Though we car crafters like to think of ourselves as a more sophisticated bunch than the average car guy, the truth is we all have a warm spot in our collective hearts for excessive and gratuitous displays of horsepower. And few things scream wretched excess more than a supercharger jutting through the hood. The bigger, the better, too. An 8-71 on a small-block Chevy-why not?
Still, it is better to take a proverbial step back before plunking down your hard-earned cash for what may be a clunker of a supercharger, which is what you will have if it is improperly matched to your engine. This article will provide a basic look at positive-displacement superchargers, hopefully giving you a good base of knowledge to refer to when planning a forced-induction build.
Positive Displacement vs. Variable Displacement
What's the difference? The an-swer is simple enough. A positive-displacement pump (whether it is pumping fluid or air) is one that moves the same amount of a substance during each revolution: One liter of air goes in, one liter is pumped out the other side no matter how fast the pump is turning. In contrast, a variable-displacement pump moves less volume at low speed than it does at high speed.
Though you can think of a centrifugal supercharger as a crank-driven turbocharger, a turbo
Here's an easy analogy we all can identify with: comparing the engine in your car with the blower motor in your HVAC system (assuming you haven't thrown it away Jeff Smith style in a quest to shave every ounce of extraneous weight). A piston engine is one type of positive displacement pump. Whether it's spinning at 6,500 rpm or you're turning it over by hand, it should pump the same amount of air on every revolution. Your heater blower motor, on the other hand, does not work that way. Like all fans, it moves more air the faster it spins. This is especially noticeable when trying to cool your car after it's been sitting in the hot sun. We all know you've got to crank the fan to high to get the air moving.
Forced induction operates the same way: Though the ultimate goal is to push more air into the engine, you can either do it with a positive- or variable-displacement pump. A generic term for these devices when used on a street machine is supercharger, and they can be driven either by the engine via a mechanical connection or by exhaust gases via a turbine wheel placed in the exhaust system. The exhaust-driven type is referred to as a turbocharger, a shortened version of the term turbosupercharger-literally a turbine [driven] supercharger. All turbochargers are variable displacement-type pumps, but beltdriven superchargers can be either positive or variable displacement. Variable displacement-type superchargers are known as centrifugal superchargers, and on the positive displacement side of things, the industry has latched on to two types-Roots and twin-screw superchargers-to get the job done.
We will not be addressing centrifugal superchargers in this article, however, because we feel the trend in the industry is focusing on the positive displacement-type superchargers. Additionally, we believe that a modern, efficient positive-displacement supercharger is currently the most economical way to make more power. Note that by economical, we are considereing the amount of time involved in installing the system as some kits are as easy as swapping an intake manifold. How much is your time worth?
In the world of positive-displacement pumps, there are a variety of shapes and styles. As we mentioned before, a piston engine is one example-you will likely find a piston pump pressurizing the compressed air tank in your garage. Refrigerant in your air conditioning system is also pushed around by pistons in your A/C compressor. Rotary pumps are another example: The oil pump in your engine is a scaled-down version, while the Wankel rotary engine found in Mazda's RX cars can make pretty good power. Vane-style pumps, like those in your power steering system and automatic transmission oil pump, are just two more examples of positive-displacement pumps you can find all over your car.
However, most of these types of pumps are not good superchargers. They generally cannot move enough air to meet the demands of an automobile engine and still be sized small enough to fit within the confines of an engine compartment. Decades ago, the industry latched on to a different design, the Roots-style pump, because it can move very large quantities of air-enough to feed a hungry V-8.
See, you can't help but look.
The ubiquitous Roots blower is the oldest and most common supercharger used in automotive applications. Its visual appeal is undeniable: Like a sucker punch to the jaw, nothing says horsepower like an oversized Roots supercharger. Though it was adapted by the transportation industry to help power two-cycle GMC diesel buses, the Roots blower has a history that dates back before the golden age of the automobile. Originally conceived by brothers Philander and Francis Roots, the device was designed as an air pump for blast furnaces. It was quickly adapted for use in a wide variety of applications, from ventilating mine shafts to pumping fluids in plumbing applications. A Roots-type pump was even used to provide the air supply (insert '70s soft-rock band joke here) to the Federal Signal Thunderbolt air raid sirens common throughout the country during the Cold War.
