can see how well the Corsa system fits under the final drive and suspension components. It sounds great and should last the life of the car.
Here’s the vacuum hose that operates the exhaust control valve. You can fold it back on itself and seal it with a zip-tie.
One thing you give up on the cat-back system is the exhaust control valve. But this does nothing for performance, and really doesn’t need to be there.
Project Results
All of the claims of big horsepower and torque gains are just marketing until you put the parts on your car and then put your car on the dyno. I installed the Corsa RSC cat-back system and took the project 328i back to the same dyno where I made the baseline tests and ran new dyno pulls under the same conditions to see how much power the cat-back really made.
This dyno run shows the stock horsepower and torque lines in blue compared to the power made with the Corsa RSC cat-back exhaust in red. You can see that the Corsa makes more power and smoother power at all levels up to 5,000 rpm, and then matches the stock system above that.
This dyno comparison is expressed relative to engine speed (RPM). The test showed that on this day, I actually lost a tiny amount of maximum horsepower compared to the test with the stock system shown in the introduction of this book. The stock setup showed maximum horsepower of 170.23, and this fell to 169.82.
We did see a maximum torque gain of almost 5 ft-lbs from 177.59 to 182.19, and the arc of torque is wonderfully flat. In addition to the gain of maximum torque, the value is more than 160 ft-lbs by 2,200 rpm and rises steadily to the maximum at about 4,400 rpm, then stays over 160 until about 5,400 rpm.
Peak horsepower and torque are not the most important goals for increasing street car performance. Rather, you should focus on increasing horsepower and particularly torque through the rev range or the total area underneath the lines. With the cat-back exhaust in place, both horsepower and torque come up quicker and smoother than with the stock system. This means more actual power that you can perceive from the driver’s seat throughout the car’s operating range.
Also, notice in the dyno chart that with the stock setup, the torque and horsepower lines are quite choppy, indicating that the stock system is not well-tuned. When I installed the Corsa exhaust with reflective sound cancellation, both the horsepower and torque lines smoothed out a great deal owing to the tuned nature of the exhaust system.
The most important thing revealed during the test was that the cat-back section of the exhaust was not the most restrictive part of the system. We would have seen bigger horsepower and torque gains if the exhaust had been the part holding the car back. The dubious distinction of the most restrictive component almost certainly goes to the mid-pipe that includes the stock 18-year-old catalytic converters or to the M52 intake manifold, but it’s also likely that the stock exhaust manifold plays a role, along with the stock air intake. I discuss modifications to those components next.
The exhaust headers attach directly to the body of the engine at the exhaust ports on the cylinder heads. On all E36 engines, the exhaust ports face the right (passenger’s) side of the car; you have two manifolds for the forward three cylinders and rearward three cylinders. For 1992–1995 325 and 1995 M3 cars, the exhaust manifolds are made of cast iron. The 1996–1998 units for the 323, 328, and M3 are made of lighter welded tubing.
The main thing you want in a header (and throughout the exhaust system) is efficient flow of gases on their way out of your car. This means a reasonably sized pipe with a few smooth bends and no restrictions. It’s good for this pipe to be wrapped or coated on the outside to keep heat in the exhaust gas until it exits your car. It’s also good for this pipe to be made of stainless steel, although stainless is brittle and prone to failure when exposed to high heat or large amounts of vibration.
For heavy-duty applications, such as racing or extensive track day use, the more malleable mild steel is a lighter and more durable choice. For street use, cosmetic purposes, or corrosion resistance in salt-air climates, stainless is a better choice. You may also choose to have the exhaust coated with ceramic, such as Jet-Hot coating, inside and out to reduce gas friction and heat loss (hotter gases are less dense and lighter), and to move gas through the system to the exit more efficiently.
The stock BMW exhaust header is very good by most standards, but torque and horsepower improvement can be obtained by a quality aftermarket replacement. Be sure that the product you select works with the mid-pipe and the cat-back system.
Finally, if you’re shopping for a header, you can find some with long primary tubes of equal length converging into two pipes. Each primary tube is generally about 1.75 inches. These headers help to tune the exhaust by making sure that each exhaust pulse comes through the system at its own time. The stock unequal-length manifolds allow pulses to come through at the same time at certain RPM.
Most BMW headers offered for the E36 are similar in length to the factory manifolds, and aftermarket headers are mid-length. It is almost impossible to fit a true long-tube header in a BMW engine bay; these are generally a custom-made part if you want them. Most of the aftermarket “long tube” headers for BMW E36 models are really mid-length headers.
Many designs are possible, but the most popular and effective header setup is to use the factory routing and factory termination point. Using this arrangement, you can fit the exhaust system with standard mid-pipes, catalysts, and so on. As a rule, cylinders 1, 2, and 3 feed one header and cylinders 4, 5, and 6 feed the other header with the exit flanges mating to the factory catalytic converter section.
When considering which headers to buy, the primary factors are the fit and finish of the item. Many of the budget aftermarket headers either do not fit, or have extremely thin walls and will melt under the higher exhaust temperatures seen in high-performance engines. Some inexpensive headers have oxygen sensor bungs placed in locations where you cannot fit the oxygen sensor, and some do not quite mate properly with the mid-pipe. High-quality exhausts manufactured by companies such as Supersprint and Akrapovic are your best option for ease of installation and usable lifespan. However, you pay more for the quality you’re getting. Another option is to use the European BMW headers. These are usually available on the used market, and are of extremely high quality and durability. They may not flow as much as some of their aftermarket counterparts, but they flow better than the U.S. market stock units and have OEM fit and finish, so they definitely work in the engine bay.
Header, Mid-Pipe, and Catalytic Converter Fitment
One factor to consider with exhaust headers is whether you are able to use them with the stock mid-pipe and catalytic. If the header does not fit, it can be expensive and difficult to modify it to fit with the stock mid-pipe. If you also replace the mid-pipe and catalytic converters, this can also be expensive and can be difficult. Your best option is to seek out an exhaust header that is a well-known name
