1. How large should I go?
First, you need to find out what's the stock size for your car and the appropriate upgrade depending on intended use and your tolerance for the increased stiffer ride vs. the tradeoff in better handling. The following table lists stock torsion bar diameter in mm:
|Year||Model||Front (mm)||Rear (mm)|
* some sources say 80-85 had 24.1 rear bars
The formula for calculating spring rate is: k=d^4G where k=spring rate, d=diameter, G=modulus of elasticity (constant for steel)so ignoring the constant:
% increase in stiffness = ( (d2^4)- (d1^4) ) / (d1^4) * 100
where d2 is new diameter and d1 is original diameter
Here are some rate increases for earlier and later cars:
Stock Front = 18.8
Stock Rear = 23
Stock Rear = 24.1
21 mm : 55.7 %
26 mm : 63 %
22 mm : 87.5 %
27 mm : 90 %
27 mm : 57 %
23 mm : 124 %
28 mm : 120 %
28 mm : 82 %
29 mm : 153 %
Some common combos are: 21/26, 21/27, 22/27, 22/28, 23/29 and 23/30.
The smaller bars are oriented to combined street/autox/track use while the big bars would most likely be a harsh ride on real-world streets but provide great handling on the track. I opted for 21/27 for my Targa which was used for about 8 PCA Driver's Education events and 10 autox per year and street driven. I went with hollow 22/28 Weltmeister bars on my coupe due to its increased stiffness over the Targa and they both have a stiff but tolerable street ride. Talk to others or better yet, get a ride before deciding. If you have a pre-78 car, you should consider installing adjustable factory spring plates from '78-86 911 (pre-G50). These can be purchased used for $50-$100 and gives some ride HEIGHT adjustability. Better yet is the aftermarket Sway-a-way adjustable spring plate which allows finer adjustment of ride HEIGHT (esp. useful when corner balancing).
This should be done with a full tank, tires correctly inflated and driver's weight in the car. First, measure the HEIGHT at the fenders front/rear. The factory spec from wheel-center to t-bar center will not help assuming the car will be lowered below Euro-HEIGHT. The car will be out of spec. You need to determine esp. at the rear how much you will be lowering so that it's still useable on the street. A good rule of thumb is you should be able to place your hand flat on the tire and feel the fender well on the upper part of your hand. I lowered mine approx 1" using this method. This is the limit for a car used on the street and you still need to be careful with driveways and speedbumps etc. You will also need to install steering rack spacers to correct for bump steer before alignment and optionally, corner balance the car. The rack spacers only cost ~$11.
2.1 Front Torsion bars
The front bars are mounted longitudinally inside the a-arm and are removed from the rear.
1. Place jackstands high enough to get under, leave wheels on. Remove underbody cover.
2. Remove torsion bars adjuster using 11mm socket.
3. Pry off rear endcap with large screwdriver.
4. Grab torsion bar splines with padded pliers/cloth and wiggle out from rear.
5. Grease new bars and install noting L and R markings for Left and Right sides.
6. Install new foam dust covers.
7. Install endcaps/adjuster, lower car and bounce suspension to check ride HEIGHT.
8. If car is still too high, turn bar clockwise to lower.
Fronts were a breeze to install and adjust. Ignore the instructions you get and the Haynes manual. They tell you to remove too much stuff. You only need to remove the front t-bar cover if the bars are broken or rusted in place. Mine came out easily even with rust. There is a foam grommet over the bar to prevent dust. I got new ones from Stoddard at $3 each before I realized I could have cut foam into a circle with scissors and make a dust cover. I did the fronts with lowering in an hour. I didn't change any bushings. If you're doing bushings, expect to significantly increase your time.
2.2. Rear Torsion Bars
1. Place car fairly high on 4 jackstands so it's level. Mark jackstands so this HEIGHT will be constant. I put rear stands under the inner torsion bar tube. Obviously, can't use outer torsion bar cover.
2. Remove lower shock bolts.
3. Remove toe-adjuster eccentric bolt/nut (and lower droplinks if you have adjustable swaybars).
4. Remove rubber strip, rocker panels and round access cover for torsion bars.
5. Remove 4 bolts holding spring plate bracket (at outer torsion bar tube). Pry off bracket.
6. Use small floor jack to support weight of control arm and gently lower arm to lowest point to relieve spring plate tension.
7. Critical step: This is basis for HEIGHT adj. Use pencil to trace outline of upper spring plate on fender well to mark original position.
8. Remove camber adjuster eccentric bolt/nut on spring plate.
9. Remove 2 19mm bolts from spring plate to control arm and raise jack to support weight of arm.
10. Pry off spring plate cover over torsion bar. If the spring plate is stuck, either use a Visegrip locking pliers to hold torsion bar or you may need to tap the spring plate with a hammer to free itself from the torsion bar splines.
11. Remove spring plate and withdraw t-bar through the access hole in the body.
3. Now comes the fun part - setting the ride HEIGHT.
The Porsche Tech Spec book lists the trailing arm angle as 36deg30'-37deg with each 1 deg change in angle resulting in 8-9mm change in vehicle HEIGHT. The torsion bar has 44 outer splines (1 spline=8deg10' and 40 inner (1 spline=9deg) therefore the smallest incremental change is 50' or about 7mm diff in ride HEIGHT. I measured the angles/ride HEIGHT before and after using a protractor and found that this method does not work if you change the spring rate. Using a change in angle of 6deg, my car still ended up being jacked up over 2" higher. However, through trial-and-error, I found that the change in untensioned spring plate HEIGHT resulted in similar change in ride HEIGHT but this will vary a little with spring rate. Therefore, I expect it will take at least 2 tries to get the ride HEIGHT right. It took me 4 tries.
12. Grease splines/bar liberally and install new t-bar noting L and R markings for Left and Right sides.
13. Install spring plate 1 " higher than base HEIGHT assuming lowering car by 1". Make sure both sides are same HEIGHT. Up to 1/8-1/4" side-side variance is normal. To change position requires rotating the inner end of the torsion bar 1 spline opposite to the spring plate.
14. Lower control arm and install bolts.
15. Install shock bolts to do a trial fitting.
16. Mount wheels/bounce suspension and check fender HEIGHT. Note correlation between change in ride HEIGHT and 1" change in spring plate spring plate HEIGHT. Car may be too low or too high on first try so you will need to reset the spring plate HEIGHT. Move the spring plate higher results in lower ride HEIGHT.
17. Lastly, get a good alignment. A good compromise track/street alignment is as follows: