Suspension upgrade

My 964

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Even a Porsche suspension is susceptible to wear and tear. After nearly 180.000km the shocks were worn. In addition to that the left front wishbone was showing signs of needing replacement sometime during the next 20.000 km. This page describes the upgrade.

Step one: What do I want?

The requirements were simple: I wanted something that would improve feel and handling at the Nürburgring Nordschleife without rattling my teeth on public roads. Specific aims were:

reduced bodyroll
improved turn-in
increased road-holding
decreased acceleration squat and brake dive

The Ring is quite bumpy when compared to a "normal" track, so it wouldn't be sensible to go for a rock-hard racing setup even without the "no teeth rattler" requirement.

There was a non-functional requirement too: it shouldn't cost an inordinate amount of money (even though that concept is a bit unfamiliar to Porsches).

Step two: Research

There aren't too many people who appear to know exactly what can and what cannot be done to improve the suspension of a '91 C2. At least, if there are, they're hard to find. John's Porsche 964 site provides some useful accounts by people who've upgraded their suspension, using Porsche 964RS components and using third-party (H&R)components. In addition, the archives of Rennlist (particularly the 964 forum) contain lots of useful tidbits. A third source of info was the Ringers list: even though it's a Ring-dedicated list it is home to quite a few people who are knowledgable about Porsches in particular and suspension in general, not to mention that they know what kind of setup suits the Ring.

In parallel to my online research, I gave some indepents (both in the Netherlands and in Germany) and my OPC the requirements to see what they came up with.

Step three: shortlist

Given the budget I came to the following shortlist:

Bilstein dampers and H&R springs (the Sport springs they make for the 964)
Koni dampers (the adjustable yellow ones) and Eibach springs (the Pro Kit stuff)
Boge dampers and H&R springs (I'm not exactly sure which springs)
Koni dampers (again the yellow ones) and original Porsche 964RS springs

Getting some idea of the spring rates of the various springs was quite difficult, but I think I had some useful results.

Step four: time to choose

The way I see it, the Eibachs lower the car to the desired height (about 40mm lower), but are very soft (particularly in front: about 65% softer than 964RS springs to begin with, still 19% softer than the RS springs when both are at their highest spring rate). At the rear they have about 50% of the RS spring rate. At the higher rates, they should be stiffer than the original C2 springs though.

The ordinary H&R's have spring rates that are 24% to 17% softer than the RS in front, 41% to 48% softer for the rear.

Unfortunately I couldn't get hold of spring rates for the Boge/H&R combination.

The Porsche RS springs weren't (surprisingly to me) not that much more expensive than the Eibach or H&R springs.

Both the Eibach kit and the ordinary H&R fell off the shortlist because of their spring rates, and because of some negative advice from somebody I trust in 964-matters, and who has first-hand experience with different 964-suspensions.

Which left the Boge/H&R setup and the Koni/Porsche. I think I would have been happy with either, but I liked the warranty of the Koni/Porsche combo, particularly when backed by my local OPC. They had done lots of good work on my car so far, and hadn't dropped the ball yet.

The Boge/H&R combo was suggested by Kaul & Will, by the way. I would have been happy to extend a Ring weekend by two days for them to get the work done, but the local solution is more convenient. Combine that with the fact that the Koni/Porsche combo has proven its worth gets us to the conclusion of the selection saga:
I went for the yellow Koni's in combination with the Porsche RS springs. Given the stiffness of the rear RS springs the standard 20mm C2 rear sway bar would be exchanged for the adjustable 18mm RS bar. Some negotiating with my OPC yielded a nice discount on a front strut brace and some other work, which never hurts.

Step five: uh oh... (Plan B)

To paraphrase a military proverb: once the work starts, the plan goes out the window. All the parts were ordered and I dropped off the car. Within two hours my phone rang: there was a bit of a problem. The springs didn't fit the dampers. Even though the research had suggested that they would: in practice they didn't... As you can see in the images, the dampers for an ordinary C2/C4 are too long for the springs (RS springs on the left, original C2 springs on the right). To put it more precisely, the top mounts in a '91 C2 are vastly different from the top mounts in an RS.

I'm very glad that Adrian Streather spent quite a bit of his time and attention looking at this problem. The result of this impromptu investigation was a list of possible solutions. During this process ThoRSten (964RS owner from Sweden) persuaded a friend of his (another 964RS owner) to go out into the cold to see if he could do some measurements on his car (ThoRSten's 964 was still on jackstands).

