I thought this was an intelligent (albeit biased?) article regarding this subject. Source at the bottom of article. (I reformatted for readability purposes)
Forced Induction: The Eternal Struggle for Superiority
Between Supercharging and Turbocharging
Introduction
So, you just went out and bought the fastest car made by your favorite manufacturer. You paid a whole year's worth of your salary on this one car, so it must be the best car ever made. You pull out of the dealership and on to the street. You get up to a red traffic light and stop. Another "sports car" pulls up next to you. "Excellent," you think to yourself, "I'll show this guy what this car can do." You rev your engine and he returns the gesture. The light turns green, and in a near instant you are seeing nothing but the taillights of the car that you were supposed to be beating. How do you solve this dilemma? More horsepower. How do you get more horsepower you ask?
Forced induction.
The engine in your car, and every other gas-powered car is basically a big air pump. The intake sucks in air, fuel injectors add gas, your pistons compress that air/gas mixture, the spark plugs ignite the mixture, and the exhaust lets all of the pressure from the explosion out. The explosion in the cylinder is what causes the piston to be forced down and thus rotates the crankshaft, which basically makes your car go. In order to make more horsepower you need a bigger explosion.
The simplest way to get much larger explosions is to increase the amount of air and gas in the cylinder when the spark plug ignites the mixture. The extra fuel is easily added by using larger fuel injectors. The problem is getting more air into the engine. Enthusiasts use a system called forced induction to actually compress the air going into the engine, thus increasing the number of air molecules. There are two types of forced induction: supercharging and turbocharging. Turbocharging is a better method than supercharging for the automotive enthusiast to use as a form of forced induction on a daily driven street vehicle--based on cost, reliability, versatility, and efficiency.
Types of Forced Induction
All forms of forced induction work on one basic principal. A compressor unit, just a pump that pressurizes air, takes in a certain volume of air and compresses (or pressurizes) it into a smaller package, which can be crammed into a smaller space. This allows more air molecules into a given volume of space, such as a cylinder in an engine. The problem that presents itself is that it takes a very large amount of power to run these compressors due to the high volume of airflow they must produce. The question is how do we power them?
One method of powering the compressor is called supercharging. This method uses a pulley attached to the compressor and to a belt driven by the engine's crankshaft pulley to power the compressor unit. The crankshaft pulley on the engine spins and thus drives the compressor wheel and makes boost-pressure for an engine's intake (Francis, 2002, p. 2).
The other method of powering the compressor is called turbocharging. This method uses a turbine attached to the compressor and the air coming out of the engine to power the compressor unit. The exhaust gases flowing out of an engine are very hot and under extremely high pressure. Heat and pressure are two types of potential energy. We channel this readily available energy through the exhaust manifold and into a turbine housing, similar to a big pinwheel or watermill and allow them to spin the turbine and consequently the compressor wheel as well (Turbocharging, 2002, p. 1).
Turbocharger System--Versatility
In order to understand the claims of the superiority of the turbocharger system over the supercharger system one must also understand the basic parts of each system. The turbocharger system uses a turbocharger unit (which is the compressor), a special exhaust manifold, and wastegate.
The turbocharger itself is actually three parts. The first part is obviously the compressor section; which includes the compressor housing and compressor wheel. This section sucks in ambient air and blows it out as pressurized air-with a slightly higher temperature due to basic physics. "When you compress a gas, its temperature will increase…" (Turbocharging, 2002, p. 1). The second part is the turbine section; which in turn includes a turbine housing and turbine wheel. This section takes in high temperature-high pressure exhaust gases and spins the turbine wheel before letting all of the gases-minus heat and pressure-out of the outlet. The third section is the bearing and shaft section. This section houses a shaft, which connects the turbine wheel to the compressor wheel and also oiled bearings, which in turn keep the shaft spinning freely.
The exhaust manifold for a turbocharger is simply a modified manifold that collects all of the hot -pressurized gases out of the engine and directs them into the turbine housing of the turbocharger. All engines have an exhaust manifold to collect the exhaust and send it out the tailpipe. The turbocharger system simply uses a slightly modified one that allows the gases to go through the turbocharger before going out the tailpipe (Francis, 2002, p. 1).
The wastegate is the last but not the least part of a turbocharger system. The wastegate serves as a bypass for exhaust gases. It allows the pressure to bypass the turbine housing and go directly to the tailpipe. The wastegate therefore determines the speed with which the turbine spins and thusly controls the level of boost being made by the compressor. By adjusting the amount of exhaust bypassed, one can also adjust the boost level (Bell, 1997, p. 142).
Supercharger System-Lack of Versatility
The opponent of the turbocharger system is the supercharger system. It includes three major components as well. It uses a supercharger (the compressor), a belt and pulley, and brackets (CamdenSupercharger, 2001, pp. 1-2).
The supercharger itself is a fairly simple unit compared to the turbocharger. It has only one major section, the compressor. The compressor section of a supercharger is identical to that of a turbocharger. It uses a compressor wheel and compressor housing to suck in ambient air and blow out pressurized air-which also has a higher temperature. Keep in mind that higher temperature air isn't a good thing, but it is a necessary evil shared by both superchargers and turbochargers because "when you compress a gas, its temperature will increase…" (Turbocharging, 2002, p. 1). Many people think that only turbochargers increase the intake temperature, because they are connected to the exhaust manifold. That is not true. Both systems increase the intake air temperature.
