Tip: With all stainless steel exhausts, any residual oils and fingerprints “burn” into the surface of the exhaust the first time you start the engine and heat the unit. It is important to properly clean all polished areas or areas that you want to maintain cosmetically with brake cleaner or a similar product before you start the car for the first time after installation.
The exhaust tips come out right where they’re supposed to be, the mark of a well-made cat-back exhaust.
Whenever you work with stainless steel, wipe all oils and fingerprints off the metal before you fire the engine. The oils in your fingerprints burn into the steel and leave permanent marks.
You 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