Peter Simpson
911 Carrera 3.2 Targa Sport
Year: 1989
Mileage: 105,957
Current Mods: Bilstein
We had been aware that there was a certain wallowing at the front end of our 911s, and we noticed that the shocks were not working the way that they should have been.
All rear-engined Porsches built from 1965 until 1989 (when the coil-sprung 964 was introduced) have essentially the same suspension systems. Torsion bars (longitudinally mounted at the front, and across the car at the rear) provide the springing medium, while damping is carried out by tubular, er, dampers. At the front, a development of the MacPherson strut (which forms the upper link of the suspension system, and also swivels with the steering) is employed. At the rear, the dampers are inclined at an angle from the lower suspension arms to location points within the engine compartment. There have, of course, been detail changes through the years. Torsion-bar diameters (and, therefore, spring rates) have changed, dependent upon model, and some cars have anti-roll bars (swaybars to our American readers) at the front and/or rear.The only major departure from the formula prior to 1989 – when coil springs replaced torsion bars – came in 1969, when Boge self-levelling hydro-pneumatic front struts, replacing the torsion bars, were offered as an option. But the system was never a success and was quietly dropped in 1973, and most cars thus equipped have now been converted to torsion bars. Boge seems to have provided the dampers for most early cars, with Koni and (sometimes) Bilstein units as options. Often the colours of the dampers are the best means of identification. Konis are usually orange and red; Boge units tend to be black or grey, and Bilsteins green. But don’t take this as gospel: modern Bilsteins come in various colours. But technology has moved on. In the early days of the 911, it was hydraulic fluid that provided the damping medium. Then the de Carbon company included a quantity of pressurised gas nitrogen in order to exert a pre-load on the fluid, and thereby improve damper response. Bilstein took the de Carbon principle a stage further, dispensing with the twin-tube set-up and perfecting a so-called monotube design, with gas at one end of the chamber, separated from the fluid by a floating piston. Our diagrams (left) show how this works, the advantage of the layout being increased strength plus more efficient damping assisted by greater pre-load pressure on the gas, and not least the absence of foaming (and the resulting loss of efficiency) because of the physical separation of the oil and gas. A slightly larger quantity of fluid also helps to prevent overheating. The layout has a further advantage – it will work upside down, which means the damper can be contained inside a cartridge inserted in the front-strut tube, with the piston rod facing downwards. In this situation, the damper tube (attached to the top mount) remains stationary, and the piston rod moves with the suspension. The large-diameter sliding member is stronger and has greater resistance to lateral forces, which means easier vertical travel and improved steering. The monotube gas-pressure unit became the preferred damper for motorsport and high-performance use from about the late 1960s, and it’s still regarded as one of the best. Technology has, of course, advanced even further since then, and there are now various alternatives, such as remote fluid reservoirs, but Bilstein has a vast catalogue, which includes replacement units for Porsches of all ages. Modern Bilstein dampers come in a variety of colours, dependent upon type and use. Direct replacements for original-equipment parts are green, while Sport units are yellow. But the strut inserts we fitted to the 911s were green for the front – and the rear on the coupé had Sport, which were yellow. If you give Euro Car Parts a call, it’s possible to find out exactly what’s available for each individual model. Fitting new dampers isn’t a particularly complicated task. The work on the 911 was carried out aided by the luxury of a hydraulic lift, but there’s no reason why the car shouldn’t simply be supported on axle stands, positioned (securely, of course) beneath the bodywork. It is necessary, however, to take the strain off the rear suspension arm when removing and refitting the lower fixing bolt. Do they work? Most certainly. We felt the difference immediately. The 911 is now a lot more stable through fast bends, steering response is improved, and the general ride – especially at higher speed – is smoother. Euro Car Parts, which distributes Bilstein in the UK, quotes £106.41 for each front strut insert, for a full front strut £401 and £123.85 for each rear unit, both plus VAT. The Sport version is slightly more expensive. Obviously, Bilstein performance and quality doesn’t come cheap. It must be added, however, that not only will Bilstein dampers last a very long time but, if we were to spend an equivalent amount of money on engine improvements, this wouldn’t increase the driving experience anywhere near as much! Look out in a future issue to see how easy they are to fit. For more information on Bilstein dampers or any Porsche parts contact: Euro Car Parts on 0870 150 6506 www.eurocarparts.com |
