Clutch replacement and Hydraulic Bearing Conversion - E34
The clutch on the car finally could not cope with the torque and would slip in high gears when the boost rolled on. The old set up used a modified single mass flywheel with a Ford falcon clutch cover. I was already running a modified release bearing and since the G290 has a deeper bell housing, the release bearing housing was already a custom set up with the old clutch and on fitting the G20 box, had to place the bearing an additional 20mm forward to give the same relative release arm position than before. Alpina call out a standard BMW T/O bearing, so you could conceivably use one of those. Pics below show the old clutch and the release bearing adapter
Hybrid Ford V8 clutch and modified T/O bearing
To deal with clutch slippage I planned to installed a new heavy duty pressure plate and cerametallic puck plate. These were sourced and the box dropped for fitment. I also planned a conversion to a hydraulically operated throw out bearing, doing away with the clutch salve cylinder and shift fork. The hardest part of this conversion is decided which bearing to use and designing and machining a special adapter that will slip over the existing guide tube and offer the bearing up to the pressure plate at the right clearance.
Bearing Selection
I decided on a Howe release bearing for stock clutches. The bearing is shown here:
I could not find any bearings in the aftermarket that would clear the diameter of the stock Getrag guide tube so the design of the adaptor had to account for this. Also, the Howe 8270 was probably not the best application as it had to be modified as the two 'ears' that have the fittings machined on them sit too far out and can interfere with the mouth of the clutch pressure plate body as will be seen later.
Adapter Design
The bearing has to sit on a custom machined adapter that is specific to the G290 deep bell housing. The adapter must locate the bearing to it has a small clearance from the clutch release fingers when installed and keep the bearing concentric. This is the CAD design I came up with:
For proper concentric location, the existing G290 guide tube was also modified so it sits in the bore (the 46mm of swallow) of the adapter. The length dimensions are selected to offer up the bearing with a spacer or two to the pressure plate fingers. The front nose O.D. is a very snug fit to the I.D. of the bearing, and the I.D. of the nose just clears the drive splines of the input shaft. The final version has some nice radii on it.
Here is the G290 guide tube and seal housing assembly with the guide tube cut back on the left and the adapter on the right.
The guide tube assembly and adapter together. Note the reaction rod (long M8 bolt with head cut off) tapped in to adapter to limit bearing rotation if it locks up. It also locates the adapter into the guide tube housing. The adapter is also held on by an M6 cap screw 180 degree opposite the rod.
Bearing Modifications
As mentioned earlier, the long connection ports on the bearing were a concern as the fittings move with the bearing and contact the opening of the pressure plate frame as they sit so far out. They are also used as reaction point to stop the bearing spinning on the adapter so some length needed to be retained on them. I cut these down and taped / plugged the ports with 1/8" NPT plugs. The bearing was taken apart and the body of the bearing was then tapped for 1/8" NPT pressure and bleed ports and AN type fittings for the bleed and pressure sides.
I originally tried -3AN for the pressure side connected through the long stock clutch circuit. The stock circuit is long and goes around the passenger side of the car as that is the side of the box the stock slave cylinder is. With the stock circuit going into a -3AN hose, fast releases of the clutch felt damped and if the pedal was sidestepped it would return lazily as the fluid moved through the long stock piping system and small -3 hose to the bearing.
-4AN size fittings solved this problem in conjunction with modifying the master cylinder hydraulic connection to a 1/8" NPT port so a -4AN right angle fitting could be installed right at the bulkhead. The cylinder was stripped and a 1/8" tap drill size run down about 7mm into the end. A 1/8" NPT tap was then run in to cut new threads. The brake type flare fitting is the same pitch of thread so it taps out to NPT nicely. The cylinder was thoroughly cleaned out and re-assembled afterwards. A one piece, relatively short -4 line now connects directly to the nose of the master cylinder straight to the release bearing through a hole in the right hand side of the bell housing. Its long enough so that the box can be dropped for clutch access without disconnecting any part of it. Clutch response is fast and clean.
-4AN size fittings solved this problem in conjunction with modifying the master cylinder hydraulic connection to a 1/8" NPT port so a -4AN right angle fitting could be installed right at the bulkhead. The cylinder was stripped and a 1/8" tap drill size run down about 7mm into the end. A 1/8" NPT tap was then run in to cut new threads. The brake type flare fitting is the same pitch of thread so it taps out to NPT nicely. The cylinder was thoroughly cleaned out and re-assembled afterwards. A one piece, relatively short -4 line now connects directly to the nose of the master cylinder straight to the release bearing through a hole in the right hand side of the bell housing. Its long enough so that the box can be dropped for clutch access without disconnecting any part of it. Clutch response is fast and clean.
Tapping the nose of the master cylinder for a NPT thread to allow installation of a 90 degree -4AN fitting.
Checking step height of complete assembly to ensure right clearance (around 1mm to zero) to clutch fingers. One spacer had to be used so my calcs were spot on.
Results: The complete set up works well. The bearing gets enough stroke with the stock 19mm master cylinder. The stoke actually has to be limited a little by the pedal stop as the fingers were going too far in and contacting the plate slightly so the clutch would release completely then grab again. I would think I am getting about 12mm of stoke now. An interesting result of this is that even with the HD cluch pressure plate, pedal effort was too light and I had to remove the over-centre helper spring on the pedal to get some nice weight back. I can only explain this as the new set up being much more efficient than the stock set up.
Clutch
I installed a 5 puck (or paddle) Exedy cerametallic sprung plate with HD pressure plate. The flywheel was machined and modified to accept larger pressure plate bolts and dowels (based on a Ford Falcon 6 bolt cover). I was aware of the drivability concerns with this type of clutch but wanted to try it to see if I could live with it. The answer to that question came very quickly when trying to reverse my car out of my very steep driveway.....NO. The clutch judders and grabs so bad, I though it would wreck something. Granted, it would smooth out over time as it beds in and I could rev the car and slip to prevent it but I could not deal with it every day. The plate was removed for a HD organic
The organic plate had awesome drivability, very smooth, but would slip in high gears under heavy boost. Undeterred, I decided that a cushioned button clutch was the only other option to try before going with with a very expensive multi-plate set up. After much discussion with my clutch man in WA (Graham at the clutch factor), we finally decided on a specially built 9 puck marcel spring cushioned button with a 5 rivet sprung centre and a heavy duty chevy diaphragm spring (apparently from Canada). This is the very solid looking plate (its not an exedy unit, note the much heavier centre section):
The pressure plate was modified with the heavier spring in preference to altering spring pivot points and such which can alter the engagement characteristics. Here is the new spring with the old one laying on top. You can see how much thicker the new spring is:
The old spring removed from pressure plate:
With is all back together, the pedal feels significantly heavier. Engagement / drivability is right between the 5 puck and the organic. It will drive pretty well if treated properly. There is a lot of chatter noise in reverse up hills, but other than that and an aggressive take up, its not too bad on a daily basis once the technique is mastered. As far as torque capacity, I have not been able to trouble this set up yet regardless of boost.
The clutch operation happens over a very small range of pedal travel. One change that probably needs to be made is to reduce the diameter of the master cylinder from 3/4" to 5/8" to allow more modulation and pedal travel. This is easily done and something that I will do in the future.
The clutch operation happens over a very small range of pedal travel. One change that probably needs to be made is to reduce the diameter of the master cylinder from 3/4" to 5/8" to allow more modulation and pedal travel. This is easily done and something that I will do in the future.