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The Many Types Of Supercharged Forced Induction - Belt-Driven Basics
A Few Fundamentals Of Supercharged Forced Induction
Keep camshaft duration short with supercharging, otherwise boost will just blow the mixture right out the exhaust port.
Show me a Mustanger who doesn't place a supercharger high on his wish list, and I'll show you a statistical oddity. Our lust for blowers is seemingly universal-whether our ride's a V-6, a GT, or a Cobra, a pushrod, or a modular-and with good reason, since the perceived effect of boost is not unlike a hefty increase in cubic inches, no matter what we're beginning with. Today's properly engineered blower kits can also be emissions-legal and have little or no effect on everyday driveability, especially in comparison to a nervously cammed, high-strung, naturally aspirated screamer capable of the same power levels. The bottom line is, superchargers are efficient power adders for race or street use, but they're also expensive, so it's important to consider your needs and understand some blower basics before making the sizeable financial commitment for one kit or another.
Different Approach,Same Result
There are two markedly different categories of supercharger vying for your Visa card's attention-centrifugal and positive displacement-and within the latter category are two distinct types.But before examining the differences, let's look at the similarities. The job of any supercharger is no more complicated than to force air into an engine's combustion chambers at higher-than-atmospheric pressure, ramming in lots of additional oxygen molecules for combustion. In simplest terms, superchargers are belt-driven air pumps (as opposed to turbochargers, which are driven by exhaust heat energy).
Since an engine's ability to produce power is directly proportional to the mass of its air/fuel mixture, our pre-vious suggestion that the effect of supercharging was comparable to an increase in displacement was not mere jour-nalistic hype. An engine's displacement can be thought of as a measurement of the total volume of air it takes to fill its cylinders, so that the eight cylinders of the current modulars, for instance, theoretically displace a total of 4.6 liters, or about 281 ci of air. Fairly basic stuff, but we say "theoretically" because the volumetric efficiency of a stock, naturally aspirated engine is typically only about 75 percent, meaning a cylinder is never really filled to capacity during the intake stroke (volumetric efficiency being a comparison of the amount of air/fuel charge that an engine could draw in, to the amount that it actually draws in). Because a supercharger forces air into the cylinders, it can raise an engine's volumetric efficiency to considerably more than 100 percent, quite literally making it act as though it has more displacement.
So the goal of a higher density of oxygen molecules in the cylinder is the same, but centrifugal and positive-displacement blowers go about achieving it in different ways, with resulting variations in boost characteristics. The design of positive-displacement superchargers, including both Roots-type and twin-screw type, is such that they displace the same volume of air with each revolution, so that their boost increases quickly in a linear fashion with rpm. With centrifugal blowers, on the other hand, the air output per revolution is not fixed, but rather increases as the square of rotational speed. Centrifugals produce comparatively little boost at low rpm and really come on as revs climb. Only you can decide if you'd prefer generous boost throughout the rev range, or a big heady wallop at the top of the tach.
Blower designs also exhibit differing levels of mechanical and thermal efficiency. Think of mechanical efficiency in terms of how many horsepower it takes to operate the supercharger at a given boost level, and thermal efficiency as how much heat is added to the air charge for every pound of boost generated. More on this as we go along. Let's look a bit closer at the different players.
Vortech, Paxton, ATI-ProCharger, and Powerdyne all offer centrifugal superchargers for the Mustang aftermarket, and all are readily identifiable by the familiar "snail" or "hair dryer" shape of their discharge ducts. Centrifugals are true compressors, compressing air within the blower itself, as opposed to within the manifold, as is the case with Roots-type blowers. At least at their maximum rated boost levels, centrifugals are more thermally and mechanically efficient than positive-displacement blowers, meaning they consume less crankshaft horsepower, and heat the discharge air less, for each pound of boost generated. This mechanical efficiency can be demonstrated by how easy it is to spin a centrifugal blower pulley by hand-there's just not much parasitic loss in the mechanism. However, to produce the high rev boost for which the centrifugal is famous, its impeller may spin at up to 50,000 rpm or higher, meaning bearing design and manufacturing tolerances are critical.
Efficiency at high boost levels makes the centrifugal blowers particularly popular in drag-race applications, where peak boost as high as 25 psi is not uncommon. The choice is not as clear-cut on the street, where this higher peak boost potential must be traded off against the design's less sparkling boost production at low engine rpm. Mash the gas on a centrifugally boosted car at 2,000 rpm and power builds gradually until the tach gets a ways clockwise.The next thing you know, you're on the rev limiter and your rear tires are just a bit smaller in diameter. There are 50-state-legal centrifugal kits designed for the street, as well as some track-only, radical-boost heavy hitters. Boost response varies significantly from unit to unit, depending largely on impeller design and step-up ratio.
