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A lot of people think that turbocharging and supercharging are the holy grails of power increases. While it’s true you can extract a large amount of power from them, people incorrectly assume that any aftermarket supercharger tossed on a BMW engine will instantly generate heaps of horsepower. As with any good, reliable means of generating horsepower, the addition of a turbocharger or supercharger needs to be carefully coordinated with your engine’s design and your desired performance characteristics.
It’s important to take a few moments to talk about turbocharging and supercharging, or “forced induction” as it is known. A forced-induction engine has some assistance in filling the combustion chamber with air/fuel mixture. On a normally aspirated engine, the maximum manifold pressure is atmospheric pressure (14.7 psi at sea level). On a forced-induction engine, the turbocharger or supercharger increases manifold pressure to a level above 14.7 psi. The result is a denser air/fuel mixture in the combustion chamber, and thus more power.
The turbocharger and supercharger are very similar in principle. Both use a compressor/blower to increase the overall pressure of gases inserted into the combustion chamber (cold side). This increase in pressure results in an air/fuel mixture that is more compressed than normal, thereby generating a more powerful piston stroke. Because of the higher density of the mixture, forced induction allows you to create a smaller-displacement engine with the same energy output that as a normally aspirated, larger-displacement engine.
What about reliability? Factory-designed forced-induction engines (like the factory BMW M12 race motors installed in the 1980s Formula 1 cars) are specially designed to accommodate the additional stresses placed on them by the added boost. Engines like these are designed from the ground up and usually have very low compression ratios to compensate for the added pressures when the car is operating under full boost. On the flip side, bolting on a turbocharger or supercharger to a stock engine will result in more wear and tear on the engine. If you plan to install a forced-induction system on your stock engine, you must plan on purchasing only high-octane fuel. The increased compression in the cylinders also increases the likelihood of detonation, which can destroy your engine very quickly.
How they work
The supercharger is powered by a pulley that attaches to the crankshaft. As the engine’s rpm increases, outside air is compressed and mixed with fuel and discharged into the intake system. There are three common types of superchargers: impeller (centrifugal), twin rotating screws (screw-type), and counter-rotating rotors (Roots-type). As the engine spins faster, the boost from the supercharger will increase. Boost is the measurement of the increase in pressure in the intake charge over normal outside atmospheric levels. Typical boost levels for a street BMW range from 6 to 9 psi. The typical supercharger experiences peak revolutions of about 15,000 rpm for the screw-type, and up to about 40,000 rpm for centrifugal-type superchargers.
The turbocharger unit drives its compressor from the excess exhaust given off by the engine. Although the back pressure on the exhaust may rob a small amount of power from the engine, the boost from the turbo is generally thought of as free boost. The turbocharger unit is very similar in operation to a centrifugal supercharger, with the exception that it is driven off of the exhaust gases versus a pulley attached to the crankshaft. Typical peak revolutions of turbos can range anywhere from 75,000 all the way up to 150,000 rpm.
Power and efficiency
Whereas the turbo runs off of the exhaust system, a supercharger takes power from the engine crankshaft to run the blower. All things being equal, superchargers sap more power overhead (40–50 horsepower to spin the blower at full boost) from the engine to run the compressor than turbochargers. Turbos, however, are not without horsepower cost—the back pressure from the turbo and restrictions from the convoluted exhaust piping act to reduce horsepower. However, these losses are minimal when compared to the horsepower cost of driving a supercharger off the crankshaft. The bottom line is that if you’re looking to squeeze the maximum amount of power out of a specific displacement (as you would if you were running in certain club racer classes), turbocharger systems win hands down over superchargers.
Reduction of lag is the top reason why superchargers are preferred over turbochargers for street cars. Since the turbocharger is spooled up by the exhaust gases from the engine, it doesn’t achieve significant boost levels until the engine’s rpm reaches a certain level. This means little or no boost in the lower rpm range. When the boost finally kicks in, it can be an unsettling experience, as the car rockets off as soon as you reach an rpm level that produces boost. This power surge can also place additional stresses on stock drivetrain and suspension components. There are several things you can do to reduce turbo lag (described further in this project), but these fixes sacrifice top-end power. A supercharger, on the other hand, connects directly to the crankshaft and spins and creates boost at all times. The screw-type and Roots-type superchargers are able to create significant boost levels at low rpm, so there’s typically not much lag. Whereas a turbocharger has power that instantly comes online at about 3,000 or –4,000 rpm, the supercharger has a nice, even boost curve that generates excellent power off of the line.
