Turbos & Superchargers - Forced Introduction

A breakdown of turbos and superchargers that everyone can understand.

IT’s no secret that forced induction is a great way to increase the power of your muscle Mustang or fast Ford, but many of you may not know the secrets behind stuffing the cylinders with extra air, and why it can add so much power, even to an otherwise-stock engine.

The purpose of this article is to not only inform you as to how and why superchargers and turbochargers work, but to also teach you about both kinds of forced induction methods.

What is forced induction?

Normal atmospheric pressure (one atmosphere) is considered to be 14.7 psi, though factors like weather (temperature and barometric pressure) and altitude play a role. Normally aspirated engines run on air that reaches the intake manifold by the downward movement of the pistons, creating a pressure drop inside the cylinders, which causes air to rush into the engine through the throttle body.

Pressure above one atmosphere is considered boost. This can only be achieved by forcing the air charge into the engine using a device such as a turbocharger or supercharger. Pressurized air, or boost, is measured in psi.

This is a front and rear view... 

   

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This is a front and rear view of a Twin-Screw Kenne Bell supercharger cutaway. Notice how the red rotor has convex (male) rotors, and the blue rotor has concave (female) rotors. Unlike a Roots-type supercharger (which has matching lobe-and-cavity rotors), the twin-screw compresses air between the rotors, rather than along the perimeter of the case of the supercharger.

Why is boost so popular?

There are countless other ways to add power to your fast Ford. And in the Mustang aftermarket, a consumer has more options for products than is possible to install on one car. Bolt-on items such as heads, camshafts, intake manifolds, and throttle bodies can help improve the volumetric efficiency of an engine, but a street engine will rarely achieve 100 percent volumetric efficiency—at least without boost.

Another way to increase power output is to increase displacement. This, however, requires the engine to be removed and either replaced with a larger displacement short-block, or rebuilt with a stroker crankshaft and/or bored to accept larger-diameter pistons. Either way, this process is time consuming and expensive.

Superchargers

Positive Displacement

Because of their simplicity, reliability, and few moving parts, they have been used in production vehicles such as the ’03-’04 SVT Cobra, ’99-’04 SVT Lightning, and ’07-and-up GT500. They are usually limited to about 25,000 rpm. Often, spinning them faster results in loss of efficiency.

The crankshaft drives the supercharger either by the front engine accessory drive (FEAD) belt or a secondary belt designated solely to drive the supercharger. These belts are often either 6-, 8-, or even 10-rib design. Higher boost pressures often require more than 180 degrees of belt wrap on the supercharger pulley to prevent belt slip. Blower size is determined by the amount of air pumped in one revolution. For instance, a 4.0L Kenne Bell can pump four liters of air per revolution (of the supercharger). This is why boost is almost instant, and max boost is reached sooner than a centrifugal supercharger or turbo.

Throttle bodies on cars or trucks equipped with positive-displacement superchargers are mounted either on or near (via an inlet elbow) the inlet port of the supercharger. This reduces air pulsation caused by the supercharger. A bypass valve is also usually integrated into the blower to allow excess inlet air to bypass the supercharger when the boost is not needed (deceleration and closed throttle). This also prevents pulsating air at the MAF, which would result in false readings.

Though the term “Roots” is commonly misused to describe all positive-displacement superchargers, Roots-style superchargers refer to only those using opposing rotors with a matching lobe-and-cavity design. The rotors draw air through the inlet and outward along the perimeter of the housing toward the outlet.

TVS, or Twin Vortices Series, superchargers are Roots-style blowers manufactured by Eaton, but are unlike normal Roots-style superchargers in that the four-lobe rotors feature 160-degree twists instead of the 60-degree twists found on normal Roots-style blowers. This helps with efficiency, reduces noise, and reduces heat production.

In more recent years, the twin-screw supercharger has gained popularity, which uses one male-lobed rotor and one female-cavity rotor (see photos) to compress the air charge. “The twin-screw compresses the air between the two rotors, instead of between the rotors and the case,” says Jim Bell of Kenne Bell. This is what makes the twin-screw different, and it also reduces heat produced by the supercharger by reducing air velocity inside the unit. Both types are very effective and will work well in most applications.

