This article is one in a series that have been released in conjunction with Wayne's new book, 101 Performance Projects for Your Porsche Boxster. The book contains 312 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 950+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Boxster owner's collection. The book is currently available and in stock now. See The Official Book Website for more details.
A lot of people think that turbo and supercharging are the holy grail of power increases. While indeed you can extract a large amount of power from them, people incorrectly assume that any aftermarket supercharger tossed on an engine will instantly generate gobs of horsepower. As with any good, reliable means of generating horsepower, the addition of a turbo or supercharger needs to be carefully coordinated with your engine's design: all the while keeping in mind your desired performance characteristics.
It's important to take a few moments here to talk about turbo and supercharging, or 'forced induction' as it is known. A forced induction engine has some 'assistance' when filling the combustion chamber with air/fuel mixture. On a normally aspirated engine, the maximum manifold pressure is atmospheric pressure (14.7 psi). On a forced induction engine, manifold pressure is increased by the supercharger (or turbocharger) to a level above 14.7 psi. The result is that a greater mass of air and fuel is injected into the combustion chamber, resulting in more power.
Both a turbocharger and supercharger are very similar in principle. Both use a compressor/blower to increase the overall pressure of gasses inserted into the combustion chamber (cold side). This increase in pressure results in an air/fuel mixture that is compressed greater than normal. The result is that the denser mixture generates a more powerful stroke. Because of the higher density of the mixture, with forced induction you can create an engine with a smaller displacement that has same energy output as you would have with a larger displacement engine (more power).
What about reliability? Factory-designed forced induction engines (like the Porsche 911 Turbo) 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. Bolting on a turbocharger or supercharger to a stock engine will result in more wear and tear on the engine. If you are planning on installing a forced-induction system on your stock engine, you must plan on purchasing only high-octane fuel. The increased compression in the cylinders will increase 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. Typical boost levels for a street Boxster range from 4 to 6 psi. Boost is the measurement of the increase in pressure the intake charge over normal outside atmospheric levels.
The turbocharger unit drives its compressor from the excess exhaust given off by the engine. Although the backpressure 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 the 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.
Power & 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 turbo chargers. Turbos are not without horsepower cost: the backpressure from the turbo and restrictions due to the convoluted exhaust piping act to reduce horsepower. However, these losses are minimal when compared to the horsepower cost of driving a supercharger off of the crankshaft. The bottom line is 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), then the turbocharger systems win hands down over the supercharger.
Power Lag: Reduction of lag is a 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 results in 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 is connected directly to the crankshaft and is spinning and creating boost at all times. 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 3000-4000 rpm, the supercharger has a nice, even boost curve that generates excellent power off of the line.
Reliability: 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 further create additional heat stress and wear on the system. When you first shut off your 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 what are known as turbo timers which 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, and thus the bearings inside have a tendency to wear out much faster. Many turbo system exhaust components are hand-made and often see welds in the seams crack with age.
Heat - The turbocharger system is powered by hot exhaust gases that have a tendency 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 density air/fuel mixture, which is the whole point of the installation of a forced induction system. To solve this problem, most turbo systems require an intercooler, which increases the complexity and cost of the system. This hot air is cycled through a large intercooler that cools the air before it is injected into the intake manifold. The cooler air helps to reduce detonation, and also increases the density of the air/fuel mixture. In addition to the heat gathered from the exhaust gases, the intake air temperature increases as well when the air is compressed. All turbochargers should be run with an intercooler, and most superchargers can also see benefits from the use of an intercooler.
Installation & Tuning: Superchargers in general are pretty easy to install. Many bolt-on kits exist that can be installed over a long weekend. The supercharger kits require only a few modifications to the fuel system mostly provided in the form of a new software map 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. Intercoolers are typically a difficult item to fit into the tight space available with the Boxster engine compartment. 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 probably run the engine using a dedicated and custom engine management system like Tec-3 or Motec.
Cost: In general, both types of systems can be expensive, costing anywhere from $6,000 for a basic kit, up to $10,000 for a complete setup installed. In general, since the turbo systems are more complicated, they tend to be slightly more expensive, particularly when you add in the modification costs associated with an intercooler. Another consideration is the cost of installation. Most supercharger setups are relatively straightforward installations, as you only need to modify one side of the engine bay and the intake system. Installation of a turbo setup is much trickier (and a bit more expensive) due to all of the effort involved with the routing and installation of 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: Both turbochargers and superchargers can produce significant power gains, although turbochargers can squeeze more total power out of the system due to the fact that they are run off of the '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 car at peak rpm all the time on the street. Most of the driving is done in the lower rpm bands, where superchargers have their 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.
In general, the most common type of supercharger installed into the Boxster is the centrifugal type. The centrifugal supercharger is most similar to a turbocharger, with the exception that it is driven off of a belt that is connected to the engine's crankshaft. The centrifugal superchargers compress air using a spun impeller. The advantage to these units is that 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 the redline of the engine. 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 are all good quality manufacturers of centrifugal superchargers.
Many people incorrectly think that a larger turbocharger will generate more boost and horsepower. In reality, this is not necessarily true. Installation of 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 the 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 turbo charger without making adjustments to the wastegate will result in no increase in maximum boost levels.
