This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
The USA spec cars were equipped with a much higher ride height than the European cars. This was to accommodate the stricter USA standards for bumper height elevation. It's a popular modification to lower the 911 back to the European specs or even lower, in order to achieve a lower center of gravity for better handling. The lowered look also looks a bit sleeker as well.
Lowering the front end of the 911 is a snap: the suspension was designed to be easily adjusted. The 911 uses a pair of torsion bars in the same manner that many cars use coil springs to support the front suspension. There is an adjustment screw on the bottom of the front suspension that resets and alters the base position of the torsion bars. This allows the adjustment of the height of the car through a significant range.
It should be noted that lowering the front of the car has some slight drawbacks. The front of the car will be lower, and hence the front spoiler or valance panel will be increasingly prone to curb damage and breakage from speed bumps. In addition, lowering the 911 front end also changes the alignment of the front suspension. Specifically, the toe-in and camber of the car is affected. It is highly recommended that if you decide to lower the front end, that you have the car professionally realigned when you are finished.
Another problem to consider is bump steer. Lowering the car will result in the tie rods no longer being perpendicular to the ground. While this shouldn't affect handling, when the car travels over a bump, the steering wheel will rotate violently in your hand. The solution for this problem is the installation of the bump steer kit (See Pelican Technical Article: Installing the Bump Steer Kit). This kit raises the steering rack back up to be aligned with the tie rods, thus reducing the amount of bump steer. I recommend that you install this kit underneath your steering rack if you lower your 911. Installing the turbo tie rods (Pelican Technical Article: Installing the Turbo Tie Rod Kit) will also help solve bump steer problems.
The lowering procedure is simple, and all adjustments should be made with the car level on the ground. Make sure that you have a tape measure handy when you start so that you can keep both sides of the car at the same level. The factory measures the height of the front suspension as the difference between the height of the center of the road wheel, and the height of the center of the torsion bar. Therefore, as this difference increases, the car rides lower to the ground. The standard USA spec for the ride height for almost all 1965-89 911s is 99mm ( 5mm. The maximum difference between the left side and the right side should be less than 5mm. The European specification is 108mm ( 5mm. To lower the car to this height, simply loosen the adjustment bolt on the front suspension. Again, make sure that the left and right sides of the suspension are equal. If you have difficulty turning the screws with the car on the ground, lift up the front suspension a bit using a floor jack. Make all measurements only after you have pushed down on the car a few times to level it. Also make sure that your trunk is empty and your spare tire is installed in the front trunk. Keep in mind that the car will also ride lower with a full tank of gas (approximately 150 lbs of extra weight). There may also be a spacer installed on the top of your shock tower that can be removed if you are lowering your car.
Balancing the left and right height of the car should also help to improve your handling. In general, the adjustment screws for the left and right sides should be dialed in roughly equal amounts. If they are not, then this might indicate that one of the torsion bars is old and worn, and needs to be replaced. Another method of checking the left to right balance is to measure the distance of the wheel to the top of the fender. This however, can sometimes give inaccurate results if one of your fenders is slightly tweaked.
Make sure that you don't loosen the screw so much that it doesn't press against the edge of the torsion bar end. If you find that you can't get the correct adjustment that you need, you may have to remove and adjust the location of the torsion bar adjustment lever that fits on the end of the bar.
The lowering of the rear suspension is a bit more complicated. In a similar manner to the front of the car, the factory measurements are based on the difference from the center of the torsion bar to the center of the road wheel. To arrive at this number, measure the height of the bottom of the torsion bar cover from the ground, and then add 1/2 the diameter of the torsion bar cover. Then subtract the height of the center of the wheel. For the European 911SC, this measurement should be 16mm ( 5mm (USA 37mm ( 5mm). On the earlier cars, this distance was 12mm. The maximum difference between the right and the left should be no more than 8mm. Check the Porsche Technical Specifications Booklet for your year car for the exact specifications.
The rear suspension uses a rear trailing arm that is sprung using a torsion bar. This torsion bar is connected to the rear of the trailing arm via a long radius arm. Adjusting the position of this radius arm with respect to the torsion bar is what changes the height of the car.
In order to adjust this height, you need to remove and reset the splines on the rear torsion bars. For more information on removing the torsion bars, see Pelican Technical Article: Replacing/Upgrading Torsion Bars. The inside end of the torsion bar has 40 splines, and the outer end of the torsion bar has 44 splines. This clever arrangement allows you to make incremental changes in the torsion bar height. Simply move the inner spline clockwise one position, and the radius arm counterclockwise, and you will lower the radium arm by about 5/6ths of a degree. Repeat this adjustment procedure, until you have achieved the desired ride height, and both sides are equal in height.