Choices abound in centrifugal blowers, and competition amongst the manufacturers keeps kit pricing quite aggressive.
Unlike centrifugals, a positive-displacement blower displaces an identical volume of air with each revolution, providing an earlier ramping up of boost. The most commonly seen positive-displacement superchargers in Ford circles are of the Roots-type, manufactured by Eaton (the Roots, by the way, is named after the two brothers who invented its design). At least two size variants-the M90 and M112-of the Eaton/Roots-type blower have been used in Ford factory applications, and these numerical designations denote their respective air displacement per revolution, in cubic inches.
Where a centrifugal supercharger compresses air internally, the Roots-type is quite literally just a blower, moving large volumes of air from its inlet to its outlet side. The compression takes place downstream in the intake manifold/ intake ports. The Roots-type is the least mechanically and thermally efficient of the three blower configurations, but anyone who has ever driven an Eaton-blown Lightning or Cobra would hardly care. As with other positive-displacement blowers, the Roots-type has the low-rpm grunt to make a small-block feel like a big-block, basically from idle on up.
There are a few aftermarket Eaton/Roots-type-based kits such as those available from BBK and AED, but Ford Racing Performance Parts does offer SOHC modular versions for both PI and non-PI heads.
There's a common misconception that all positive-displacement blowers are Roots-type. Not true; there's a whole separate category known as twin-screw blowers, so named because their parallel intermeshing rotors "screw" the air from inlet to outlet, compressing it internally along the way. Twin-screw superchargers are manufactured in Sweden by Lysholm and Autorotor and, in contrast to Roots-type blowers, are demonstrably more efficient, producing somewhat more boost per rpm at lower air-charge temperatures. They are also quieter and somewhat more expensive.
As with the Roots-type, however, the twin-screw ramps up to full boost by around 2,000 rpm, for nice, fat torque and horsepower curves that basically eliminate the need to ever gear down again.
With Power Comes Responsibility
As with any other power adder, supercharging comes with some basic caveats. While a denser air mass in the cylinders provides more oxygen molecules for combustion, these must be accompanied by more fuel in order to maintain the desired combustion temperatures of a proper air/ fuel ratio, a situation exacerbated by the higher combustion-chamber temperatures found under boost. Failure to provide the cooling effect of this additional fuel while under boost will result in the destructive heat of a too-lean air/fuel ration in short order. Whereas a normally aspirated engine might have a wide-open air/fuel ratioin the area of 13.5:1, tuners aim for 12.5:1 or even 11.5:1 with street-forced induction. The more boost, the more fuel required, so the cure may come from something as simple as revised EEC programming to something as comprehensive as a wholesale upgrade of a car's fuel system. The good news is that, when cruising along not under boost, fuel mileage should be about the same as before.
One of the inescapable laws of physics is that to compress air requires work, and the air being compressed absorbs the energy of that work as heat. The more you compress it, the more you heat it. Though different types of blowers have different thermal efficiencies (that is to say, the amount of heat produced per pound of boost), you simply cannot produce boost without introducing some level of temperature increase to the air charge that fills the engine's cylinders. Also, with more air forced into the cylinder at bottom dead center, but with the piston still moving the same distance as usual to compress it, the effective compression ratio is substantially increased under boost, therefore creating even more heat. The higher the boost, the higher the heat within the chamber, increasing the chance of the dreaded detonation.
Spark timing also becomes critical under the elevated chamber pressures of higher boost levels. Under the increased heat and compression ratio of boost, what would otherwise be considered normal levels of spark advance will occur too early in the compression stroke, causing the same pneumatic shock loading of the advancing piston as heat-induced detonation. The solution is to proportionally delay firing of the spark plug to closer to piston top dead center as boost increases. The degree of neces-sary timing retard per pound of boost will depend on a number of factors, including static compression, available fuel quality (octane), and the temperature of the intake air charge.
With the proper precautions and tuning considerations, supercharging is a safe and efficient way to make whatever engine resides under the hood of your Mustang act up to one-and-a-half times its size.
Gaining A Few Pounds At The Bar
In North America, we tend to describe our blowers, and label our boost gauges, in terms of pounds-or psi-of boost. Just to be sure we're all on the same page, what we're really referring to is the amount of pressure the supercharger will create beyond normal atmospheric-or barometric-pressure. Standard sea-level atmospheric pressure is about 14.7 pounds per square inch, reflecting the average weight of an inch-square column of air extending up to the top of the atmosphere.
For example, a 7-pound blower at maximum boost will push air into the cylinders with a pressure 7 pounds above normal atmospheric, or about 21.7 pounds per square inch of absolute pressure.In Europe, the tendency is to describe boost in terms of "bars," with one bar equaling standard barometric pressure. On a European boost gauge, that same 7-pound blower would register about 1.5 bar-or 1.5 times standard baro-metric pressure-at full boost.