Project Spec E46: Rear Subframe, Differential and Fuel Tank

With the cage freshly in the car and the car up on jack stands for the first of a thousand times, it’s starting to look and feel more like a racecar. My decision to attack the rear of the car was strategic, to make the best use of the time I had with the car before its scheduled time to go to a shop for electrical work.

I’ve never really wrenched on an E46 chassis very much. At least, I hadn’t participated in any major projects. For me, this was something of a “try it and find out” session that I was having with the car. While under the car looking at things, it appeared to me that I could probably drop the entire rear subframe without removing any of the components that were attached to it. I thought I could just start by dropping the stock exhaust system, which turned out to be fairly easy. I was surprised at how much the stock system weighed.

The stock exhaust comes off easily enough. Just a few hangers, a brace and a vacuum line at the inside top. The surprising part was how heavy it is.

Obviously, I would need to disconnect the driveshaft from the differential. I then removed the stock heat shields from the subframe where the exhaust was located. Then I would need to disconnect the stock brake lines and drain the dark-as-Guiness-stout brake fluid — or it would just continue to drip-drip-drip brake fluid on you the entire time you are under the car.

Inside the car, I disconnected the parking brake cables and removed and discarded the parking brake handle. Then I unbolted the rear shocks and removed the stock springs. The last step before you can drop the rear subframe is to unplug each of the ABS wheel sensors on each side and unplug the brake pad wear sensor on the passenger side. Once that is done, the rear subframe drops down with a little persuasion on each side as it slides down off of the two rear subframe pins.

I’m confident this is the opposite way the subframe was installed at the factory. It was a fairly quick and painless process to remove it. I thank gravity for the help. Once out from under the car, I continued stripping off all of the parts from the subframe. The rear shock absorbers were installed at the factory and were in such bad shape that when I unbolted the passenger side, the rod fell down into the shock! Clunk!

A personal observation on the stock rear lower control arms: They are flimsy looking as can be! They are simply two stamped pieces of thin steel with a bushing on one end and tack welded together and that’s all that is keeping things in place. No adjustments are available on them other than slots where they mount and the cam bolt that attaches it. I opted for adjustable units that will definitely make setting the rear suspension geometry the way I want it much easier, and provide peace of mind that any sort of jolt wont taco the stock arms.

Adjustable lower control arms are superior in every to the stock pieces.

I removed the parking brake assemblies and the dust shields along with the parking brake cables. Then I removed the stock rear swaybar. My mechanical background stems from aviation, so I tend to like having really clean parts and doing clean builds. I broke out the pressure washer and went to town on everything after a dousing with some Simple Green, and scrubbed everything with a worn out scotch brite pad. For the aluminum upper control arms, I used a new Scotch Brite pad and more Simple Green to restore some of the original aluminum color. I know nobody will see them, but I’ll know they are not only clean, but also free from any cracks or other damage that can hide behind dirt.

The rear subframe has a total of seven bushings that need to be replaced. A couple of them were actually in serviceable condition, but not for a racecar. I’ve studied many different types of suspension bushing materials, and a long story short, I found that a Delrin bushing is as stiff that you can get without it being a metal bushings. The problem with metal bushings is they don’t have any give, whatsoever. And when they do give, they are then permanently damaged. Poly bushings are much softer than metal or Delrin material, and as such, a little more compliant, but I’ve had some bad experiences with poly bushings crumbling or distorting and having a short lifespan. My trials with Delrin have all been good, so for that reason, I prefer to use Delrin where I can on this car.

Getting The Old Bushings Out

I’m sure if you did a search, there are many sources on the topic of removing subframe bushings. I know of a few methods, and the one method I always avoid is the use of a torch to burn the rubber away. Yes the burning does dispose of the rubber portion of the bushing that prevents an easy removal of the bushing itself. However, in the process, I believe you affect the metal in whatever item you are trying to remove the bushing from and makes it brittle, making it prone to cracking and failure.

To get the center sleeves out, I drilled the rubber bushing around the full circumference of the inner sleeve.

Most of these factory bushings have a metal center section that make removal a little more difficult. I’ve used one method that uses a hacksaw blade, or to be more modern, a Sawzall and cut around the center of the bushing. Only cutting the rubber, which cuts easily and quickly. The center section comes out, which allows you to use a chisel or a punch to fold in the edges of the metal insert. This lessens its grip on the housing and, if done right, you actually shrink the diameter of the bushing housing and it comes out, eventually.

Another method is after you have removed the center section, you cut the metal of just the bushing with a saws all or hacksaw blade, but not so much that you cut through into the metal of the housing itself. Just slot the bushing. Then the bushing comes out using a chisel or punch. I tried to use my small portable press, but I couldn’t maneuver the subframe to a position where I could use it effectively.

Because the center portion of the bushings are a solid dense rubber, I used a drill bit and drilled a couple dozen holes in the rubber all around the center support metal bushing. All the while changing the angle as I went to effectively cut more of the rubber in the bushing. That’s how I got the metal centers out of all of the bushings for this job. Then I cheated and I used an air chisel with a notched bit. I had the subframe stripped of all of its bushings in about an hour. Alternatively, one could just drop the subframe off at a machine shop and ask them to press out the bushings for you. That’s an easy button always available to you!