The belt and pulley system on a supercharger are connected to the compressor wheel in lieu of the turbine section found on a turbocharger. This belt and pulley vary in power directly to the speed at which the engine turns. This directly influences boost pressure. The only way to get more boost is to either rev up the engine or increase the ratio of the pulleys on the crankshaft and supercharger by taking off one pulley and putting on another pulley that costs around $165 each time you want a different boost level (Francis, 2002, p. 2).
The third component of a supercharger system, which you cannot do without, is brackets. You must have some form of bracket to mount the supercharger to the engine and additional brackets for belt tensioners for the drive belt and pulley system. Turbos use the manifold itself as the mounting bracket.
Costs
The first and perhaps most obvious benefit of turbocharging, only to an accountant, is the difference in cost between a supercharger and turbocharger system. One basic comparison in cost comes from an anonymous article on turbocharging a Ford Mustang versus supercharging a Ford Mustang. In this comparison and many other similar ones the turbocharger system is less expensive but only to the trained eye. Superchargers appear to be cheaper because the basic kit only costs $3000 whereas the turbocharger basic kit costs $4000. However the supercharger companies are very good at concealing the truth about costs (Turbocharging, 2002, p. 3).
The basic kit for a supercharger does not include the following required items for forced induction. Fuel injectors, priced at $210, are required to increase the amount of fuel in the air/fuel mixture; after all we need more fuel too not just more air. Headers, another name for the previously mentioned exhaust manifolds, are also needed and they cost around $159. The problem with putting more air into an engine is that you also have to allow more air out of an engine; these do just that. The mass air meter is a meter that is found inline between the compressor unit and the intake of the engine. It can be a major restriction if it is too small. It must be upgraded to an 80mm unit which costs $400. Spark plug wires, another $50, give you increased spark power to ignite the additional air and fuel in the cylinders. Cold air induction, which is generally priced around $250, gives you a colder initial intake air temperature, which in turn, lowers the final air intake temperature-i.e. cooler air = more power. Total price of supercharger system basic kit plus all of the additional required parts is $4070 (Turbocharging, 2002, p. 3).
Now the two systems seem to have very similar costs: turbocharger $4000, and supercharger $4070. But there is also one other thing that the turbo kit includes that the supercharger kit doesn't: an intercooler. Intercoolers cost around $1400 for a sufficient unit. Although intercoolers aren't required, they do add a considerable amount of horsepower to an engine and value to a kit. When you take all of this into account-as only a very keen eye would-you can see that a turbocharger system is actually cheaper than a supercharger system (Turbocharging, 2002, p. 3).
Efficiency--Reliability
Another fundamental advantage of turbochargers over superchargers is efficiency. The turbocharger system steals almost no power from the engine itself to make boost. Supercharger systems on the other hand can take as much as 20% of the engines power to make boost. It seems almost comical to think that someone would design a unit to increase power on an engine that initially takes away 20% of its power (Francis, 2002, p. 1).
Turbochargers use the exhaust gas heat and pressure from an engine to generate power for the compressor and make boost. "A turbo captures some of the energy in hot gas that normally is wasted out the tailpipe" (Robinson, 1999, p. 1). This heat and pressure would otherwise be wasted out the tailpipe and in to the atmosphere. This is "free" energy that we are harnessing here (Francis, 2002, p. 1).
On the other hand, superchargers use valuable power from the crankshaft of the engine itself to power the compressor and make boost. This power is not free and thus is a loss in horsepower that must be subtracted from the net gain in power caused by the system (Francis, 2002, p. 1).
If both systems are making 10 lbs of boost and thus generating a 40% increase in engine output on the same base 200hp engine, then the turbocharged setup would be producing 280HP and the supercharged setup would be producing 240HP. Which system would you want? The supercharger would have to increase boost pressure in order to make the same amount of power as the turbocharger. This extra boost pressure obviously will cause greater wear on the engine itself thus resulting in shorter engine life and less reliability (Francis, 2002, p. 1).
Another factor which causes superchargers to have less reliability than turbochargers is the fact that the supercharger system puts increase stress on the front half of the crankshaft due to the tension placed on it by the pulleys and belt that drive the supercharger.
Conclusion
When comparing supercharging and turbocharging systems and weighing out one's options on what to buy to make one's high-performance street machine faster, one must take into account the efficiency, reliability, versatility, and cost of his options. If you choose to do so, you will undoubtedly see the overwhelming superiority of turbocharging to supercharging. Many arguments have been raised at cruise-ins, dinner tables, performance shops, internet forums, and even grocery stores. But the only way to solve this argument is at the track. Time and time again, turbocharged cars have given out beatings to supercharged cars, and until the majority of racers start seeing through the facade created by supercharger companies to cover up their shortcomings, I will be included in the few that are proud to hand out those beatings-with my turbocharged racecar.
See the original here:
http://www.hotrodders.com/forum/turbo-vs-supercharger-write-up-29060.html