James Bricken of Pelican Parts, offers the following tip on achieving the rear height and balance that you are looking for:
Often one end of the torsion bar, or the other, will stick in place causing a loss of base line (which means you have to start from scratch. The way I do it (and I do it often), is to remove the lower-rearward bolt on the torsion bar cover and the lower shock bolt. This allows you to fully drop the rear suspension. Then I use a Smart Tool (but a simple leveling protractor works as well) and measure the inclination of the spring plate. Next, I scribe a line on the trailing arm where the spring plate meets it. This gives me a good approximation of where it should be when I reassemble it. Then I take the whole thing apart, generously lube the new torsion bars and install them. I then install the spring plates on the torsion bars just slightly and take measurements with the Smart Tool, adjusting them until I get the least amount of change from the original setting. Once I am satisfied that I am close, I put the rest of the suspension together, without fully tightening anything except for the camber and toe bolts. Once the car is on the ground, and I am happy with the results, everything else is then tightened down.
An excellent method for synchronizing the settings of the left and right rear torsion bars is to use a level to determine the amount of adjustment required. Aside from measuring the total height of the rear suspension, you can also measure the angle that the rear radius arm makes with the horizontal. Place the car on a flat and level surface, and then place a bubble level tool on the radius arm. You can adjust the height of the bubble level tool by placing small spacers underneath each end of the level. Once the level is adjusted, compare the angle to the opposite side by taking the level and the spacers and placing them on the opposite radius arm. Adjust the setting of the torsion bars as needed to make sure that the level of the left and right suspension are exactly equal.
If you have a later model 911, then you're in luck, as you don't have to remove the torsion bar covers and radius arms to get at least some degree of adjustment. On these cars, the radius arms were equipped with an adjustment screw. To raise or lower the rear of the car, simply loosen the large nut and bolt closest to the torsion bar. Then rotate the other bolt located next to it. This bolt is eccentric, and will cause the rear of the car to be raised or lowered as you turn it. Adjust the height as described previously, and then tighten the bolt located nearest to the torsion bar. Make sure that you don't touch the two nuts located towards the rear of the car: these adjust the toe-in and the camber for the rear suspension, and should only be adjusted by a trained professional using an alignment rack.
A third method of adjusting your rear suspension is to use an aftermarket radius arm. These arms, available for about $400/pair, have a built-in adjustment screw that works similarly to the adjustment screw for the front suspension. This radius arm allows you to quickly make adjustments, and is designed more for racers, than the casual everyday driver. They also allow for quick and easy change of the torsion bars.
After you've made all these adjustments to the ride height of the car, it's a wise idea to have an alignment expert check and align the car, especially if you have had to remove the rear radius arm. Make sure that you have the ride height set before you take the car in, as subsequent adjustments will alter the alignment specifications. Drive the car for a few miles, and then double-check all your measurements. Make sure that the left to right height of the car is the same after driving. When adjusting your 911 ride height, it's smart to consider the weight balance of the car. With the correct alignment dialed in, a properly set up 911 will have approximately 40% of the weight on the front wheels and 60 % on the rear, with the chassis riding very slightly lower in the front.
Here is the adjustment bolt for the front suspension. Make sure that you don't loosen the bolt so much that it is no longer pressed against the torsion bar end. A simple wrench can be used to lower the car in seconds. Be aware though, that changes in the height of the front suspension will affect the toe-in alignment and camber specifications. Remember to have the car realigned after any height adjustment. Sometime this bolt gets a bit rusty - use generous amounts of WD-40 before attempting to remove it.
The rear radius arm on this 911SC contains an assortment of adjustment bolts. The bolt located closest to the torsion bar cover keeps the ride height secure (green arrow). The bolt second from the right is used to adjust the height of the radius arm (red arrow). There is some degree of adjustment available, but not as much as can be achieved with the removal of the torsion bar. The bolt that is furthest to the rear of the car is the rear camber adjustment (white arrow). The bolt next to it adjusts the rear toe-in specification for the car (yellow arrow).
As determined by the factory, the measurement for determining the front ride height is calculated by subtracting the height of the center of the road wheel from the height of the center of the torsion bar. The larger the number, the lower the car sits to the ground.
The rear height measurement is similar to the front end measurement. The specification for the rear height is determined by the difference in the height from the center of the torsion bar to the height of the center of the road wheel.