After drilling out the center section, an air chisel with a notched bit knocked out the sleeves.

The New Bushings

I ordered a bunch of parts from Condor Speed Shop. I plug them not because they have paid me to do so, but because this particular shop also races with NASA, builds some great parts for BMWs and they are very easy to deal with.

Just recently, the owner of the shop, Carlos Mendez, competed in the 2021 NASA Championships at Daytona, and won Spec E30 class and Super Touring 4! An impressive feat to win one class, but to dominate two classes is superb. They are easy to find online and browse their online catalog, and the “how to” videos on installing their parts on your own car. They truly make things easier for you and easier to understand how things go, especially when you just don’t know.

The air chisel can make short work of removing the outer sleeves of the bushings.

Condor makes all the Delrin bushings, which I installed in the subframe. I’ll also use them anywhere else I can replace a bushing. Including motor and transmission mounts. I also ordered a complete stainless brake line kit, shifter bushings and even a set of brass bushings for the rear brake calipers plus a lot more, as we will see. Once all the bushings were installed in the rear subframe, I set it aside.

Delrin bushings from Condor Speed Shop will help hold the rear subframe in place.

The rear differential is open in this class. You can do anything you want to do with it, but it must retain the stock housing, flanges and mounting points. Otherwise, anything goes. I researched what can be done to the rear differential to increase bite coming off the corners. Too stiff a differential, like one with welded spider gears, for example, can do more harm than good. Conversely, an open differential also hurts performance by spinning the unloaded tire in corners. I don’t intend to make this a segment on what a differential is and can be made to do, although it’s an interesting subject and there are lot of details, but rather this is what I went with.

A four-clutch differential from BimmerWorld and DiffsOnline was chosen for performance and longevity.

Through BimmerWorld and DiffsOnline, I went with one of their four-clutch setups. It’s not their most expensive unit they sell, but darn close. For me, the additional clutches not only even out the performance from side to side, but also allow for, hopefully, more even wear as the car sees lots of racing miles. I also added a finned rear cover. These diffs are on the cusp of needing a differential cooler, so anything that helps keep the differential as cool as possible while in the heat of battle, is worth it.

A finned differential cover adds cooling and increases fluid capacity.

If I were to do enduros, then for sure I would install a rear diff cooler, but for now, just doing sprint races, this will be all that’s required in keeping the diff cool under fire. Just before I bolted the diff cover, on I added the Castrol 75×140 fluid that DiffsOnline recommends, and then I sealed up the unit. You can reuse the fill plug and all but one of the bolts for the new diff cover. You will need to source just one M10 X 1.5 bolt that’s 50 mm long to complete the hardware package. Then, of course, I set the diff aside. Part of my madness is that I like having components ready to go in all at once.

Before any welding of the rear subframe can occur, the stock gas tank needs to come out. Sigh. Nobody likes removing gas tanks. But in my case, it’s a blessing because I ordered the BimmerWorld fuel starvation kit. Yes, one of the problems with the E36 and E46 is they tend to fuel-starve even with lots of fuel onboard due to the saddlebag design of the gas tank. There is a factory siphon tube that draws fuel from one side of the tank to the other, which is how you burn all the fuel in a street car. In a racecar, it’s not a fast enough process for the car to be able to use all of the fuel available in the tank and can cut short a race, or simply require one to carry much more fuel —thus weight — in the car than is really needed to be able to finish a race and not starve for fuel.

Before we could weld in the chassis bracing for the differential mounting pins, we had to remove the fuel tank.

While the tank was out, we installed the fuel starvation kit from BimmerWorld, which uses an E36 fuel pump to piggyback into the stock pump. One pumps fuel from one side of the tank over to the other side of the tank and that pump sends fuel to the engine. Very similar to the stock system only this transfer system is forced. Thus keeping fuel in the section with the pump feeding the engine, which allows the car to burn more of the fuel on board.

The fuel starvation kit from Bimmerworld uses an E36 fuel pump fuel from one side of the tank to the other to avoid starvation in high-g turns.

Now that I’ve installed one, if I were to do enduros, I’d probably add a pressure switch on the feed line so when the transfer pumps runs low on fuel pressure, I’d have an indicator light on the dash that would let me know I’m getting low on fuel. You could get more sophisticated and put an actual pressure gauge on it to read fuel pressure, but then the driver would have to monitor it — and we all know how well drivers watch gauges.

I replaced the stock 190,000 mile fuel pump, so both pumps were new and again, more peace of mind. With the tank out of the car it was a fairly easy job to do. I drained all of the fuel out of the tank so I could inspect inside of the plastic tank and was pleased to find a very sterile interior. The instructions are written to be done in the car, which is fine and works well, but with the tank out of the car I was able to secure the wiring and transfer hose to the existing lines to limit any possible chaffing that might occur. The last thing to be done was the four wires and the hoses once the tank was installed. But this can only happen after the rear subframe reinforcement kit was installed. That’s next! Until then, I set the gas tank aside.

The Bimmerworld fuel starvation kit keeps the side of the tank that supplies the engine fully fueled to avoid starvation in corners.
Image courtesy of Shawn Meze

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