Turbocharger systems are somewhat complex, and thus are considered less reliable than superchargers. In addition, all of the turbo system components work with exhaust gases, which creates additional heat stress and wear on the system. When you first shut off a car with a turbo system, the temperatures can spike inside the turbo and you can experience problems with the impeller bearings being cooked by this high heat (some people install turbo timers that let the engine run at idle and cool down for a minute or so before shutting off). Turbochargers spin at much higher rpm than superchargers; thus, the bearings inside tend to wear out much faster. Many turbo system exhaust components are hand-made and as such, weld seams often crack with age.
The turbocharger system is powered by hot exhaust gases, which tend to inadvertently heat up the intake mixture charge. Hot air expands and becomes less dense, so this heating effect works against the compressing action of the turbocharger. Cooler air means a higher air/fuel mixture density, which is the whole point of installing a forced-induction system. To solve this problem, most turbos require an intercooler, which increases the complexity and cost of the system. Hot air is cycled through a large intercooler, cooling the air before it goes into the intake manifold. The cooler air helps to reduce detonation and also increases the density of the air/fuel mixture. Just about all the intercoolers I’ve seen on the 3 Series cars have been installed in the front bumper—not a simple task. In addition to the heat gathered from the exhaust gases, the intake air temperature increases when the air is compressed. All turbochargers should be run with an intercooler, and most superchargers can also benefit from the use of an intercooler.
Installation and tuning
Superchargers in general are very easy to install. Many bolt-on kits exist that can be installed over a weekend, and BMW inline engines have plenty of room in the engine compartment. Supercharger kits often require only a few modifications to the fuel system or a new fuel injection mapping chip for the DME to get the engine up and running well. On the other hand, most turbo installations involve complex routing of exhaust pipes, oil lines, and other components, many of which must be modified to fit properly. Intercoolers are typically difficult to fit in the front bumper of the 3 Series cars. Although turbo systems can be made to run with the stock fuel and ignition systems (DME), in order to extract the most power out of a turbocharger system, you should run the engine with an engine management system (see our article on Engine Management Systems).
In general, both types of systems can be expensive, costing anywhere from $3,000 for a basic kit up to $10,000 for a complete setup and installation. Since the turbo systems are more complicated, they tend to be slightly more expensive, particularly when you add the cost of a front-mounted intercooler. Another consideration is the cost of installation. Most supercharger installations are relatively straightforward, as you only need to modify the left side of the engine bay and the intake system. Installation of a turbo setup is much trickier (and a bit more expensive) due to the effort involved with routing and installing the exhaust pipes. Despite what many manufacturers may say, turbo kits are almost never a straight bolt-on installation. The pipes and brackets are almost always hand-made and often require some tweaking to fit.
Power output and streetability
Turbochargers and superchargers can both produce significant power gains, although turbochargers can squeeze more total power out of the system due to the fact that they run off free energy from the exhaust system. Because the turbocharger units operate at very high rpm, they can produce very high levels of boost in the upper rpm range and deliver much more peak horsepower at these levels. However, most people don’t drive their cars at peak rpm on the street. Most of the driving is done in the lower rpm bands, where superchargers have the power advantage. If you want to drive around town with more power off of the line, then a supercharger kit is probably the best choice. If you are going to be racing the car on a track or you want maximum top-end power on the highway, then a turbocharger will allow you to squeeze the most power out of your engine.
There are two types of superchargers that are commonly installed onto BMW inline engines: centrifugal and screw-type. Of the two, the centrifugal supercharger is most like a turbocharger, with the exception that a belt connected to the engine’s crankshaft drives it. Centrifugal superchargers compress air using a spinning impeller. With centrifugal superchargers, you can often swap out impeller sizes and change the drive pulley to customize the boost curve for your particular needs. Centrifugal superchargers are typically set to generate their peak boost at or near engine redline. In general, they develop more of their boost at higher rpm and offer less boost on the low end of the rpm range. Paxton, Powerdyne, ProCharger, and Vortech all manufacture quality centrifugal superchargers.
Screw-type superchargers are an improvement on the design of the older Roots-type supercharger. A twin-screw mechanism geared off the front pulley compresses air as it moves between the two screw blades. These are also often called positive-displacement units because they move a fixed amount of air per revolution. This design creates good compression at lower rpm, resulting in a significant increase in power from idle all the way through the rest of the power range. Drag racers who want instant boost off the line typically go with the screw-type supercharger. One problem with the screw-type, however, is that it takes up significantly more space than a centrifugal supercharger. With BMW four-cylinder engines like the 318i, this is not a huge problem, as there is extra room in the engine compartment for the intake manifold adapters (see Photo 1). Downing Atlanta makes an excellent supercharger kit that utilizes a screw-type blower to extract more than 200 horsepower from a stock four-cylinder engine. Other manufacturers of screw-type superchargers include Kenne Belle, Eaton, and Whipple.