How does the size of the turbo charger affect performance? The numeric digits used to describe the turbo charger (K24, K26, K27, etc.) usually corresponds to the actual size of the turbo exhaust fan wheel inside the turbo charger (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 can alter the overall personality of the turbo charger 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 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 combined with a large blower wheel can be used together. The downside 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 downside 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 where it can begin to have an effect on the intake pressures. This is what is commonly known as turbo-lag. In a race engine, turbo lag is typically not a major issue, since the transmission gearing and overall setup of the engine is usually designed to operate within a narrow power band in the high 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 turbo chargers: each one changes the performance characteristics slightly different from the next. Perform some research and ask others who have installed various units on their Boxsters before you spend a large amount of money on a new turbocharger. Adding an intercooler, or upgrading your existing one will also increase your overall performance. Simply dialing in more boost from the turbo charger (by changing the wastegate relief valve setting) can give you an immediate performance improvement. Also, increasing the compression in your 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 turbo charging, see the book "Turbochargers" by Hugh MacInnes or "Maximum Boost" by Corky Bell.
How Much Boost?
This is an age-old question that 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 previously, 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 that you're running at highly boosted levels for extended periods of time. With a centrifugal supercharger, it 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 setup.
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, then the fuel will explode prematurely, resulting what is commonly known as engine knocking, or detonation. When the mixture in the combustion chamber explodes, it increases the pressure in the cylinder and pushes down on the piston. When detonation occurs, the piston is still rising and still compressing the mixture. Thus, when ignition occurs, the pressure builds and has no release. The pressure is pushing down on the piston as it's rising, creating a tremendous amount of pressure that has no where to go. Unchecked, detonation will destroy pistons, and blow out head gaskets. It's the number one killer of forced induction engines. The solution is to 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 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 some 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 are increasing the amount of air that is injected into the combustion chamber. Most of the time, this will cause the air / fuel mixture to become lean. You must compensate for this by increasing the amount of fuel that is combined with the air mixture, since that mixture is now compressed and thus more dense. According to modern fuel injection theory, fuel and air combustion achieves its maximum efficiency at a ratio of 14.67:1. Although this ratio may be optimum 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 at about 14.2:1 to 14.3:1. On boosted engines this maximum power ratio is more in the range of 12.2 to 12.4. If your boosted engine is running too lean, this 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 without performing a complete remapping of the air-fuel mixture program (see Pelican Technical Article: Installing Performance DME Map Software).
What is the compression ratio? Engines that start out with a high compression ratio (like the 2000 Boxster at 11:1) cannot be boosted as much as engines with lower ratios. To properly integrate forced induction into any engine, it should be designed from the ground up with forced induction in mind. The higher compression ratios of the Boxster and 996 Carrera engines, 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 that you can generate more horsepower from maximizing the boost from the turbo than you can with higher compression. If you run higher compression, then you will be forced to run with less boost at the higher end to avoid destroying your engine. You want to design your engine to have low compression, so that you can run higher boost at higher RPMs, and generate more horsepower. You can lower the compression ratio by a variety of methods: adding a thicker head gasket, installing lower compression custom-made pistons, etc.
How old is the engine? Bearings and clearances wear over years of use. Most companies that sell superchargers don't recommend installing them on a tired engine. The chances that you will blow out your head gasket are quite high 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's a few rules of thumbs 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:
|Boost Level||Compression Ratio|
|*running on 91 octane with a BMW inline engine.|
Which forced induction unit to install really depends upon your overall goals, which include ease of installation and budget limitations. If you ask 10 different enthusiasts out there what their preferences are, you will get ten completely different answers. There are people who are turbo fans and there are people 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.
There are some generalizations that you can make though regarding the relative performance of these two systems. If you want a drag car with lots of power off of the line, then you should probably go with a supercharger system. This 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, then 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 when you're at redline. With a turbocharger, you will have nearly full boost all the way from about 3000-4000 rpm to redline. If you like the feel of a rush of power and 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 owner's requirements: 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. Of course, if you want more overall power without the hassle of forced induction, then just put a 996 Carrera engine in your Boxster (see Pelican Technical Article: Boxster Engine Conversion Project).
In terms of installation ease, the supercharger systems win overwhelmingly. Turbocharger systems can be made to be better performers, but they generally require more time, money, and installation effort to achieve this.
Here's a shot of the supercharger kit developed by Stephen Kaspar for Imagine Auto, installed on a Boxster 2.5. These compact cars are very light and very nimble, and create quite a performance machine when you bump up the horsepower with one of these kits. Because of the lack of room in the engine compartment, this supercharger kit requires the use of a lot of custom parts, including a whole set of new intake plenums. It's a tough squeeze, but the centrifugal supercharger fits quite nicely tacked onto the side of the camshaft cover. The engine compartment retains its "stock look", and the kit simply bolts into place under the car. A new belt is required of course, as well as a complete remapping of the DME.
Here are the main contents of the supercharger kit described in the previous photo. Although it takes thousands of hours and testing to successfully design and implement a good bolt-on kit, the actual installation and assembly process can be completed typically over a long weekend by a semi-experienced do-it-yourselfer.
This graph shows clearly why Imagine Auto's supercharger kits are so popular. They take the mild-mannered Boxster 2.5 liter engines, and turn them into monsters. When combined with a lightweight chassis like the Boxster, the combination makes for a powerful and nimble track car with performance exceeding that of a Boxster S.
This chart shows the different boost levels between a centrifugal supercharger, a twin-screw supercharger, and a turbocharger system. As you can see, the twin-screw generates more boost per rpm than the centrifugal unit. The turbocharger graph shows low boost levels until about 3500 where it rockets up to full boost in a hurry. Steve Anderson
Shown here is a boost gauge mounted on the driver's side A-pillar. I recommend installing one of these in your car if you're running a forced induction system. A boost gauge will give you a snapshot of the health of your induction system, and will also alert you to boost levels that may be too high.