Centrifugal

“A centrifugal supercharger compresses the air aerodynamically. The impeller, which spins as fast as 60,000 rpm, pulls air in and accelerates it into the surrounding compressor housing and then discharges it via the supercharger outlet. The faster the impeller spins, the more air it pulls in, compresses, and then flows to the intercooler and ultimately the engine,” says Jeff Lacina of ProCharger. Naturally, there are many impeller and housing designs to meet all desirable horsepower levels.

Because most centrifugal superchargers are mounted to the front of the engine, it is necessary to have sturdy mounting hardware. This will prevent deflection of the blower under strain of the belt. Manufacturers of centrifugal supercharger systems have taken this into consideration and provide heavy-duty mounting hardware. This mounting point also makes these systems relatively easy to install, even for someone with basic hand tools in their driveway.

Unlike positive-displacement superchargers, centrifugal superchargers don’t have a fixed output that increases directly with rpm. In other words, as rpm increases, output potential of the supercharger increases exponentially. This is why centrifugal superchargers are more efficient at higher rpm but are known for being less efficient down low.

Centrifugal superchargers are also known for being noisy, which some consider cool. Since they must be spun at a higher rpm to be efficient, most are driven by an internal transmission or step-up gears (to multiply impeller shaft speed). This gear set can be noisy, though some manufacturers have opted for helical-cut gears, which reduces noise for street applications.

To relieve excess boost, a bypass valve is normally located in the outlet tubing. Using engine vacuum as a gauge, the bypass valve bleeds off excess boost pressure to the low-pressure side of the system before the supercharger inlet.

The operation of a centrifugal supercharger creates less heat than that of a positive displacement supercharger. “Centrifugal superchargers produce significantly cooler air-charge temperatures due to their design and overall advantages in efficiently compressing air,” says Lacina.

Centrifugal superchargers have been a staple of the Mustang aftermarket since the early ’90s, before turbo kits or positive displacement superchargers were widely available. Companies like Vortech, Paxton, and ProCharger have been at the forefront of technology, and all provide a wide array of kits for almost every late-model Mustang application.

Turbos

A turbo basically consists of a turbine wheel, a compressor wheel, and a center housing. The exhaust-driven turbine converts heat and pressure into rotational force by applying pressure on the impeller and turbine shaft, causing it to spin. The turbine shaft is connected directly to the compressor through the center housing (which houses the bearings). On the other end of the turbine shaft is the compressor wheel. Like a centrifugal supercharger, the compressor wheel draws inlet air through a snail-shell-shaped housing (compressor housing) and to the intake system of the engine.

Often oiled and cooled by engine oil, turbochargers can spin up to 100,000 to 150,000 rpm. This requires solid engineering when developing bearings, housings, and turbine and compressor wheels. The main turbocharger manufacturers are Garrett Turbo, Borg-Warner, Precision, Turbonetics, and Master Power. Typically, the larger the inducer diameter (measured tip to tip in millimeters on the induction side of the compressor wheel), the more potential the turbo has to create power.

As important, though, are the other components of a turbocharger system that allow it to work. Also required are hot piping (exhaust side), a wastegate, cold piping (inlet side), and a blow-off valve. Unlike a supercharger system, turbocharger systems often require significant (and perhaps complicated) piping, and either welds or clamps to seal the tubes, preventing either exhaust gases or compressed air from leaking out.

Hot piping is what routes the exhaust gases to the turbine housing. Often necessary, or preferred, is the manufacture or modification of exhaust manifolds. Because a turbo/turbos are typically mounted near the front of the engine (to take advantage of cool, fresh air from outside the engine bay), most turbo exhaust manifolds point forward.

In the hot piping before reaching the turbo housing is the wastegate. It bleeds off excess exhaust gases into the atmosphere or the exhaust system (after the turbo). The wastegate controls boost pressure by either a set-weight spring connected to a valve, and/or being controlled by a boost controller.

After the fresh air is compressed by the turbo(s), it travels through the cold piping to either an intercooler (which we’ll discuss later) or directly to the intake system. This is often clamped together with V-bands to reduce loss of boost pressure.

In the cold piping is the blow-off valve. This is the piece that makes the cool psssshhhhh noise. It bleeds off excessive boost under decel and when the throttle is closed. Otherwise, the pressure in the cold piping would cause the turbine wheel to pulse, which can damage the shaft bearings.