Roots pumps are extremely simple machines. A case houses two rotors that are machined to tight tolerances, though the rotors usually do not touch each other, nor do they touch the case. The rotors are connected by a pair of gears, and a shaft extends out from one of the gears and is driven by the crankshaft. That shaft turns the gears causing the rotors to spin in opposite directions. An opening in the top of the case allows air to fill the empty space in the cavity of the rotors. Then as they turn, the rotors trap that air between themselves and the case of the pump, pushing it around and down through the case and out the other side.
The simplicity of the Roots supercharger may be its ultimate shortcoming as well. Dustin Whipple of Whipple Industries tells us that Roots pumps are a "no pressure" design. "They were never intended to pressurize air-only to pump it," he says. As a result, the Roots supercharger is one of the most inefficient. More about that later.
The twin-screw supercharger is the other commonly used positive-displacement model. It is similar in appearance to a Roots blower but different on the inside. The design was patented in the '30s by Swedish engineer Alf Lysholm, who-like the Roots brothers-was designing a pump for industrial applications. Decades later, his design would be put to use making cars go faster. The main difference between a Roots supercharger and a twin-screw or Lysholm-style pump is that the twin-screw supercharger is a true compressor becasue it compresses and pressurizes air inside its case.
Like a Roots supercharger, the twin-screw design also relies on spinning rotors, but its shape is much different. There is a male and female rotor, and the lobe count is different between the two, usually three lobes on the male rotor and five on the female. Like in the Roots superchargers, the rotors never touch each other or their housing, but unlike in the Roots, the rotors turn toward each other. Air enters the case from the rear, filling up an opening between the rotors. As the rotors turn, their meshing action pushes the air through a cavity that gets smaller toward the end, compressing the air along the way.
Here is a Whipple twin-screw supercharger installed on an '06 Ford Mustang.
Boost is Not Always Good
We car guys usually equate boost with horsepower: The more boost the better. This is not always true, though. We are going to demonstrate that we should be concerned with the density of the air the supercharger delivers, rather than how much boost it makes.
The purpose of a supercharger is to force more air into an engine than it would otherwise be able to ingest naturally aspirated. Instead of operating with vacuum in the intake manifold, this extra air pressurizes the intake; the extra air pressure is referred to as boost. In basic terms, boost represents an increase in air pressure that is above ambient atmospheric pressure. Ambient pressure is around 14.7 pounds per square inch at sea level but varies slightly due to elevation and barometric conditions.
Extra pressure does not always mean there is actually more air in the intake, since air expands at high temperatures and contracts at low temperatures. In a closed system, heating the air in a fixed volume of space will cause an increase in pressure because the air molecules are trying to expand, but the amount of air or oxygen remains the same.
To illustrate this point, we performed an easy experiment using the air in our tires. We wanted to see how much the tire pressure changed after a few minutes of hard cornering (OK, more like 45 minutes of hard cornering). Before we started driving, we had 34 pounds of air in the tires. After a blast through one of our favorite canyon roads, the tire gauge read about 39 psi. Looking at it another way, you could call the original reading of 34 psi our ambient air pressure. Doing nothing but heating the air in the tires increased the pressure 5 psi. If we were to attach a boost gauge (calibrated to be zeroed out at our ambient pressure of 34 psi), it would read 5 pounds of boost in our tires.
See what we're getting at? Within the confines of your intake manifold, 200-degree air takes up more space than air at 80 degrees. But the 200-degree air has less oxygen at a given volume and mixes with less fuel than the 80-degree air does. At the same boost levels, the 80-degree air will make more power than the 200-degree air because there is more air to burn. The point? The best supercharger is the one that can increase pressure with the least amount of temperature rise.
High-Helix And TVS
As we mentioned before, Roots superchargers don't make pressure in and of themselves. They generate boost by feeding more air into the engine than the engine can consume, essentially pressurizing the intake manifold with extra air. They become less efficient at higher boost levels because air begins to leak past the rotors back up through the supercharger, causing turbulence in the case. Turbulence increases the friction between air molecules, and we all know that more friction equals more heat. This is why we stated earlier that Roots superchargers were not very efficient. Traditional GMC-style Roots superchargers, i.e., the familiar 6- and 8-71 models, are great for drag racing. They operate best at low pressure levels-wide-open throttle at high engine speeds-but their efficiency can fall to around 30 percent when pressure levels in the intake manifold increase.
