Muscle Mustangs & Fast FordsHow To Engine
All About Blowers
The ins and outs of compressing air.
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There are hordes of late-model Mustang owners who will swear a blower is the only way to fly. These folks have felt the magic of supercharging, be it the sight of an intimidating polished huffer, having heard the inrush of air whistling at high pitch, or perhaps they have felt the good graces of g-force created from boost and unchained horsepower.
Ah, yes, superchargers, those miraculous devices that can easily double the power of a stock engine or help you build a beast capable of 2,000 hp or more.
Today, with technology being so advanced, making those ponies is not all that hard. But there's a science to how it all works and it takes a bit of knowledge to make big power and keep it alive. And that science involves getting the air into the cylinders and providing the best diet of fuel, timing and octane.
Ever since the late-model Mustang gained huge popularity in the mid-'80s, the supercharger has been one of the most sought-after, add-on accessories. Back in the day, blowers were far less common than they are today. They had an almost mystical aura about them, even in the early '90s. Today, you can only describe their popularity as a mania. We at MM&FF know how popular they are, because over the course of each month we get dozens of letters and e-mails asking about superchargers and the effect they will have on a particular car or engine. The questioning includes, "How much power will a blower add? Will a blower hurt my engine?" And, "Which is the best one?"
I'll tell you right off the bat that we've yet to find one single "best" supercharger. This may disappoint those looking for a clear-cut winner, but the bottom line is that most blowers work well, both in theory and practice, and all have good points and bad. And, while there are ways to directly compare blowers, it's more important to understand how they work and what to expect from each one. Different vehicles may benefit from one type or another so it's important to know what you're looking for.
When picking a supercharger, you'll need to consider the size of the unit, the type of drive system, airflow potential/boost capabilities and the drive speed. In addition, consider the weight of the vehicle, transmission type, gearing and the manner in which the vehicle will be used. Is it a drag-only vehicle or is it mainly street driven? If it's heavy, like a Lightning, it will love low-rpm torque, and if it's light (3,000 pounds or less), it can afford to give up on torque in order to make more high-rpm horsepower.
There are enough types, sizes, makes and models to confuse even the brightest blower backer. Superchargers are relatively simple machines, however, finding maximum power and keeping the engine alive is a different thing entirely. This part of the equation is complicated indeed, but we plan to diffuse the important facts and expose the myths. So fret not fearless Stang bangers, because in the next few pages we plan to take a journey through the land of supercharging. But we caution you. Reading this may just "force" you to get blown.
A supercharger, by definition in Webster's Dictionary, is "an apparatus consisting of a pump, compressor, or blower used to increase the volume of air over and above that which would normally be drawn into an internal combustion engine due to the action of its pistons."
When the engine is naturally aspirated it relies on the pumping action of the pistons to draw air into the cylinders. The negative pressure in the cylinders combined with the ram effect of the intake manifold, camshaft overlap and exhaust scavenging, allows each cylinder to draw in the air/fuel mixture every time the intake valve opens. With supercharging, air is constantly being packed into the intake manifold under pressure (called boost), so the air is forced into the cylinders when the intake valve opens, rather than being drawn in solely based on the pressure differential between that of the cylinders and the atmosphere in the manifold. This additional air provided by the supercharger simply permits the engine to burn more fuel, thus creating more cylinder pressure and, with any luck, more power. Supercharging is really that simple--on the surface.
Since a supercharger is a compressor driven by the crankshaft, the output of the blower changes with rpm (speed). Generally, the output will increase until the point of peak efficiency, and then output falls off. Eventually the blower will reach the point where it makes no more boost, just extra heat.
Supercharging has been around for quite some time and has been used on all types of engines, including piston-driven aircraft and generators in big industry. In auto racing (and in high-performance street applications), we've learned to apply supercharger technology to help our internal combustion engines achieve incredible power levels. With a blower, volumetric efficiency can easily exceed 100 percent, while most naturally aspirated engines struggle to achieve 60-80 percent VE.
Another benefit is that owners can retain the stock cam, heads and, in some cases, the induction system, therefore, retaining much of the OE driveability, yet still realize a huge increase in performance.
The most extreme supercharged engines can be found in drag racing, namely in the Top Fuel and Funny Car ranks. These extreme machines utilize 500-cubic-inch engines with hemispherical combustion chambers and they burn a specialized fuel called nitromethane. Using large 14-71 Roots-style blowers, these "fuel burners" produce 45-50 psi of boost and generate upwards of 8,000 hp. Today's Top Fuel cars run quarter-mile times in 4.40s at speeds over 330 mph.
