Ford Mustang Supercharger Tech Guide

• Although designs have changed over the years, the two most common designs are centrifugal blowers and positive displacement superchargers
• Centrifugal superchargers operate similar to a turbocharger, but spool the incoming air(instead of exhaust) before "forcing" it into the engine. This creates more power higher in the RMP band, much like a turbo would
• Two types of positive displacement blowers--roots and twin-screw--are common on Mustangs
• Blow off valves, intercoolers and bypass valves are typically installed when supercharging a Mustang
• Picking a supercharger depends on what your looking for from your Mustang

The centrifugal supercharger became a very popular option for the Fox-Body Mustang, although its first use on the Mustang date back several decades ago. Carroll Shelby starting using Paxton superchargers on certain Shelby Mustangs back in the sixties. These particular Shelbys command a very high price tag among car collectors today.

The centrifugal supercharger is best suited for producing higher RPM horsepower, as it generally starts providing noticeable boost around 3,000 RPM. This made the centrifugal supercharger ideal for the 5.0 Mustang, as that pushrod V8 engine was known for producing plenty of low-end torque on its own.

The centrifugal supercharger is placed beside or in front of the engine, with air entering the front of the supercharger. A pulley is mounted directly opposite to the air inlet, and is driven by the engine’s accessory belt system. This pulley spins the impeller, which is found inside the supercharger. The impeller looks like a series of blades, and can be seen through the air inlet. As the impeller is spun increasingly faster, the motor's RPM increases as well, and the impeller produces enough force to overcome the vacuum that normally pulls air into the motor. The supercharger then forces air into the motor. It’s this force that is commonly referred to as boost. Boost is represented in psi (pounds per square inch). Typical boost is in the 5-10 psi range, although highly modified engines can see boost much higher than this typical range.

The biggest advantage of the centrifugal supercharger is fitment. It mounts to the front of the engine much like an accessory would. They generally use an air- to- air intercooler. The compressed air is pumped through the intercooler before it enters the intake plenum. The intercooler mounts to the front of the engine bay and is also similar to a radiator just like the air-to-water intercooler. The disadvantage is full power potential isn’t reached until you’re pretty much shifting gears.

A centrifugal supercharger spins at very high speeds, thus, creating heat. To combat this, oil is required to lubricate it. Some centrifugal superchargers tap into the vehicle’s engine oil supply, while other centrifugal superchargers use oil that is self-contained in the supercharger. At specified intervals, the superchargers using self-contained oil must have their oil changed, just as you would also change your engine’s oil.

The official correct way to change supercharger oil is to remove the blower from the engine. Then, remove the plug, and drain the oil. This can sometimes, depending on the model, need the supercharger to be tilted a certain way for a long period of time. This is to let all the oil drain out. Once completely drained, fill with the correct specification.

The two most commonly used centrifugal superchargers are Paxton and Vortech, although there are still many other manufacturers to choose from. It should also be noted that a centrifugal supercharger and a turbo share the same basic operational concept. The chief difference is that the centrifugal supercharger is driven by the engine’s accessory belt system, and a turbocharger is driven by the engine’s exhaust.

Positive displacement superchargers are easily recognized as they sit directly on top of the motor. There are some exotic cars, however, that use a positive displacement charger mounted on the side of the engine. Positive displacement chargers still use the basic supercharger concept of compressing air and overcoming the engine’s vacuum, but achieves this is a slightly different way than the centrifugal supercharger does. With a positive displacement supercharger, air enters it, and ends up at the rotors. From there, the rotors (whether twin-screw or the rounded Roots-style) compress the air as they spin inside the supercharger. The rotors or screws are powered by a pulley at the front of the supercharger that is driven by the engine’s accessory belt system.

When the air reaches the rotors, they compress it, forcing it out the underside of the supercharger’s body. From there, it enters directly into the intake of the engine. As with the centrifugal supercharger, you’ll see a minimum of 5-10 psi of boost with modified and strengthened motors, often sporting much more boost. 

The first positive displacement supercharger on record was the Roots-type supercharger, which has origins dating back to the mid-1800’s. Roots style blowers offer excellent torque at a blip of the throttle and provide boost all through the rpm band. Ford uses them as their stock option supercharger for their durability and consistency. The twin-screw style superchargers perform similarly, but twin-screws have tighter tolerances than the roots. While great for high boost applications, the tighter tolerances means more heat produced by compressing incoming air. Needless to say, excessive heat can hamper performance and require increased cooling, so there's a tradeoff. 

Depending upon the exact type of positive displacement supercharger, there may be more than two rotors used. With the modern positive displacement superchargers used on Mustangs, twin rotors are normally employed. Just as with the centrifugal supercharger, the rotors spin at high speeds and lubrication is required inside the supercharger. Positive displacement nearly always use a self-contained oil system, which will requires oil changes on a regular basis.