Two ways of solving the problem would be to get the RS top mounts (expensive), or to figure out a way to raise the spring perches on the shocks. The third option, shortening the rod on the shocks, was not feasible because the adjusters are in the way.

I figured that both courses would take lots of time and therefore money to get things right, so it was time to start considering alternative options.

Option 1 would be to get RS top mounts and possibly mess around with the spring perches.
Option 2 would be to mess around with the spring perches bigtime, particularly in the rear.
Option 3 was to find some Eibach RS springs (comparable to Porsche RS springs, but with a suitable length).
Option 4 was to come up with a Plan B.
Of course, there was still Option 0: put the original springs back in (they would work well with the Koni's anyway).

The decision process was quite straightforward: Option 1 was too expensive. Option 2 was too expensive and too experimental for my tastes. Option 3 would be perfect, but the Eibach RS springs were long out of production and nowhere to be found. Option 0 wasn't particularly attractive from a handling improvement point of view. Which meant that Option 4 was the way to move forward.

Another uk oh... (Plan B Mk.2)

While this was going on, Adrian came up with another important piece of information. One of the overall goals of the suspension upgrade was to find a setup where the parts worked well together. Therefore the plan was to fit the RS 18mm 3-way adjustable rear sway bar too. However, fitting such a bar to a standard 964 might result in a dangerous situation. Only one of the three positions is usable. If you use the other two positions you run the risk of having the sway bar hit the bodywork, with similar effects as bottoming out the suspension. Not good.

In case you wondered: the front sway bar suffers from similar problems, but isn't easily fitted to a C2 anyway.

Suffice it to say that I refrained from installing any RS sway bars to my car, instead staying with the original bars for now. A future upgrade might be to go to a slightly thinner rear bar.

Strut brace

One aspect of the upgrade went without any problems whatsoever: the fitting of a front strut brace. Stiffening the front suspension increases the stress on the bodywork. Adding a front strut brace helps the front suspension towers to remain at the same position relative to each other.

Yet another uh oh... moment

After checking with Jeff Curtis (yet another Rennlister) to make doubly sure that the H&R Turbo springs would fit a 1991 C2, I decided to let the Porsche RS springs be and go for the out-of-the-box H&R Turbo solution.

After a short wait I got a call that the new springs had arrived. One more day of fitting bits and pieces and getting the car aligned again, I went to pick it up. It certainly looked very good: the front was lowered by about 30mm; the rear sat about 10mm lower. Perfect. The pictures below show the original ride hight and the new ride height.

The front end felt a bit nervous over bumps, but that was easily solved by adjusting the shocks to a softer setting. Body roll was significantly reduced, there was less squat and less dive, and although bumps were transmitted more clearly, it was definitely easy on the fillings.

Back home I decided it might be a good idea to see if I could make some pictures of the new springs. Thanks to the car sitting lower, this wasn't very easy. What I did see was that the springs were Green. Which they aren't supposed to be. H&R sport springs for a Turbo are supposed to be Red. A quick call to the dealer confirmed what I feared: they'd put normal C2/C4 sport springs (which are Green) under my car instead of the ones I had asked for (which are Red). Despite my providing them with the correct H&R part number. Marvellous.

The advantage of this was that I could do a full roadtest of the Green springs myself, then switch to Red springs and compare them to the Green ones. Testing was (of course) done at the Nordschleife of the Nürburgring. After a few laps I softened the front dampers a touch more, and didn't mess around with them after that. The car didn't bottom out in the Karussell, road holding was improved, and it was easier to feel what the car was doing. All in all, a big improvement.

Project complete at last

While I was having fun with my Green springs, the dealer had received the correct springs from H&R. These were fitted two weeks after testing the Green ones at the Ring during Easter. May 4 was the premiere of the Red springs. Below is a picture of me at the Ring, taken by Alexander Neurohr.

If you like you can admire the shiny red springs even better by going to the brake upgrade page.

The 300km trip to the Ring told me that the Red springs are quite a bit firmer than the Greens. This is very noticeable when cornering: the car stays much flatter and turns in better. Naturally, squat and dive are less than with the Greens. Again there's no problem bottoming out when going through the compression at Fuchsröhre or when going round the Karussell. Even though the Red springs are quite firm, they don't rattle teeth unless you drive over a sharp ridge that's perpendicular to your direction. Road holding has improved a lot. The ride height is the same as with the Greens.

Conclusion: a spectacular improvement over the old setup, and a clear improvement over the Green H&R's.