I want to preface this with a small note. I think i'm probably one of the few people who have owned (and therefore used extensively) all of the following 993 suspension choices: standard Monroes, KW variant 3, Bilstein PSS10. I have also test driven (on multiple occasions) the very popular Bilstein HD, 993RS suspension, Porsche M030 and the standard 993TT suspension. I've also driven Targas as well. Not sure if they have different suspension for their increased mass but just in case they do ....
993 Suspension advice is something that gets asked for a lot especially on the internet but sadly its also throws up a lot of poor information. Most people are just keen to recommend what THEY PERSONALLY OWN, without necessarily considering the wishes, needs and past car history/driving style of the person seeking the advice. We all like to think of our own choices as being 'the best' and the natural tendency for many is to seek verification through internet chatrooms. The chances are though many people only ever owned the one suspension system, or if you're lucky one other as well which was already well past its best when they changed it (so not worth referring to in the first instance). To complicate matters further there are usually one or two other individuals who are keen to just stir up trouble and confusion in general whilst masquerading as someone knowledgable with something genuninely worthwhile to contribute.
Well, having owned 3 of the most popular and leading 'prosumer' suspension choices and tried numerous others including the factory Porsche options whether they be on standard 993's, RS's, Targas or turbocharged models I'll add in my unbiased two cents here and now. None of the options I know of or that we've talked about here are a bad choice and it really is so subjective and up to the individual and what he/she likes. Go out, get some test drives, meet some heads, sit in people's cars and make up your own mind. I've driven cars that I've found perfectly acceptable that others have hated with a vengenance. I also found certain 993's pretty appaling, the very same cars that others have marvelled at and treasured over every square inch of UK bitumen they've ever swallowed. Don't take advice, go out and try, and when people use words like hard, soft, comfortable, smooth, tramilining, bumpy.... these words don't mean anything unless you are THAT actual person speaking them.
Now that's out the way, i'll offer my personal opinions for what they are worth. Feel free to disregard or tally them with your own findings. My own suspension choice was firstly pegged by the fact that I wanted full adjustability. Secondly I wanted something modern and up to date in design and current know how. Finally I wanted a car that felt VERY planted and rock solid in a similar manner to other cars i've owned like EVO's, caterhams and the like. My feeling is that KW and PSS10 are the good choices if this is what you're after but as ever one mans meat is anothers poison etc. My belief is that most people will NOT want a car as focused as mine. That said, PSS10 or KW alone do not necessarily make for a focused car and the state of mine is definitely a combination of many other things: ride height, uprated anti roll bars, increased unsprung weight, geometry amongst others.
Again, draw your own conclusions but if you want my central overview on the 993 after trying 3 differents rims,different geo's, varying ride heights and all manner of suspension choice, then it's pretty simple: if you want a proper 'real world' road based 993 for UK A and B roads that just works, then consider keeping the car on 17" rims and no lower than 'sport' height and standard ARBS and bushes. My opinion is that the 993 doesn't work too well on 18" rims in any guise, it's just too old and if you're anything like me (having been reared on Lotuses and supple compliant sports cars rather than those that are guided by fashion) then the above combination will be the best starting point.
Going more hardcore with a 993, from rim size to suspension to uprated arbs to ride height, stiffer bushes etc. is a road that MANY people go down, only to find that the car is pretty poor on many surfaces, desperately annoying around town and suitable for little more than a 'weekend-only' role, preferably a few hours at a time. Of course there are people like me who aren't prepared to forego the RS look/spec and don't mind a hardcore setup but just be warned of what you are dealing with. I wouldn't describe my car now as everyday and on orught roads it can still be a real animal if it so chooses !
If you're interested I'll now take you over my decision making process here. I am writing this in March 2011 but I actually changed over to PSS10 late last year around October time. The reason I deided to change over to PSS10 from KW is simply because with the more aggressive RS specification that I wish to run my 993 at (see below) I found that overall the car lacked compliance giving rise to poor road manners over less than perfect surfaces.
low RS ride height
The best way to solve this was to lessen the spring rates and introduce a little more suppleness and give into the car. Attacking it from that angle meant that I could keep my bling rims, brakes, arbs and sexy RS ride height.
Now I could have changed the springs on the KW for something a little more relaxed but when weighing up the costs it actually made more sense to go for a fresh set of Bilstein PSS10 which are known to have slightly softer spring rates.
I was fortunate enough to try out another owners 993 which was fitted with PSS10. The owner in question very kindly let me drive his car and although his was at a different ride height again and with different spec geo, rims etc.. it didn't take me very long to understand how the PSS10 generally compare to KW. There seems to be a lot of internet mythology surrounding this especially the rumour that PSS10 are in some way more track orientated. My opinion for what it's worth is that it's definitely the other way round.
As you may have read over some of my previous pages, my KW variant 3 did work very well in certain configurations and I don't want my directions or my webpages here to put anyone off any particular choice. I have driven other cars with KW var3 at RS+10mm ride height and sensible geometry and in the case of one of those (a car fettled by Chris at centre of gravity) it was probably the best all round road 993 I have ever driven. Planted and very secure with masses of grip but smooth and comfy especially at low speed.