Centrifugal Mustang Superchargers

• Excellent for owners looking for peak power high in the RPM band
• Easy to intercool because of location
• Usually need to be integrated into the engine's oil lubrication system
• Lower off the line power than positive displacement chargers

Roots Superchargers

• Great for off the line torque
• Consistent power
• Good balance between cost, performance, and durability
• Looser tolerances with fewer lobes which means less power
  
  

Twin-screw Superchargers

• Consistent power like a Roots-style charger
• Tight tolerances for high boost applications
• Excellent balance between Roots-style and centrifugal chargers
• Risk of excessive heat in low throttle conditions

Intercooler: One of the side effects of supercharging is when air is compressed, it also heats up. Heat lessens the density of the air, and can potentially increase the heat in the engine. To combat this, an intercooler is used. The intercooler is typically placed after the supercharger, and the compressed air is forced through it. Intercoolers are fed by engine coolant most of the time, although some race setups have the ability to facilitate the use of ice. While the air passing through the intercooler will cause some minor loss of boost pressure, the benefits of cooling the incoming air far outweigh the disadvantages of an intercooler.

A centrifugal supercharger uses an intercooler that is placed into the piping that connects the supercharger to the intake. In the case of a positive displacement supercharger, the intercooler is placed between the supercharger and the motor. The air exits the underside of the top mount supercharger, is moved through the intercooler, and then into the intake. Sometimes intercoolers are referred to as heat exchangers.

As a side note a typical intercooler in use today is more akin to an “aftercooler” since it cools the air after it has been compressed. Early intercoolers were placed before the supercharger/turbocharger, and used normal airflow to cool the incoming air charge. The name "intercooler" has stuck through the years and persists to this day.

The chief disadvantage to a supercharger is that the incoming air is heated as it’s compressed. Nitrous, as a power adder, doesn’t have this disadvantage, as cooling is a side effect of the change nitrous undergoes when it switches from a liquid to a gas. Superchargers are always driven by the engine as well, adding a small amount of load on the engine, even if they aren’t producing boost. While the use of a bypass valve has helped with this aspect, there is still a small impact to fuel economy.

A supercharger has several advantages in its favor, however. The first of these is the fact that, unlike with nitrous, bottle refills are not necessary. The power provided by a supercharger is also always ready, requiring nothing more that the throttle to be opened up. Because of this, and the fact that the use of nitrous on the street may be illegal on some areas, supercharging is one of the most popular options for street-driven Mustangs. The supercharger may seem expensive at first, but when it comes to weighing the horsepower gained by dollar spent, it’s quickly seen as a much more viable option than replacing engine components. It’s also a much quicker install than diving into and changing parts of the motor as well, as most supercharger installations aren’t too difficult, although tuning after the fact may be required.

• Larger fuel injectors to compensate for the extra air
• Higher volume fuel pump
• A fuel pressure booster or a boost-a-pump
• Underdrive pulleys
• A tuner to make everything work together

With an increased amount of air pushed into the engine, fuel delivery must also be increased. This can be accomplished through a higher volume fuel pump, or a pressure booster like the Kenne Bell boost-a-pump. Larger fuel injectors may also be needed depending on the application.

Both supercharger types are belt driven, meaning they operate off a pulley. The pulley usually spins faster than the engine, but is directly proportional to engine speed. If engine speed doubles, so does the supercharger’s speed. The size of the pulley dictates the boost levels. A smaller pulley will spin faster as it is being “overdriven”. Many aftermarket companies such as Metco offer different sized pulleys to attain different boost levels.

Since the 2003-2004 Cobra Mustang came stock with a supercharger, a popular modification is swapping out the pulley on the nose of the supercharger. Like the underdrive pulleys mentioned above, this changes the ratio the crankshaft spins the supercharger. Some manufacturers claim changing the pulley can net you 50 HP! 

However there's one big hang-up: the serpentine belt. What size do you get? Below is a handy chart for the 03-04 Mustang's. For each crankshaft pulley and supercharger pulley combination we've listed the appropriate serpentine belt size.

As with any engine modification, the need for a tune cannot be stressed enough. A good tune will not only keep the engine operating safely, but will also take full advantage of all the available horsepower. The tuner will be able to set timing and fuel parameters to get most out of the boost, without having the engine go poof. Nothing worse than having your new set-up pop a piston because the air/fuel mixture went too lean.

If high levels of boost are the goal, be prepared to install a forged rotating assembly in the block. The more boost, the more power. Not to mention some pretty extreme pressures on the engine internals. The stock rotating assemble can generally handle about 8psi with a good tune. That is enough boost to really throw you back in your seat.