Many people incorrectly think a larger turbocharger alone will generate more boost and horsepower. In reality, this is not necessarily true. A larger forced-induction unit must also accompany other important changes in the engine. Maximum boost pressure is limited by a pressure relief valve called a “wastegate.” The wastegate acts to release exhaust gas pressure, slowing the turbine so the engine doesn’t suffer from too much boost being applied. Installing a larger turbocharger without making adjustments to the wastegate will result in no increase in maximum boost levels.
How does the size of the turbocharger affect performance? The numeric digits used to describe turbos (K24, K26, K27, etc.) usually correspond to the actual size of the exhaust fan wheel inside the turbocharger (called the “hot side”). In addition, there is the wheel on the intake (“cold side”) that compresses the air to create the actual boost. Changing the sizes of the two wheels alters the overall personality of the turbocharger and can be used to tailor the turbo response to your specific application.
For example, a small turbine wheel in the exhaust, combined with a small impeller wheel on the compressor side, will spin the turbo up quickly and generate a quick throttle response, but it will also tend to drop off power on the top end. A small turbine in the exhaust with a large blower will generate a good compromise between throttle response and top-end power. To obtain the best top-end performance, a large turbo wheel can be combined with a large blower wheel. The down side is that throttle response will suffer in the lower rpm range.
Installing a smaller turbine wheel in the exhaust means that it will spin up much faster than a larger one. The ideal turbo configuration for everyday street driving is to have a smaller turbine on the hot side and a larger blower turbine on the cold side. This particular configuration is a good compromise between low-end throttle response and high-end power. The down side to this configuration is that it takes a certain level of exhaust pressure at a minimum rpm to spin up the exhaust (hot side) turbine to the point that it can begin to impact the intake pressures. This is what is commonly known as “turbo lag.” In a race engine, turbo lag is not a major issue, since the transmission gearing and overall setup of the engine are usually designed to operate within a narrow power band high in the rpm range.
How do you improve performance? Swapping the turbocharger with one that has a different ratio between the wheels can change your turbo engine’s characteristics. There are numerous options for turbochargers, and each one changes the performance characteristics differently than the next. Do some research and ask others who have installed various units on their BMWs before you spend a large amount of money on a new turbocharger. Adding an intercooler, or upgrading your existing one, will also increase the overall performance. Simply dialing in more boost from the turbocharger (by changing the wastegate relief valve setting) can give you an immediate performance improvement. Also, increasing the compression in the engine will give you more low-end power. However, these approaches can be extremely hazardous to your engine. Severe detonation from poor-quality gas can cause pistons to overheat, and the engine can literally blow itself apart. For more information on turbocharging, see the book Turbochargers by Hugh MacInnes or Maximum Boost by Corky Bell.
How much boost?
This age-old question is answered with the old saying, “There’s no such thing as a free lunch.” How much boost you run on your forced-induction system depends upon a wide variety of factors.
What type of induction is it? As mentioned, turbo systems come to full boost capability and then bleed off excess with a wastegate. While this creates great power in the upper rpm range, it also means you’re running at highly boosted levels for extended periods of time. A centrifugal supercharger only reaches maximum boost at the highest rpm, and then only for a few seconds. So, you can run much higher peak boost levels on a centrifugal supercharger than you can with a turbo or screw-type supercharger system.
Which fuel octane? Running a boosted engine puts a lot of stress on the internals of the engine, as you are pushing more and more power through the drivetrain. However, the real killer for these engines is detonation. If the octane is too low and the compression of the engine too high, the fuel will explode prematurely, resulting in what is commonly known as “engine knocking,” or “detonation.” When the mixture in the combustion chamber burns, it increases the pressure in the cylinder and pushes down on the piston. When detonation occurs, the piston is still rising and compressing the mixture. Thus, when ignition occurs, the pressure builds and has no release. The pressure pushes down on the piston as it’s rising, creating a tremendous amount of pressure that has nowhere to go. Unchecked, detonation will destroy pistons and blow out head gaskets. It’s the number one killer of forced-induction engines. To prevent destruction, reduce your boost levels so that the engine no longer detonates. The engine management system (DME) normally adjusts timing and ignition in response to signals received from the knock sensor to reduce detonation in the cylinders. However, running really high amounts of boost with lower octane fuel can overwhelm the stock system and confuse it. The bottom line is that the higher the boost you wish to run, the higher the octane of fuel you will have to buy. If you want to head to the drag strip and run all out with as much boost as you possibly can, be prepared to buy race fuel with octane ratings in the 105–110 range.