When my car was at sport ride height with a gentle geometry and with the preload taken off it worked pretty well. On 17" rims with plenty of compliance introduced into the mix via the tall sidewalls it worked fabulously well and if I ever had to hillclimb or rally one of these things or wanted a B road 993 then thats the config i'd be looking at, so devastating was it at soaking up rough terrain. But I was always determined to modify my car into an 'RS homage' that is close in spec to the actual rennsport M002 factory car. I found that with each RS upgrade I made, I degraded the stability over bumps and road manners of the car a little bit more.
By the time I was at more agressive camber than RS, stiff RS arbs, heavy RS calipers/discs/rims and an RS ride height I discovered that there was very little compliance left and the car was quite snatchy and wayward over bumps and undulating surfaces. The RS arbs in particular changed the cars manners dramatically. After driving a friends PSS10-shod 993 my thinking was that going the Bilstein route might introduce the right amount of compliance back into the car and restore some much needed give between the car and the rough roads, the lumps and the bumps
I was able to install my PSS10 fairly painlessley. With the adjustable droplink bracket at the rear I am now able to use standard RS rear doplinks which I had previously bought. This makes it nice and neat when using the RS arbs ... no custom brackets or custom machined droplinks required.
A quick word on those rear droplink brackets, make sure they are very tight as they can easily come undone if not tightened properly and you can easily break something should they ever come loose.
A word on the fronts as well. When you compress the damper there is a flat section near the top of the shaft that can lodge itself against the upside of the top hat if the shaft is not perfectly concentric with the hole in the top hat. This will mean that the top threaded end of the damper will not rise up enough and if you install them like this then one day when you go to full droop the shaft will get itself concentric with the top hat and the damper will come crashing through and then be very loose with a whole load of play. I guess you could easily break something if you ever did this and it could be quite dangerous. My tip is to compress the spring and then assemble into your top mount. Make sure the threaded end is situated quite high into the top mount when the strut nut it tightened.
After fitting the PSS10 my car's behaviour improved quite dramatically. It is now a lot more planted and predictable, less perturbed and unsettled on bumpy and undulating roads. Less squirming and side to side movement when the going gets rough. In a bend the chassis remains stable regardless of the surface underneath and you don't have to indefinitely adjust the steering to counteract constantly changing passive rear steer. At 120mph+ on the autobahn she is lovely and stable. As predicted there is a fair amount of squidgyness there once up to speed.
The net effect is a car that can be driven with a lot more trust and at far greater speed. I am confident that this setup is the answer for me and is properly real world. I run at around setting rear = 4, front = 5 for the road so there is plenty more stiffness in the damper if I take get on circuit. This stage in my development process now paves the way for further RS mods such as the rear arms and the monoball top mounts. Sometimes I do miss the ultra solid and stiffly sprung feel of the KW and I can tell you that the grip I had with the KW was MIGHTY, but you can't have your cake and eat it too. The springs rates of the standard 993 RS suspension are significantly softer than KW variant 3 and now I understand why. The only caveat is that for road use the front bar needs to be soft otherwise you get too much deflection and stifness at the front wheels and a ton of understeer too.
So there you have it. I have had a lot of mails asking me if I have changed my opinion of KW variant 3 but the answer is the same as given above. They are a great set at a good price. At a more 'gentle' spec on a narrow bodied 993 I think that they work very well indeed and give a very taut, controlled and planted car particularly on 17" rims. However, if you want to bias your car like mine with compliance taken away from all other parts of the suspension system then my experience is that they don't work as well. A car has to have a way of isolating you from the inevitable unwanted effects of rough roads. Tyres are a big part here as is ride height and spring rate but if all of these are instead optimised for stiffness and lack of relative movement, then you are fighting a losing battle in your quest for creating a decent road car.
Huge subjective warning here but seeing as i've been asked here goes. In terms of a direct shootout if that's at all possible (trying to discount the particular nature of my car) I would first of all note that the KW are significantly better at low speed. They seem to be smoother and not as jarring or crashy over bumps when travelling slowly. This is more crawling through village/town stuff though rather than anything 30mph+. The PSS10 can send strong thuds up through the car over low speed sharp bumps and if you hit a deep pothole prepare to wince.
Once up to speed though I think that the damper response of the bilsteins is a little better/quicker with it being an inverted monotube and the car feels more a bit better controlled. The PSS10 only have one adjustment (versus the bump and rebound on the KW) but I do feel that the adjustment range is better matched to the spring rates. I found a fair bit of the KW's rebound adjustment redundant given the stiff springs, the harder rebound settings and the extreme soft settings are all but unusable in my experience. Of course this isn't an issue once you dial them in but those are my findings nevertheless.
Ultimately the PSS10 are a fair bit more expensive so you might expect slightly better performance in certain areas. Ride height adjustments take place by pushing against the helper so the main spring preload never changes which is a setup I vastly prefer. The PSS10 only have one adjuster. Sadly its not on the top of the damper bodies but at the bottom but it's a large dial with clear numbers and a whole lot easier to get to and adjust than the KW bump screw. If you have a small mirror you can easily tweak them without jacking the car.