Читать книгу BMW 3-Series (E36) 1992-1999 - Eddie Nakato - Страница 9
ОглавлениеThe fuel charge in an engine does its work by expanding as it burns, pushing a piston down in the cylinder. Once the fuel charge has accomplished this task, an efficient engine evacuates spent exhaust gases as quickly as possible, although turbocharged cars scavenge a bit more energy out of the gas on the way out. The exhaust system performs this very simple function, and the exhaust is a great place for you to easily bolt on some low-cost horsepower and torque.
Tip: A note of common sense: Always replace all gaskets and worn fasteners when replacing exhaust components. Exhaust leaks are easy to avoid and a pain to fix.
But before you work on the exhaust, bear in mind that the exhaust system is also a critical emissions control system. Both federal and state laws govern what you can and cannot do to your car.
The catalytic converter has been the cornerstone of automotive emission controls for the past 35 years. Controlling emissions from street cars is an important environmental concern. Cars today are far cleaner than in decades past, thanks in large part to improvements in catalyst technology.
The catalytic converter is a muffler-type device that uses a ceramic or stainless steel alloy web that holds reactive catalyzing material (usually palladium or rhodium). When the exhaust gases pass through the catalytic converter and heat the catalyst, a chemical reaction occurs that helps change carbon monoxide to carbon dioxide, and helps to burn off any unburned hydrocarbons that remain in the exhaust stream before it exits your car.
The exhaust system does much more than just produce a sexy sound. This is a critical performance item in the build, and you should consider your purchases carefully, and in concert with the other engine modifications.
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 mufflers.
All aftermarket exhausts fall into the same general envelope of performance enhancement; there’s only so much power you can get out of a cat-back. If you go with a well-known and respected brand, you have a good unit that installs easily with the stock hangers. Custom exhaust systems with weld-in mufflers, such as Flowmaster, generally require more work to install, and you won’t really get any additional power benefits.
Project: Upgrading a Cat-Back Exhaust
The heart of the Corsa exhaust is the RSC mufflers, and the great-looking exhaust tips. The reflective sound cancellation system is designed to prevent exhaust drone by shaping the interior of the muffler to reflect and cancel sound that would otherwise build into a standing wave. One other reason to spend the money on a reputable, quality cat-back is the fit: A cheap unit may not have the proper clearances and can melt the bumper cover.
This project installs the Corsa RSC (Reflective Sound Cancellation) cat-back exhaust system on the project 1996 328i. This product uses twin 2.25-inch OD T-304 stainless tubes and dual mufflers. Inside the mufflers, the exhaust gases pass straight through; the mufflers are designed to reduce noise without impeding gas flow. With the previously installed cold-air intake, this is an obvious next step in performance modification.
Tip: If you’re considering installing a limited-slip differential or reinforcing the rear subframe mounts, now is a great time to do that, as you have to remove the exhaust from the car to do those projects.
If you have access to a safe automotive lift, this project is easy and convenient. You can do the work using only jack stands, but it takes longer and is more difficult. Always follow the manufacturer’s specific instructions.
Follow These Steps
Disconnect the battery and raise the car. There’s no risk of electric shock, but this resets the DME so it recalibrates to the new exhaust. To ensure that the DME is completely drained and reset, disconnect both battery leads and remove the battery from the car. Then zip-tie the positive and negative battery leads together with the clamps touching for about 30 minutes to completely drain the DME capacitor while you are working.
Unbolt the stock cat-back exhaust segment from the rear end of the catalytic converter. Remove the four bolts that connect the two pipes. These are 13-mm bolts and nuts, so you need two wrenches. One pipe connection is a slip-fit and the other uses a ring gasket, but be sure to catch and save that gasket as the pieces come apart! Next, unbolt the exhaust hangers on either side of the mufflers, while supporting the rear end of the exhaust. With the hangers loose, the exhaust should come loose and drop out easily.
Tip: Spray some WD-40 lubricant on the rubber exhaust hangers and work them off gently because the rubber hangers are easy to break.
To begin the installation, place the car in the air. You can see the old, stock cat-back on the bottom.
Exhaust systems tend to rust far more than other areas because their heat cycles encourage the metal to soak up moisture from the air, and because of water splashed up from underneath the car. Plan on some work to lossen these nuts and have some penetrating oil and a heat source, such as a handheld propane torch, ready to go.
Apply the oil and wait, then apply heat from the torch to the nut. Patience helps loosen a lot of rusty parts, but if you have to cut the system apart, an angle grinder is the easiest and least expensive method. Just plug in the angle grinder and turn it on. Then hold the grinding surface to the stuck nuts and grind them until the part comes free.
With the exhaust bolts disconnected (or broken, or cut through) you can start to separate the cat-back section from the rest of the exhaust.
Remove the exhaust hangers from the stock exhaust unit and place them on the aftermarket replacement. The stock hangers can be reused on the outboard sides of the mufflers if they are in good condition. New OEM replacements should be used if the old hangers are suspect or broken. The hangers are not symmetrical and the larger space should be oriented toward the exhaust tips.
Exhaust hangers have metal frames supporting rubber mounts, to help let the exhaust move in relation to the other components, but to keep it within boundaries.
Use the shop stand (or make one) to support the front end of the cat-back exhaust while you’re undoing the hangers at the rear of the car.
Simply install the existing hangers back onto the Corsa cat-back exhaust. If the rubber is decayed or broken, replace the hanger.
Position the new exhaust to the car and adjust the fitment of all parts. Make sure you put the ring gasket back into its space between the catalyst section and the cat-back section. Then, working front to back, tighten the bolts throughout the system. Make sure the exhaust is centered in its space at the bumper and that adequate clearance is provided all around. Run your hands along the top of the exhaust path and anywhere that the heat shielding is too close to let your fingers by, press it up and out of the way.
One side of the exhaust slides into the new cat-back, while the other side simply mates with a ring seal. This is to allow for easy installation.
Installing the ring seal into the exhaust is important to prevent stinky and annoying exhaust leaks. It’s a tricky fit-up process sometimes so take your time.
Tighten the side with the ring seal first. The slide-in section of the other side holds itself in place while you work.
Reconnect the battery and lower the car to the ground. You are ready to test-fire the car. Listen for rattles and exhaust leaks, which often make a ticking sound. Your car should sound absolutely fantastic, with a throaty purr that changes to a roar when you go to wide-open throttle.
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.
You may have a vacuu-mactuated exhaust valve on your E36, and this valve is not present on an aftermarket unit. Simply fold this tube back on itself and zip-tie it closed, then tuck it up out of the way.
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 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 brand that is known to match the stock mid-pipe. These can be found new or used.
Finally, it’s important to recognize that an exhaust system is indeed a system, and piecing together headers, pipes, and catalytic converters is never as effective as a designed system. You want to keep an appropriate amount of resistance (generally called back pressure) in the system, and make the pipes small enough to keep exhaust gases flowing at the right velocity. The exact optimum setup varies from engine type to engine type, and by the RPM range that makes the horsepower and torque. Unless you buy a complete kit (such as those offered by well-known BMW suppliers) or have a lot of time and resources to dyno-test multiple configurations, you’re generally limited to the commercially available products.
For racing, the only considerations are flow and meeting the noise limits set by the racetrack, and all racetracks have them. A large muffler generally restricts exhaust sound while flowing very well.