What is the air/fuel mixture?When you install a forced-induction system onto an engine, you increase the amount of air injected into the combustion chamber. Most of the time, this will cause the air/fuel mixture to become lean. You must compensate by increasing the amount of fuel that is combined with the air, since that air is now compressed and thus more dense. According to modern fuel injection theory, combustion achieves its maximum efficiency at an air/fuel ratio of 14.67:1. Although this ratio may be optimal for good fuel economy, it’s not best for maximizing power. On a normally aspirated engine at full throttle, maximum power is achieved with an air/fuel ratio set from about 14.2:1 to 14.3:1. On boosted engines, this maximum power ratio is closer to the range of 12.2:1 to 12.4:1. If your boosted engine is running too lean, it will increase the likelihood of detonation and also will increase the operating temperature of the cylinder head. It’s very important to make sure that your engine is running on the rich side. I recommend running an aftermarket air/fuel mixture gauge to monitor and protect against the engine running lean.
What modifications have been done to the DME?The DME (digital motor electronics) controls the ignition and air/fuel ratio injected into the engine. Installing a forced-induction system is such a major change to the engine that it’s difficult to adapt the computer to correctly compensate for the compressed intake charge. The Downing Atlanta supercharger kit uses an auxiliary fuel pressure regulator that increases fuel pressure in direct relation to the amount of boost the supercharger is outputting. This is a simple yet clever way to adjust the fuel mixture without having to reprogram any chips or sensors.
What is the compression ratio? Engines that start with a high compression ratio (like the 1996–1999 E36 M3 at 10.5:1) cannot be boosted as much as engines with lower ratios. To properly integrate forced induction, the engine should be designed from the ground up with forced induction in mind. The higher compression ratios of the M3 (and the 328 with 10.2:1) don’t naturally lend themselves to forced-induction kits. In general, forced-induction engines are blueprinted to have a very low compression ratio (like the venerable Porsche 911 Turbo with 7.0:1). The bottom line is you can generate more horsepower by maximizing boost from the turbo than you can with higher compression. If you run higher compression, you will be forced to run with less boost at the higher end to avoid destroying the engine. Thus, you want to design the engine to have low compression, so that you can run higher boost at higher rpm and generate more horsepower. You can lower the compression ratio by a variety of methods, such as adding a thicker head gasket, installing lower-compression custom-made pistons, etc.
How old is the engine? Bearings and clearances wear out over years of use. Most companies that sell superchargers don’t recommend installing them on a tired engine. The chances you will blow out your head gasket increase as the engine gets older. Increasing the overall compression inside the combustion chamber increases the wear and tear on all the parts in the drivetrain.
The bottom line?There are a few rules of thumb when it comes to running forced-induction engines. The following table gives you a broad outline of what boost levels you can run for a variety of compression ratios:
*running on 91 octane with a BMW inline engine
Which forced-induction unit you install really depends upon your overall goals, which should include ease of installation and budget limitations. If you ask 10 different enthusiasts what their preferences are, you will get 10 completely different answers. There are those who are turbo fans, and there are others who are die-hard supercharger recruits. Obviously, this project can only scratch the surface of what’s involved in designing and implementing a turbocharger or supercharger system.
Some closing remarks regarding the relative performance of these two systems: If you want a drag car with lots of power off of the line, you should probably go with a supercharger system. It will give you boost at low rpm and a predictable power curve. If you’re looking for top speed on the Autobahn, where you want to squeeze out all the power you can, I recommend a turbocharger system. It will give you maximum power at the higher end of your rpm range. With most supercharger systems, you will achieve maximum boost only when you’re at redline. With a turbocharger, you will have nearly full boost all the way from about 3,000 to –4,000 rpm to redline. If you like to feel the rush of power and want the ability to create gobs of peak horsepower, go with a turbocharger. Turbo systems are also considered to be more flexible in that they can often be designed to fit most owners’ requirements, while superchargers can be a bit more limited. If you want your car to feel somewhat stock with a big push on the high-end, then go with a centrifugal supercharger. If you just want more even power across the entire rpm range, then install a positive-displacement (twin-screw) supercharger.
In terms of installation ease, the supercharger systems win by a mile. Turbocharger systems can be made to perform better, but they generally require more time, money, and installation effort to achieve this end.
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