Air goes in, things go bang, and the air goes out. Sounds simple, doesn’t it? The process of an engine working can actually be broken down to that simple concept. The internal combustion engine is merely an air compressor that moves a given amount of air to create power. So, what happens if you add more air to the engine than it can physically draw in by its own created vacuum?
For years, people have been adding forced induction to just about every style of engine that has been created, and with today’s cars as easily “tunable” as they are, the possibilities are endless. But when it comes to forced induction, what’s right for you? Are you a supercharger or turbocharger kind of guy?
To make that decision, you first need to know what a turbo and supercharger have in common, and what their differences are. Both systems use a centrifugally-driven “compressor” to compress the engine’s incoming air to higher than normal atmospheric pressure, which is 14.7 psi by the way. When the compressor increases engine cylinder pressure, this means that more air is filling the cylinders. If there is more air present in the cylinders, that means that more fuel can also be added. This isn’t an overly technical explanation, but you get the point.
When you compress air, a byproduct of this compression is that the temperature of that air increases. When forcing air into an engine, the trick is to not heat the air to a point that power is hampered. That’s why many boosted applications utilize an intercooler to cool the air before it enters the engine. Intercooling plays a huge role in controlling air temperature, but even the efficiency of the turbo or supercharger plays a part. The efficiency of a forced-induction system is rated by how much heat is introduced into the system as compared to the level of air compression that is created.
Since both a turbocharger and supercharger compress air, what’s the difference? The major difference is in the drive of the “charger.” A turbocharger’s compressor is connected directly to a turbine housing that is operated by the exhaust. The turbine takes exhaust energy and converts it to motion. Since the turbine and the compressor are connected by a shaft, when the turbine spins, the compressor spins at the same rate as the turbine. Since powered by the exhaust, a turbocharger does not place a direct mechanical load on the engine. Many feel that this is more efficient, because it uses the exhaust to drive the compressor instead of actual energy supplied by the engine. We also need to mention what is referred to as “turbo lag” or “spool time.” This is the amount of time between the throttle being opened, and the turbocharger actually providing increased intake pressure, thereby increasing horsepower. Turbo lag is inevitable, and occurs because turbochargers rely on the buildup of exhaust pressure to spin the turbine section of the turbocharger. In an automobile engine, the exhaust-gas pressure at idle and/or lower engine speed is not sufficient to drive the turbine fast enough to create boost. Boost (or pressure), is only developed when the engine rpm has reached a sufficient speed that the turbine starts to spin fast enough to produce pressure in the intake.
A turbo is not entirely “free” horsepower though. The turbine is literally a “blockage” in the exhaust, so some additional back pressure (exhaust flow restriction) is created by the turbine. But once the turbo starts to spin, the “blockage” is minimal. What this means is that the power “robbed” by the extra back pressure is very small.
Unlike a turbocharger, a supercharger is not powered by the exhaust gas, but is mechanically driven by the engine. While belts used to drive a supercharger place a mechanical load on the engine, the power used is more than overcome by the output of the supercharger. We must make note, though, that one disadvantage of supercharging is a lower adiabatic efficiency as compared to turbocharging. Adiabatic efficiency is the measure of a compressors ability to compress air without adding an excessive amount of heat to that air. The most widely used superchargers tend to use compressors that create significantly more heat to the air than their turbocharged counterparts. What this means is that for a given pressure and volume of air, turbocharged air is inherently cooler, which relates to a denser charge. A denser charge has the potential to create more power.
So what does this all mean? In theory, a turbo kit producing 8 psi of boost should produce more overall peak power, because a supercharger is using engine power just to get it spinning. What is more important when discussing a street car application is what’s known as “power under the curve,” or total average horsepower produced. A street car is very seldom driven at full throttle; therefore, the amount of power it creates while at less than full throttle -- or “under the curve” -- will directly affect performance, more so than while the engine is at full throttle. When more power is added to the bottom -- or lower end of the rpm range -- the car will be faster, even if peak power doesn’t change. This is where the turbo outperforms a supercharger, since you can have full boost at a lower rpm.
We decided that since we had covered supercharging in a previous issue (December 2013), we needed to find out if anyone reputable had developed a turbo kit for Mopars. We didn’t have to search very hard, and we found Arrington Performance/Shophemi.com. The guys at Shophemi.com have developed a kit that mounts two Precision turbos directly to the factory exhaust manifolds. Mounting the turbos close to the engine like this virtually eliminates turbo lag, it keeps the turbos well above the scrub line of the car, and when installed, it is virtually undetectable from looking under the car. Now, there are some things to keep in mind when using a turbo kit. Oiling the turbos is a huge concern. Without proper oiling, they will not last. With the turbos mounted low, the guys at shophemi.com developed a proprietary oiling system that keeps the turbos well-oiled and happy. There are different factors that need to be considered when you decide to install a shophemi.com turbo kit, and a single phone call to them will ensure that you get what you need.
Costs for PN API-TTK-62: *$10,699.95
* Includes everything required. (Note: at this boost level it does not include a boost controller).