A good portion of the massive power comes directly from the fuel and its explosive force. Unlike leaded racing gasoline, nitromethane carries its own oxygen and requires a nearly equal 1.7:1 air/fuel ratio. Compare that to a gasoline-burning supercharged engine that will require a 11.5-12.0:1 a/f ratio.
At wide open throttle, the nitro burner's fuel flow is equivalent to a garden hose with the nozzle held wide open--and that's per cylinder. If you've watched drag racing on TV you've seen the header flames, but at times there is raw fuel pouring out of the headers during a run. The engine is said to have "dropped a cylinder" and it happens when the spark plugs can no longer light the massive quantity of fuel. This can lead to all kinds of problems. One is engine hydraulicing and that's often followed by a massive explosion and ensuing fireball. But Top Fuel engines aren't the only supercharged beasts that go boom in the night when things go wrong.
We've seen many Mustangs and Lightnings have failures as a result of boost, however, this isn't a fair way to explain all the blow-ups. Read on and I'll explain. Remember one of the questions that I said we often get, "Will a blower hurt my engine?" The answer here is yes and no. (And I'm not trying to be a wise guy, either.)
First I'll cover the "yes." A supercharger can cause engine damage that otherwise wouldn't occur, but the common failures (blown head gaskets, cracked blocks and burned pistons) are often the result of a poor engine tune-up, gasoline with insufficient octane rating, abuse from the driver or any combination of the three.
"First off you have to consider the condition of the engine," stated Ricky Best, the Race/Media Relations Manager for Vortech Engineering. "Most bolt-on systems making about 8 pounds of boost are designed and tuned to be able to be applied to completely stock, as well as modified engines, without having any adverse effects on the motor itself. If the engine is ailing or high in mileage and is using oil, then the supercharger is only going to help your engine find an early grave."
With today's kits, you can easily add 100-150 hp to a stock engine, and anytime you take a component and push it that far past the factory-designed capabilities there is a chance of failure. When it comes to building and tuning your own supercharged car, you must carefully select each piece of the puzzle. This includes the short-block, the heads, the fuel system, and don't forget the fuel management system, the type of gaskets, the intake manifold, ignition and, of course, the type of fuel.
Still, what people fail to realize is that despite the power increase, most problems come from a poor tune-up. In addition, remember that as horsepower increases there is additional heat as a result. It takes extra air and fuel to make extra power and burning extra fuel means more heat. Compressing the air also creates heat, and the stock 5.0 or 4.6 engine was not originally designed to dissipate all that extra heat, so you have to manage it properly. Mismanaging it leads to problems and one example of mismanaging is beating the snot out of your supercharged engine with it cooking hot. Another is turning up the boost without re-tuning.
"Tuning is a big thing," adds Best. "Assuming the engine is healthy and the kit has been installed correctly, the number one killer of most supercharged applications is bad tuning. Bad tuning can be anything from improper air/fuel ratio, too much or not enough ignition timing, too much or not enough fuel pressure, improper FMU, and incorrect spark plugs. Lastly is boost pressure. It's really easy to get your first supercharger and get it installed and running and have a lot of fun, only to find out that you can swap a pulley and get more boost. What you need to keep in mind is that adding boost means more airflow and that usually means more heat. And more heat requires more fuel and less timing. The bottom line is that all of the variables need to be addressed."
With this you can also see how the answer to the original question could also be no. A blower alone can't hurt the engine, because all it's doing is feeding in more air. In other words, it's the tune and the ability of the driver to pay attention and use the equipment in a mature fashion that will help longevity.
When it comes to the tune, care must be taken to ensure that the compressed air (read: boost) is matched with the correct ratio of fuel and the proper timing advance curve. Force-feeding the engine presents us with challenges that must be overcome in order to maintain good power, driveability and reliability. The most notable demon is detonation. By its nature, compressing air creates heat and this potentially results in engine-damaging detonation.
"We've worked with flame-front experts and we know that auto-ignition or detonation creates cylinder pressures that are up to 10 times that of normal combustion," stated Dan Jones of ATI/ProCharger. "Pre-ignition or unscheduled spark ignition occurs when the piston is traveling up the bore and is not yet at the point for the scheduled ignition but ignition occurs without the spark plug firing."
