Note: Remember to observe the laws in your state. Whether or not you need a catalytic converter in the line is completely on you to do the research.
The exhaust section can be done so many different ways. The heads of the E23, E24, E28 are slightly different in that they will clear the passenger side motor mount bracket. While this may be true, I have not tried the install in this fashion. I will simply document what I did to get the exhaust functional.
18.104.22.168 TOOLS REQUIRED
22.214.171.124 TIME & $$ 8Hrs & $110
The front exhaust manifold needs to be from an older (E24, E23, E28) car in order to clear the passenger side motor mount bracket. If you go this route, you will also need the downpipes from the same car. Unless, of course you make your own custom downpipes. For the purposes of the imperative section of the write-up, this is the cheapest option to get an exhaust on your car.
All parts here are to be sourced at a local junkyard.
Note that you will come into difficulty attempting to buy the downpipes used. However, if you can achieve this, it is preferred at your local junkyard.
The exhaust headers from the E28,E23, E24 are DIFFERENT than the headers from the E32, E34. These older M30 headers have 3 bolts that hook up to the downpipes instead of 2 as shown in Figure 30. Those pipes that have 3 holes that connect to the downpipes are from an E38/E23/E24 and are shown in Figure 31. They are not compatible, so whichever exhaust manifolds you get, get the same downpipes.
If you refer to Figure 30, you will see the 2 bolts that bolt on to any aftermarket exhaust you decide to put, but just remember that the passenger side motor mount bracket, given its very awkward position and design, will most likely interfere with stock downpipes. You can see the dichotomy here when referring Figure 30.
Note: if you let off the gas when coasting your car, and you hear backfiring, then more than likely you have an exhaust leak somewhere… probably closer to the engine. When it sucks fresh air in when you let of the pedal, it backfires.
The downpipes shall be sourced to match the headers from the same engine. These can be found at a local junkyard or ebay. The E30 M20 downpipes are not appropriate as they will not fit the headers from the E28 535i. Figure 32 shows that the yellow-highlighted section shall be kept and everything after that shall be cut off, including the catalytic converter.
The midpipes for this shall be custom and purchased from places like Jegs or Summit. This will involve cutting/welding dual 2” exhaust from these midpipes directly to your muffler. See below for a rough Parts List of what to buy to build the mid-pipes.
Straight Tubing: WLK-47972 7’ 2” Straight Tubing - $15
Tubing Bends: 2x SUM-622001 2” Tubing 180Degree - $30
Make Subframe Hanger: Home Depot Steel - $10
X-Pipe: JEX-XP2 2” $23
Note that the x-pipe is crucial for any dual exhaust system and shall be installed. Welding is more thorough, but the sound and power output is very impressive and well worth the extra work.
The aftermarket 2” inlet muffler shall be sourced from either online, a junkyard, or someone you know. A rusty junkyard muffler is not an option here. Your 2” exhaust shall weld up to the muffler. Please follow the steps highlighted below for installing the muffler in the correct orientation using the existing E30 Muffler Straps.
1. Install Muffler in the position you desire where it fits appropriately in the E30 underside. You must ensure that the muffler inlet piping is identical to that of your 2” mid pipes. All of the following welding that I specify should be done after the X-Pipe.
a. Take note on its lateral as well as longitudinal position so that the muffler tips are adequately spaced from the body.
b. The muffler shall be suspended by the existing E30 muffler straps.
c. Use a car-jack to push the muffler in the exact spot you want the muffler to settle. Once it’s held in its position, you can now start to match up the mid-pipes to the muffler.
d. First do a single mid-pipe and tack it together. Once the muffler is in the position you desire, then you weld the second mid-pipe. Once it’s all completely tacked, you remove the exhaust and completely weld.
The result is an impressively aggressive, free-flow exhaust system that allows your M30 to realize its full potential.