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Forced Induction - Why Boost It?
Everything you've ever wanted to know about forced induction on your fast Ford.
Positive Displacement Superchargers
Most commonly mounted atop the engine in place of the factory intake manifold, positive displacement (PD) superchargers are driven by a belt connected to the crankshaft and generally proved a huge increase of tip-in throttle response and horsepower. Unlike a centrifugal supercharger, which compresses air by diffusing it, a positive displacement supercharger simply collects and delivers air by either a set of screws (twin-screw) or a pair of lobes (Roots- or TVS-type).
It's important to note that screw blowers compress air in the housing (using the actual screw lobes), whereas a Roots or TVS blower compresses air in the manifold. Either positive-displacement design delivers a fixed amount of air for every revolution, typically measured in liters, and because air delivery isn't rpm dependent, can produce significant increases in airflow at very low engine rpm.
For example, a 2.3-liter supercharger can deliver—you guessed it—2.3 liters of air for every revolution. This is fixed and—although some leakage between the rotors does occur—doesn't change with speed. PD blowers can create such instant torque as soon as you hit the gas because they provide excellent cylinder-filling capabilities at low rpm.
Low-rpm torque production: Both Roots-type and twin-screw PD superchargers work by moving a fixed volume of air per revolution. At low rpm, this airflow is significant, and forcing it into the combustion chambers results in a major increase in power production. If you're looking for stump-pulling, get-off-the-line power, a PD blower will do it.
Packaging: A positive displacement supercharger replaces the factory intake manifold on practically every late-model Ford, and doing so makes packaging very straightforward and simple. Thanks to the built-in air-to-water intercooler found on most OE and aftermarket systems, almost everything is contained in one area and is easy to install or remove for service.
Heat and efficiency: Positive displacement supercharging generates significant heat, both under the hood and within the compressed air being delivered to the combustion chambers, and this heat can be difficult to handle. Air-to-water intercooling is essential, but care must be taken to cool the unit properly between runs to maintain consistent power delivery. Some companies have devised cooling systems that will reduce inlet air temp, and we're seeing more technology in this area lately.
Clearance: The larger PD superchargers require the addition of an aftermarket cowl hood to clear the tall supercharger assembly. This adds cost to some systems and can take away from the desired look of your project. Or, it could be awesome if you've always wanted one.
Traction: Good luck.
The speed at which the internal rotors rotate is determined by the pulley ratio set by the upper and lower pulleys. Driven by a belt attached to the crankshaft, both the crank pulley (the lower) and the supercharger pulley (the upper) work together to set overall boost.
A turbocharger is technically a type of supercharger—engine exhaust gasses, instead of the engine's crankshaft, drive the impeller. Connected to an engine's exhaust manifolds through a series of pipes, a turbocharger features two wheels connected by a shaft, which spin at the same rpm. The turbine wheel is powered by the expansion of exhaust gasses across its blades, while the impeller wheel is tasked with grabbing fresh air and compressing it within the volute. By using exhaust gas instead of a set of engine-driven pulleys, turbochargers offer increased efficiency and unmatched adjustability, although they are significantly more complicated to plumb and install.
Efficiency: Turbochargers are both internally and externally efficient. Modern impeller wheel and compressor housing designs create significant airflow at moderate temperatures, which results in a cool, reliable air charge within the intake tract. And, since they are driven by spent exhaust gas instead of a crankshaft, turbochargers do not require as much power to be driven (there are some losses), which can result in a net addition of several hundred horsepower compared to a similarly sized supercharger.
Power delivery: Turbochargers are extremely controllable—power delivery, boost, and impeller speed can be controlled down to the tenth of a psi. This allows drivers, racers, and tuners to dial in exactly the power they need when they need it to deal with traction issues or driveability on the street.
Lag: Turbochargers are not driven by engine rpm and their output is not linear, which means an improperly sized turbocharger may take several thousand rpm to come up into boost. This results in a sluggish feeling at take-off. Properly sizing a set of turbochargers to an engine combination can easily solve the lag issue, although it requires a user to set realistic goals and chose a turbocharger wisely.
Packaging: A turbo system is complicated as there is a lot of piping—and the rerouting of the exhaust manifolds to the turbos—plus the intercooler and the inlet. This requires a significant amount of room under the hood of a modern Mustang. Plumbing can be difficult, and routing pipes—sometimes up to 5 inches in diameter—through a Mustang bay usually requires custom fabrication or removal of factory parts.
Cost: Turbocharged systems generally cost more than supercharged systems, if for no other reason than the amount of parts involved. You're essentially buying an exhaust system, a power adder, a boost control system, and an intercooler all at once, which can be tough on the wallet.
Adjusting turbocharger boost levels is done by opening or closing the wastegate. Plumbed inline with the turbocharger, the wastegate works by diverting exhaust gasses away from the turbocharger turbine wheel, which slows it down (or speeds it up) and increases or decreases the boost level. With aftermarket electronic boost controllers, drivers can adjust boost levels on the fly, which is great for a street/strip project, or one you want to turn it up on the weekends and back down during the week.
In typical aftermarket and/or stock forced-induction applications, there are two types of intercoolers used to pull heat away from the compressed air. Air-to-air intercoolers feature a front-mounted aluminum unit (typically behind the front grille, in front of the radiator) that flows compressed air through a set of tubes, which are chilled by a set of fins that are exposed to ambient air pressure.
Air-to-air intercooling is simple and great for street cars that have ample airflow over the front while driving at speed. Air-to-water intercoolers, on the other hand, transfer heat by flowing a coolant over a set of tubes that contain the compressed air. Many OEM applications ship from the factory with air-to-water units that utilize a front-mounted heat exchanger (think radiator) to keep the coolant at a normalized level. In race applications, the heat exchanger is typically removed and an ice/water mix is used to cool everything. Of course, once the ice melts and the water heats, the system can experience heat soak, which is not desirable.