By design, Lysholm-type superchargers have a clear advantage here. By twisting the rotors and routing the air longitudinally through the case, turbulence and friction are greatly reduced. Plus, because they compress the air within the rotors, they are delivering high-pressure air into the intake manifold, which also helps keep air from blowing backward through the case. They are more efficient at higher boost levels and are ideally suited to street cars operating in part- to medium-throttle at higher boost levels.
Don't count out a Roots supercharger, though. A lot of R&D has been done to the GMC blower, and today's Roots blowers are much more efficient than their predecessors. Over the years, engineers have messed around with the rotor design, adding extra lobes and a 60-degree twist called the High Helix. All these improvements have boosted the efficiency of the pumps.
Photo by Whipple Superchargers
Recently, Eaton took things to a higher level with the introduction of its Twin Vortices Series (TVS) superchargers. Eaton is an OE-level supplier with an operating budget we can only dream of. The company added even more twist to the old Roots design, making rotors with a 160-degree twist along their length. Like a Lysholm, the TVS superchargers draw in air from the back of the case, pushing it out of an opening on the bottom of the opposite end. Like a traditional Roots design, though, the rotors still turn away from each other, and the air is still pumped between the rotor and the case. But the higher degree of twist prevents leakage past the rotors better than the old Roots design did.
At the end of all this is the issue of efficiency. So which supercharger type is the best? That obviously depends on your application and the type of driving you do, but we do have at our disposal several tools to help judge which is the best-we can measure a supercharger's efficiency. The two efficiency ratings to consider are the volumetric and adiabatic efficiency numbers.
Volumetric efficiency measures how much of a substance a pump can flow versus how much it actually does flow. We're familiar with this term as it applies to the automobile engine. Under perfect conditions, a 5.7L engine should pump 5.7 liters of air for each turn of the crankshaft. This rarely happens in the real world, though. Unless you have a big carburetor and exceptionally good-flowing cylinder heads, chances are an engine turning at 6,000 rpm will not be filling its cylinders as completely as it could. Atmospheric pressure can only push air in the intake, through the ports, and past the valves so quickly.
Photo by Magnuson Superchargers
The same principle applies to superchargers. While they are busy forcing air into the engine, superchargers rely on atmospheric pressure to push air into them. Things like rough castings, poorly designed rotors, and inlet and exhaust ports all affect airflow through the supercharger and can diminish its volumetric efficiency.
The adiabatic efficiency rating deals with the issue of heat. The easy explanation is to say adiabatic efficiency is a measure of how much a supercharger heats the air before delivering it to the intake port. That's too simple, though. Like the volumetric number, adiabatic efficiency measures the actual increase in heat versus the ideal. For any machine to do work, you can expect a certain amount of heat to be generated-usually due to friction. Bearings generate friction as they turn and air molecules generate friction as they tumble around inside the supercharger-friction is unavoidable. One can measure all these parameters to predict how much heat should be generated by this machine as it works. Then you measure the actual difference and divide the two to come up with the adiabatic efficiency rating for that given supercharger.
Like turbochargers, compressor maps are available for superchargers, showing the rpm and pressure levels they are the most efficient at. Though they look confusing, spend some time learning how to read them. They really aren't difficult to understand.
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How to Buy
The best supercharger is the one that matches the airflow demands of your engine. Get one that is sized to deliver the right amount of air, and you can fine-tune boost levels with pulley changes. Tell your sales rep the specs on your engine-cylinder heads, compression ratio, cast or forged internals-and your primary driving style. Also be sure to tell him how much power you want to make, how fast you want your car to go, and what type of fuel you will run. The more information you give, the better he can match his product to your application.
For a mainly street-driven car within the positive-displacement family, we like either a twin-screw or an Eaton TVS supercharger. These new-tech blowers are much more efficient than GMC-style Roots blowers. And their compact size allows them to fit a wide variety of engines and engine compartments. For a big drag engine, stick with the GMC-Roots.
We also want to stress the importance of running a charge cooler with the supercharger, though most of the kits available come with some sort of heat exchanger usually fitted in the manifold below the supercharger. The cooler the intake charge, the more power you will make. A supercharger is an excellent performance buy, and you'll typically drop on 50 to 100 hp in a weekend's work.
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