A replacement high-flow catalyst can really help improve airflow through the system, which means more power under your right foot. If you drive on the public roads, be sure to stay on the right side of the law by running a catalyst. Modern ones have no performance drawback.
For a modern car designed to work with at least one catalytic converter, the presence of a catalyst does not mean you can’t increase horsepower. The entire engine management system is designed to work with the catalyst and provide good performance. If you are concerned about the catalyst being restrictive, or you think it might be plugged-up, you can buy a new high-flow replacement unit.
Prices for new direct-fit catalytic converters for E36 models range from $500 to more than $1,000, depending on your model, so this is not a decision to be made lightly. Aftermarket “universal” catalysts cost much less and may be welded into place, but results vary widely. At a minimum, make sure that the inlet and outlet diameters for any aftermarket catalyst match those on your car. And bear in mind that with older cars such as the E36, some changes may have already been made to the factory parts. Check your car’s actual system. Also, check to be sure that any aftermarket catalyst fits in the available space and does not knock against the car’s floor-pan. In general, you are better off spending the money on a direct-fit catalyst designed for your E36.
Although extreme high-performance applications generally include removing one or more catalysts from the exhaust system, it is a violation of U.S. federal law to do so on a car registered for use on public roads. Moreover, removing the catalysts makes it harder, if not impossible, for your car to pass emissions testing, and removing the catalyst on any OBD-II car (1996 and newer) will likely cause a check engine light.
Be aware that an illuminated check engine light is an automatic emissions testing failure on all OBD-II cars. Simply clearing the light and driving to testing while the light is still off does not bypass this requirement, as the OBD-II system reports that the car has not driven enough miles to report reliably on engine condition. Testers see this and fail the car for emissions testing wherever such tests are required.
Legal Aspects of Catalytic Converters
The laws that govern catalytic converters are strict. It is illegal to remove a functional catalytic converter even to replace it with a newer or better one. It is also illegal to tamper with emission control devices. The anti-tampering law applies to individuals as well as to businesses. Individuals may be fined as much as $2,500 for each vehicle tampered with, and businesses are subject to fines of up to $25,000.
Generally, you are allowed to install an aftermarket converter for these three reasons only:
1. If the converter is missing from the vehicle when the car is brought in for exhaust system repair.
2. If an inspection has determined that the existing converter has been lead-poisoned, damaged, or otherwise needs replacement.
3. If the vehicle is more than 5 years old or has more than 50,000 miles (8 years/80,000 miles for 1995 and newer vehicles) and a legitimate need for replacement has been established and appropriately documented (such as a plugged converter or unrepairable exhaust leaks).
Manufacturers of new aftermarket converters are required to offer a 5-year/50,000-mile warranty on the converter shell and end pipes. New catalysts must also be guaranteed to meet the Environmental Protection Agency (EPA) emission performance standards for 25,000 miles when the vehicle is properly used and maintained.
The EPA requires that a new, legal replacement catalytic converter must be properly labeled. Required labels on the converters have a series of letters and numbers in the following format:
N/XX/YYYY/ZZZZ
Here’s what that label means:
N = a new converter
XX = manufacturer’s code issued by EPA
YYYY = a numerical designation of the vehicle application or part number
ZZZZ = numeric month and year of manufacture
Converters manufactured for sale in California may have the letters “CA” in the first position. Because California standards are more stringent than federal EPA standards, these converters meet EPA requirements and are usually known as “50-state” units. Catalysts that comply with federal laws but not California regulations are known as “49-State” units. If the new catalytic converter does not have this kind of label it may not be a legal replacement part for any U.S. application, placing you in violation of federal law.
As the name implies, a cat-back system replaces the factory exhaust from the last catalytic converter to the exhaust tips at the rear of the car. This section of the exhaust system is where the mufflers are found. Desirable features in this part of the power system are high exhaust gas velocity and free flow with as few bends as possible.
Most aftermarket exhausts keep the basic tube size close to stock for the model of car to maintain exhaust gas velocity. Because of chassis size and underbody differences, exhausts that fit the 325/328/M3 models do not generally fit the 318 or 323 models. However, cat-backs fit two-door or four-door E36 models alike.
When choosing an aftermarket exhaust, apart from tubing size you need to look for smooth bends and joints in the pipe and unrestricted mufflers. Generally speaking, the freer the flow, the louder the exhaust, so there’s a tradeoff you need to understand before you buy. If you plan to drive your car daily, excess noise can become annoying and may be illegal. If you are building a dedicated track car, noise is usually less of an issue (within the bounds of the track’s noise limits).
The Corsa (RSC Reflective Sound Cancellation) cat-back exhaust for the BMW 328i is a quality piece of equipment. The stainless steel construction, TIG welding, great exhaust note, and performance enhancement all make this modification worth doing.
Horsepower and Torque Gains from Cat-Back Systems
Exhaust manufacturers often claim gains of 5 to 15 hp and a similar torque improvement from a good cat-back exhaust. The actual gain depends on several factors beyond the product you choose:
• What model of E36 are you upgrading? An M3 probably sees more benefit than a 325i or 328i because of the increased amount of air already flowing through the system.
• Have you already upgraded the cold-air intake, intake manifold, exhaust header, and catalytic converter mid-pipe?
• Engine tuning is necessary to fully realize the benefits of a less-restrictive exhaust.
In general, you see the best results if the exhaust is the most restrictive part of the system before the upgrade, but most people perform the easy cat-back exhaust upgrade before they do the more difficult and expensive header and catalyst upgrades. The catalyst is usually the restriction point, and as noted above, for legal reasons a catalyst delete pipe may not be an option.
The market offers many quality cat-back exhausts, and the main thing you care about is the way they sound. Some products have a drone at certain RPM levels, and some are quieter than others. Much of that difference depends on what’s been done to the engine already, because all aftermarket systems were designed and tested using otherwise stock cars. As with other parts, you’re going to get what you pay for, so beware of false economy. Akrapovic, Dinan, Magnaflow, Corsa, and Supersprint are generally well known for excellent results and they fit like original equipment. Off-brands tend to have fitment issues and use thinner metal and lower quality