When this occurs, the mixture begins to burn prematurely and cylinder pressure skyrockets. The piston is still on the way up, but it's fighting the cylinder pressure that's not supposed to be present. Then, the spark plug fires and the scheduled flamefront begins. Ultimately the two flamefronts collide and the sound is heard as a knocking or pinging, i.e. detonation. This not only creates tremendous pressure in the cylinder, but also creates unwanted harmonics throughout the engine. And over time (sometimes a really, really short time) these harmonics cause major engine parts to fail. But even when pistons or head gaskets do survive detonation, the related effect places severe load on the crankshaft and the bearings, as well as the rings and the block.
According to Jones, the problems associated with detonation far exceed the alleged problem of extra load on the engine imposed from simply making extra horsepower and torque.
"When the engine is running properly there is a very short period in degrees of crankshaft rotation where big force is applied to the pistons. This occurs during the power stroke and for a short time just after the mixture is combusted as the piston crosses TDC," said Jones. "It's important to note that with forced-induction engines there is less total peak rod/piston/crank load, but the duration of the power application is longer than with naturally aspirated engines. This means the force is being applied closer to the point at which the crank nears the 90-degree angle and this means more torque can be applied."
Roots and Screws
There are three common types of superchargers (centrifugal, Roots and screw-type or twin-screw) and each one of them is designed to do the same thing--make boost. But while they all force air into the engine, they possess different characteristics.
Hot rodders are familiar with Roots-style blowers seen perched atop Pro Street or Top Fuel engines. Roots blowers have been around since the 1800s and they fall in the "positive displacement" or "fixed displacement" family. Twin-screw blowers are also positive-displacement blowers.
Positive-displacement superchargers are labeled as such because they move a fixed displacement of air per each revolution of the blower. As the rotors or lobes spin, a fixed amount of air is trapped and that air can not reverse in flow. An increase or decrease in airflow through the blower will be noticed based on the position of the throttle, but once the air enters the blower and is sealed between the rotors and the case (or the screw lobes) the amount of air per revolution can't change.
On the other hand, a centrifugal blower can allow a backflow of air because the air is not sealed within the compressor at any point. It's also important, but not critical, to note that screw and centrifugal superchargers compress the air within their housings, whereas Roots blowers force air through the blower and the compressing is done in the manifold.
Each blower has a case, usually cast from aluminum, with a machined inside, which houses two (or three) rotors. The rotors will have either two or three lobes, and some units, like the Eaton blower found on the Lightning and Cobra, have the lobes twisted. Air is drawn in at one point and guided towards the rotors. The rotors accelerate the air towards the outlet, where it is carried and fed into the intake manifold.
The blower or compressor is attached to a plenum that serves as the intake manifold. There's usually a machined surface where the blower sits, a plenum and runners to supply the ports. In most cases, the ports are short, due to space limitations and because a long runner is just not necessary when you have boost pressure. Roots blowers do a tremendous job of making instant boost, thus filling the cylinders at low rpm and this helps to generate great throttle response and torque.
Disadvantages to this style of supercharger are that (in most applications) they sit atop the engine, which can create packaging or hood clearance problems and that they generate lots of heat. There's not much you can do to solve the clearance problem, but with the use of intercoolers, aftercoolers and heat exchangers, the issue of heat is not as prevalent.
"Roots blowers are good reliable units, but the twin-screw is a much more efficient design. And that's why Ford has gone to a twin-screw on the new GT," stated Jim Bell of Kenne Bell Inc.
A screw blower looks similar from the outside, but the internals are completely different. With a twin-screw there are male lobes that intermesh with female lobes. Both sets rotate inward and as air is drawn in it is compressed and "screwed" forward towards the front of the case. According to Bell, rotor speed can approach 24,000 rpm.
Bell also said that by design the Roots is about 30 percent less efficient. Therefore, it must be 30 percent larger to pump the same amount of air. And larger blowers take more energy to turn so there are greater parasitic losses and more heat.
Because of the way each of the three blowers arrives at making boost, there are great debates as to which system is more efficient. The generation and dissipation of heat within a blower system has to do with thermodynamics (the physics of relationships between heat and other forms of energy) and this is quite the complicated subject.
Any time you compress air its temperature rises. You can't avoid this--it's one of the laws of physics. You also have heat generated by the blower itself due to internal friction, or more technically, by the work necessary to get the air from its natural pressure up to the desired boost pressure.