Single-turbo - The most popular turbo kits are the single-turbo systems. Because of their simplicity (in relation to twins), they are most widely used in OEM and aftermarket applications. A misconception about single-turbo systems is that a larger turbo is always better. If a turbocharger is too large for an application, it will not “spool up” (achieve its most efficient operating speed) quickly enough for the engine to take advantage of its larger diameter wheel.

There are formulas used to determine what size turbo would be ideal for your application, but there is not enough room in this article to even begin to discuss this. Just remember that bigger is not always better, and to consult the manufacturer regarding turbo size.

Though the piping on a twin-turbo system may be more complex, it is better at reducing heat loss in the hot piping. Heat loss in the hot piping is energy loss, resulting in less potential of the turbocharger(s).

Intercoolers

An intercooler is a heat exchanger, which cools the inlet charge. This occurs by condensing the compressed air charge from a turbocharger or supercharger system. Molecules of air are closer together when cold, allowing more room for more molecules to fit at a given time. This, combined with the right amount of fuel, will allow the engine to make more power. Also, a cooler air charge decreases the chance of knock (detonation). A hot air charge requires ignition timing to be retarded to prevent knock, thus decreasing power.

There are a number of ways to cool a compressed air charge in a forced-induction vehicle. The most popular is the air-to-air intercooler. Another growing method is the air-to-water intercooler. A less-conventional method is through water/methanol injection.

Air-to-Air - Like coolant flowing through a radiator, the compressed air flows through a block of tubes separated by fins. Cool air flows across these fins, cooling the air charge. Because of its simplicity and wide availability, the air-to-air intercooler is the most popular. Unlike air-to-water and water/methanol injection, an air-to-air intercooler’s ability to cool is constant as long as the vehicle is moving or dedicated fans are used, and most can cool an air charge by 30-40 percent.

Also, minimal maintenance, affordable price, and reliability are all major positive features of the air-to-air intercooler. These factors make the air-to-air intercooler the choice for most street applications.

Air-to-Water - Air-to-water intercoolers use ice water or ethylene glycol, stored in a reservoir and pumped through the system to cool the inlet charge. In this case, the intercooler acts more like the evaporator core of an A/C system, where the air charge actually flows over the coils of the intercooler instead of through it. The water collects heat from the air charge and is then pumped through the tubes of a heat exchanger (mounted in front of the radiator), where air then flows through fins to cool the water.

This method is extremely effective (more than an air-to-air) for a short period of time. Then, the water/coolant begins to become heat soaked. To maintain maximum cooling capabilities, the ice must be replenished, or the coolant must be allowed to cool.

Air-to-water intercoolers are most popular on Roots-type and twin-screw superchargers and are usually mounted in the valley of the engine below the supercharger. Air is forced directly through the intercooler as it exits the outlet port of the supercharger.

Water/Methanol Injection - Lately, some companies have developed systems that inject a mixture of water and methanol fuel into the intake tract of a forced induction engine. “Think of it as chemical intercooling,” says Matt Snow of Snow Performance. “It’s ideal for cars with no room for intercoolers.” On a street application making 15 psi of boost, water/meth injection can reduce air temps from 180 to 190 degrees to 120 to 130 degrees—a 60-degree drop.

Not only does it cool and condense inlet air, but it also raises the effective octane of the fuel. “With 91 octane pump gas and a 50/50 water/meth mixture, we can meet or beat 116 octane race fuel,” says Snow. The mixture slows combustion, making combustion more controlled, and allowing a leaner air/fuel mixture and more-aggressive ignition timing.

The disadvantage to water/methanol injection is that is it is consumable. Once your two-and-a-half-gallon container is empty, you must refill it to regain benefit.

Wrap It Up

With the amount of support provided by aftermarket manufacturers, Mustang and Ford owners are living the good life when it comes to forced induction system options, price, and reliability. There are already nearly a dozen kits available for the ’11 5.0L Mustang GT, with many more to come. There’s no end in sight, and it will be exciting to see how these systems improve even more over the coming years.

(Author’s Note—When writing this story, the following publications provided invaluable information: Maximum Boost and Supercharged!, both by Corky Bell.)