The following note was created by Marc Belanger.
The most challenging (read: difficult) task in upgrading my car's suspension was the procedure for replacing the rear torsion bars. During my quest for documentation on the detailed procedure for the 944, I consistently came up short. The intent of this article is to share with you the step by step procedure I followed during my recent successful experience at torsion bar replacement in my 1986 normally-aspirated 944 (from 23.5 to 26mm).
Be warned that torsion bar replacement for a 944 is not a job for the faint of heart. In fact even experienced specialists will reluctantly admit that they have never replaced the torsion bars on a 944. So youíve been warned, proceed at your own risk. Please note that this particular procedure applies to my 1986 944; your procedures may vary slightly (i.e. different calipers, ABS sensors, etc.). Please also note that this procedure will require that you disconnect brake lines, so all usual brake system safety procedures will apply, i.e. bleeding, etc.
The next step is to insert and properly index the new torsion bars. In order to properly accomplish this task you will require a sound understanding of the index adjustment procedures for splined-end torsion bars. 911 (pre-964) guys and gals have understood this for years - God bless them.
At this stage, after having removed the torsion bars from your 944 as described above, hold the torsion bar in your hand and take a good look at it. You will immediately notice the splinned ends. Take a closer look and you will notice that one end has more splines than the other. In fact the inner end has 40 splines and the outer has 44.
Now here comes the fundamental concept of the torsion bar adjustment (i.e. indexing). Knowing that a complete circle has 360 degrees, imagine that each end of the torsion bar is actually 360 degrees divided by the respective number of splines. The inner end has 40 splines therefore each spline is seperated by 9 degrees (360 divided by 40) and the barís outer 44 splines are each seperated by 8 and 1/6th degrees (360/44) or 8 degrees and 10 minutes. (Remember your high school geometry? There are 60 minutes in a degree.)
Here comes the real leap of faith. Letís image you are at the driverís side rear facing the spring plate. If we fix the outer spring plate on the outer end of the torsion bar and then move the torsion bar inner by one spline (in the counter-clockwise direction - moves the spring plate up and lowers the car), we will have changed the position of the spring plate (relative to the ground) by 9 degrees. Now if we fix the inner splines and now relocate the spring plate (in the opposite direction, clockwise - moves the spring plate down raising the car) we will be essentialy removing 8 degrees 10 minutes from the 9 degrees we advanced on the inner splines, thus resulting in a net 50 minute change in the spring plate position. Think about it, and reread this paragraph until you get it. Due to the 4-spline difference between the inner and outer splines of the torsion bar, adding then subracting these relative angles allow us to adjust the ride height to within fractions of an inch.
If you understand the meaning of the preceeding paragraph you are ready to index your own torsion bars. If youíre lost or confused at this point, get help. Your next question will probably be "But how much do I raise or lower the spring plate in order to get the ride height Iím looking for?" If you are not changing the torsion bars and you are starting from the factory torsion bar position (you did take notes when you took it apart didnít you?), you can Ďestimateí the ride height change (approx 6.5mm for a 50 minute change) by using basic priciples of geometry a the rules of the great queen SOH-CAH-TOA. If you are replacing the torsion bars with ones of different rates or did not note the location of the spring plate (dumb-dumb) before removing the torsion bar, the answer to this question, Iím sad to say is: "Trial and error my friend, trial and error." When I bought my new torsion bars, and dared ask this Ďstupidí question, the customer service rep at Automotion gave me that answer. I thought he was nuts! He also told me it would probably take three tries to get right. I whimpered then cried. My better half Kristine smacked on the head and snapped me back to reality. "Thanks Kris, I needed that."
Unfortunately, he and she are right. It will take about three tries and a good smack on the head (maybe more) to get the ride height right. This effort is aggravated by the fact that you must basically re-assemble the entire rear torsion bar assembly, remount the wheels, then roll the car along enough to settle the suspension before you can take the a valid ride height measurement. Once you take your measurement you then have to disassemble the rear suspension once again to make any necessary corrections to the ride height. You repeat this process until you get it right. (Lord take me now!)
The 944 is a wonderful car, but the design of its rear torsion bar housing does not allow for easy adjustment. While I have no first hand experience with a 944 rear end coil-over setup, it would certainly be a great if you plan to change your rear ride height on a regular basis.
Hope this helps with your attempt at turning your 944 into the ultimate handling machine of your dreams.