For instance, heat is generated at the bearings, within the blower's internal drive system, by the drivebelt and even by the friction of the air flowing through the blower. Centrifugal blower manufacturers tell us these units tend to be the more efficient because they are not bolted directly to the intake, so less heat is transferred to the engine. They are also easier to intercool, and intercoolers are an important tool. Intercooling (or aftercooling) reduces the temperature of the intake air charge and allows tuners to dial in more boost pressure and more ignition timing without as much fear of detonation.
In direct contrast to the Roots or screw blower is the centrifugal supercharger. There is a huge difference between the designs of the two. Where the Roots and screw units are positive-displacement blowers, the centrifugal blower's displacement is not fixed.
Like their Roots cousins, centrifugal superchargers are also driven by the crankshaft, however, they are generally much smaller and are usually mounted at the front of the engine rather than on top (although there are some Roots blowers that are front-mounted and driven directly by the crank). This allows them to adapt easily to EFI engines because the owner can retain his or her complete throttle body and intake manifold system. In most cases, just the inlet tube (or system) needs to be modified.
The centrifugal housing is shaped similarly to a turbocharger and in place of rotors or screws, it uses an impeller (also similar to a turbo) to draw in air and direct it to the housing. "Centrifugal blowers are true compressors," stated Best. "As the supercharger draws in air it accelerates and compresses the air internally. The scroll collects the compressed air and forces it into the discharged tube and then into the intake manifold. A well designed compressor stage exhibits much higher efficiency than the Roots design, resulting in much greater net gains due to lower charge air temperature and parasitic loss," he added.
Centrifugal blowers accelerate the air due to centrifugal force, hence the name. The impeller wheel is driven by an internal transmission with a "step-up ratio" and a drive pulley system, therefore it can drive the impeller much faster than the actual engine rpm. Impeller speeds are generally in the range of 50,000-65,000 rpm.
"Centrifugal blowers take in air and the impeller carries or directs the air and accelerates it. It whips it up to speed dramatically, but the impeller doesn't compress the air or generate boost. The flow of air exits the impeller and enters a vaneless diffuser where it is straightened out and sent into the scroll. Air then slows down and pressure is created," explained Jones.
And since there are a virtually unlimited number of applications, blower manufacturers have developed a variety of housings and impeller types to suit the needs of everything from a stock 3.8- or 5-liter engine to a 6-second, 200-mph Pro racer. The size of the housing and the shape of the impeller blades (or fins) have a great affect on the boost curve and changes to the impeller can be made to fine tune this curve to maximize airflow and boost for a specific application.
If there is a downside to the centrifugal superchargers, it's that they rely on rpm to make boost and they give up low-rpm performance to the Roots and screw units in this department. Nevertheless, they are generally more efficient at making boost in the higher rpm ranges.
Today there is a huge number of blower kits available to Mustang and Lightning owners. The choices can be overwhelming, but we've found that picking the right blower requires nothing more than a little research on your part. The key to finding the best one for your combination is to select a unit that can supply the most efficient level of boost in the rpm range that you're building your engine for. In addition, consider the combination as a whole. For instance, heavier vehicles need more torque than lighter ones do and that's why the Eaton, Magnum Powers or Kenne Bell is the best choice for a 4,500-pound Lightning. But a Paxton, Powerdyne, ProCharger, Vortech may be the way to go for your 3,000-pound LX.
And lastly, remember that boost is awesome for a street-driven car and peak power numbers are important, but reliability and driveability should outweigh maximum power. We've seen too many people shoot for the moon, you know, for that last ounce of power and end up with a worthless pile of pistons, rods and crankshafts.
People often ask how long their stock short-block will last with a blower. To them, we point out our "Ice Box" '01 Mustang GT project car. It's had a Vortech SQ on it virtually since it was new. Thanks to a ported set of stock heads, Comp cams and a ported Bullitt intake, it now makes 542 rwhp and runs 11.2 at 126 on pump gas. It is driven daily year round on 93- or 94-octane fuel and has eclipsed 40,000 trouble-free miles on the untouched factory short-block.
Much of the credit has to go to its conservative JDM Engineering tune up. When tuning the car, JDM proprietor Jim D'Amore told us he could make more horsepower, but didn't feel comfortable going higher with the factory rods.
In other words, common sense should prevail. Always run good gas, keep a check on fuel pressure and timing and remember there's a time and a place to hold the gas to the floor.