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993 gearbox whine and aquaplaning.

89se7en

89se7en

New member
Hello everyone. I bought a wonderful 993 c4s two years or so ago (just before first lockdown) but haven't really managed to put many miles on it until this year. We've been to Classic Lemans and Inverness in the last few weeks and as I live in Northumberland, that's equated to around 3000 miles.
Car has just clocked up 119000 miles today. Its oil tight, has been very well looked after (thanks Les), engine is faultless, as is most everything else.
Two thing I've noticed on these trips.
1 There's a whine coming from gearbox (or diffs) at around an indicated 80mph (legal limit on French autoroute). It seems to start once everything is nice and warm. It seems to stop when I lift off, which negates my hopeful theory that it was just wind noise from the massive turbo spoiler that's been fitted. I had intended to get the diff and gearbox oils changed this year anyway. Is this noise normal?
2 In standing water, the thing is LETHAL. Anything over 55mph and it skips and slides all over the place. It's aquaplaning. In anything but standing water it's solid and trustworthy.
Car is running Billy PSS10s, on softest damping settings, but ride height is at the BOTTOM of RS spec and that means a lot of rear neg camber. Tyres are P Zeros, with plenty tread but not sure how old they are.
Any thoughts?

Thanks in advance
Jeff(James)
 
The front diff on my C4 whines a bit but it has done for many tens of thousands of miles with no issue. But the gearbox/diff haas always been relatively silent, so can't help you there.

The suspension geometry sounds like it needs to be properly set up - somebody that didn't know what they are doing has probably faffed about with the kinematic arm setting when setting the rear geometry. This can induce tram lining, aquaplaning and excessive wear on the rear tyres. That said, it could be that the geometry has been deliberate set up to try and dial out the understeer due to the four wheel drive, making the rear a bit looser than would normally be the case but it sounds like they've taken it a bit too far if it scares the bajeebers out of you in the wet. Finally, the ride height also sounds like it's too low if you haven't also got the RS goodies on it that helps to get a proper camber adjustment - looks nice but won't drive too good and the ride is probably overly hard as well.
 
Thanks Clyde, much appreciated.
A geometry thing fits, turn in is very crisp, almost scary quick, but in the dry the back end is very planted. It's nervous on British country roads too, but it doesn't seem like bump steer, and the front doesn't have a lot of neg camber in it, perhaps it' has been modded for the extreme ride height. I've been thinking of raising the ride height to top end of RS spec to see if that helps. I'll do that first, then play with damping.

Now, anyone know of an air-cooled specialist in my area who knows suspension set up? I refuse to use my local OPC. They're "plug it in, swap an (expensive) unit" types with no apparent idea of anything other than standard factory spec and what it says in the book.
My trusted specialists are great, but they're not familiar with the foibles of 993.
 
The whine isn't normal. First port of call is to check the levels of the fluids front and rear. I appreciate that you say the car is dry underneath but do check the levels. Next would be to change the fluids.

With respect to the tyres - check the date codes. I don't like Pirelli tyres - they always seem too eager to flat spot if a car isn't used regularly and on a limited mileage car they seem to age harden and develop dry rot cracks long before other brands. Check the date codes and look carefully for fine cracks between the tread blocks. Anything older than 8 years probably ought to be replaced. If in doubt replace.

Do you have a geo printout for the car? Loads of negative camber will make the car keen to change direction - great for spirited driving in the dry but it can be a handful in the wet, especially if the tyres are past their prime. What is the tread wear like across the widths on all 4 corners? If the tyres are suspect replace them and then see how the car feels.

 
Just saw your comment about bump steer - that's a side effect of lowering a 993-era car without changing the (expensive) front uprights to eliminate the bump steer. A low ride height looks great and is fine for the track but is far from ideal for street use. If the car ever bottoms out in normal use then it's set too low.

 
Good to see another 993 in the north, first impression is it’s too low, and maybe a bit of an extreme geo set up for the road, I’m at RS +10mm and it tracks straight and true.
Bin the tyres and pop some Michelin‘s on there and get the geo done at the same time. Centre Gravity are fabulous and worth the trip down.
Not sure about the diff wine (if that is the cause) but as you have said start with oil changes. ????
Give me a shout when you are over in Cumbria.
 
89se7en said:
Now, anyone know of an air-cooled specialist in my area who knows suspension set up? I refuse to use my local OPC. They're "plug it in, swap an (expensive) unit" types with no apparent idea of anything other than standard factory spec and what it says in the book.
My trusted specialists are great, but they're not familiar with the foibles of 993.
Click to expand...


If you're anywhere near the midlands I would take it to Center Gravity, Chris set mine up a while back and it drove the best it had ever done. He takes the time to work out what your driving style is and what you want from the car during a pre-adjustment test drive rather than just guessing
 
+1 for a visit to Center Gravity.

I'd echo what Clyde said on the whine - although I note others disagree. Specifically the whine on my C4 is something that really worried/annoyed my mate when we drove to Le Mans a few years ago - so it's something about the French autoroutes! I tend not to notice it these day, but it has been there most/all of the time I've owned the car and does not seem to have got worse over time/miles on the clock.
 
...I refuse to use my local OPC. They're "plug it in, swap an (expensive) unit" types with no apparent idea of anything other than standard factory spec and what it says in the book.
Click to expand...


Not surprising at all - most OPCs wouldn’t know their 4rse from their elbow with a 993 anyway [8|] they’re geared up for new owners and the replacement ethic.

As people have said (but don’t try guessing / adjusting in the meantime), take it to Centre Gravity and Chris should sort it for you [;)]
 
100% agree with all the above, a lowered 993 will bump steer like crazy (and when you hit water thats basically what you are doing)

RS uprights and track rods will dial that out, also worth checking the front wishbone joints as these can make the car a problem if worn. If we knew where you are we could advise better!
 
Fellas! Thanks so much for your responses, these are great and much food for thought. I really appreciate this.
Given I'm not sure of the history of the suspension, I think getting it down to Centre Gravity will be most sensible first option.

However, the tyres are 2016 and 2017, and the rears are close to their wear limits.
Which Michelins would you all suggest? Car gets used most of the year round, if only to give it a proper warm thru every fortnight during the bad months, so gets out in the rain a lot.
BARRY I've used nearly all this years 5k mileage cap already, the trip to Centre Gravity will finish it off, so unlikely to be across your way soon. However, I'm in Corbridge, so if you're hitting Alston, Stanhope, Blanchland etc, give me a shout for a cuppa and a biscuit.
 
I'm also on PS2s and very happy with them. They are now discontinued so I'm hoping PS4s will be equally planted. I do need a new set very soon.

When it was a daily driver I also had a set of winter tyres. They have not been on the car for probably 4 years as I hardly use it in the winter nowadays, but my memory is that they were better (than the PS2s) at stopping the car in cold weather and that made me swap them every October/March.
 
I’d run it into D&G Porsche Specialists just along from the Newcastle Quayside - they’ve got a full alignment setup and are well versed in dealing with 993s and the air cooled stuff. They had a 993 Turbo in and a 993 RS for pretty substantial work so will know what’s what and will help you out - ask for Gary and tell them that I sent you.
 
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