wheels on good grip it didnt seem to transfer the power and this was on several cars so i think they were typical and not faulty.I know that had that been a 964 with the diffs locked, that it would have been a walk in the park.This is the price it seems for a more neutral handling characteristic and of cousre to save money and weight!! regards C

 

[Dave Wilkinson]

ORIGINAL: totyfinalist mot manual states that all four wheel drive 911 models including 964 must not be tested in the rollers, they must use the tapley decellarometer(gmeter) out on the road because of their mechanical 4wd system,this also applies to lots of other older 4wd vehicles like sierra 4x4 ,cavalier 4x4 ,405 4.4 and of course the range rover(WHICH HAVE NO CENTER DIFF).All 911 models from the 993 forwards can use the roller brake tester to check the brake efficency as long as the roller speed is 6km/h or less, their are some  testers that are slightly faster although i have never seen any.So a 993 should be no problem.

This seems to be completely at odds to the experiences mentioned above!! When I questioned the tester at my recent MOT (964 C4) they told me that as long as the car had an unlocked centre diff there was no problem in using the rollers - and I can confirm that doing so was no problem at all for my C4. They DID confirm that the RR is a problem as, indeed, is any car with a locked (or even partially locked) centre diff. My expectation with the 993 C4 is that as long as the centre diff is cold it will be fine. If the car has been working the diff hard on the drive to the MOT station it may be another story. Bear in mind that the whole point of having a diff in the centre is to allow the front and rear wheels to turn independently. Only when the diff is locked is there likely to be a problem. Regards Dave