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Supercharging Theory
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One of the most basic tenets of the old-school hot rodder and today’s tuner is a strong distrust, or
downright disbelief, in theory as opposed to practical application. The modus operandi of hot-rod
engineering is to look at what has worked the best in the past, figure out a few practical tricks to
make it work a little better, and then bolt it on the car and give it a try. If it doesn’t work better, you
go back to what you know through experience will work.
Although the field of automotive performance modification is becoming more and more
sophisticated these days, the ultimate test is what you can feel behind the wheel, or in what the time
slip says at the drag strip. The manufacturers of speed parts these days rely on flow benches,
dynamometers, and computers to design and test new products, but the average enthusiast – as
well as most professional racers – still assert, “you can’t drive a dyno.”
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Hot-rodders have known since the 1950s that superchargers work. You bolt one on an engine and
it makes horsepower. Spin it faster, or bolt on a bigger one, and it’ll make even more horsepower.
Go too far and you’ll blow head gaskets or melt pistons. Hot rodders have also learned that,
although turbochargers make more peak horsepower than Roots blowers, they just don’t deliver the
same instant horsepower on the street.
Unfortunately, this mindset among hot rodders and the new generation of tuners can be both
commendable and misleading. The new breed of tuners, like their hot-rodding predecessors, has
done things with cars and engines that have left the theoretical engineers flabbergasted. These
enthusiasts have often succeeded where a highly educated designer would never have attempted
a project, just because they didn’t know that it wasn’t supposed to work. If a perpetual-motion
machine or a perfectly efficient internal combustion engine could be built, it would be a dedicated
gearhead that would figure out how through sheer perseverance and enthusiasm.
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However, the harsh reality is that a perpetual-motion machine will remain an impossibility, and even
the best internal combustion engine will be far from perfect. The laws of conservation of matter and
energy are true: you cannot get more out of a system than you put into it. Second, and more
important, a law called entropy applies: you can never even get as much out of a system as you put
into it when you are converting matter to energy and energy to work as you are in an engine.
This is all very pertinent to supercharging, because a blower can increase the volumetric efficiency
of an engine above 100 percent. And although a belt-driven blower uses up a certain percentage
of the engine’s horsepower in order to run, it produces much more power in the engine than it
takes. These facts can fool us into thinking that a supercharger is some magical apparatus that can
defy the laws of physics – but it can’t.
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The reason why a blower seems to work miracles on an internal combustion engine is that the
engine is so inefficient to begin with. A good gasoline engine is lucky if it can convert 30 percent of
the energy contained in the fuel into actual work. The addition of a supercharger, in most cases,
actually lowers this figure (because it requires a greater percentage of fuel to make more power in
the motor. . . we’ll explain in a moment). And as I pointed out earlier, superchargers are similar to
internal combustion engines in that they are both types of air pumps. All superchargers are
themselves far from 100 percent efficient in terms of the amount of energy they consume compared
to the amount of air they pump, and in terms of the mass (weight) of air they pump compared to the
size of the blower and the volume of air it displaces.
We won’t be able to fully explore the intricacies of supercharger design and efficiency here. In fact,
a surprisingly small amount of research has actually been compiled on superchargers, especially
Roots blowers, and the handful of existing books and the several engineers I consulted during this
project either had few answers or openly contradicted each other. What I want to do in this chapter
is give you just enough engineering background to understand what blowers can do and what they
can’t. I want to dispel some myths about superchargers, and to give you a more solid basis from
which to choose the type of blower system that will be best for your vehicle and the way you drive it.
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This dyno chart from Jackson Racing
compares a ’95 Honda Civic EX’s
power output before and after the
installation of one of its superchargers.
It shows just how effective
supercharging really is.
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Density, Volume, and Pressure
Myth number one is that a blower’s primary function is to increase the volume or the pressure of the
air in the intake system of the engine. The real purpose of the supercharger is to increase the
density of that air.
Density is the mass, or weight, of a substance in a given volume. For most solids and liquids, weight
is nearly proportional to volume – increase the volume, and you increase the weight a proportional
amount. In other words, the density of a given solid or liquid does not change very much under
normal conditions. Consequently, we tend to think of an increase in volume of a substance as an
increase in the amount (mass) of that substance.
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However, such thinking is not correct, and it certainly does not apply to gases. The density of a gas
such as air changes considerably as its temperature or pressure changes. Increase the
temperature and the density of the gas decreases (if it’s not in a closed container), because the
gas expands. Increase the pressure, and the density will usually increase because a greater
amount (mass) of the gas will be compressed into a smaller space (volume). I say “usually” because
we are dealing with more than one variable at the same time when we are dealing with the state of a
gas. If a gas is both compressed (which should increase its density) and heated (which should
decrease its density) concurrently, the net result could be an increase, a decrease, or no change
at all in the density of the gas.
These facts are extremely important to an understanding of supercharging, because a
supercharger does in fact both compress and heat the air entering the engine, in addition to
increasing the mass of air passing through the intake system
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The Gas Law
Before we go any further, let’s take a quick look at the basic law that describes the relationship
between the pressure, volume, weight, and temperature of a gas. Known as the Ideal Gas
Equation, or just the Gas Law, it is usually stated by the equation:
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PV = nRT
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In this formula “P” is pressure, “V” is volume, “n” is the weight of the gas in moles, “R” is a constant
(called the Molar Gas Constant, which has different values depending on the units used for the
other quantities), and “T” is the temperature. It isn’t within the scope of this book to fully explore
how this equation can be used, or how gases act in general. What this equation should immediately
show you, assuming you have a basic familiarity with algebra, is the relationship between the
pressure, volume, temperature, and weight of a given gas.
If we’re discussing a given amount of gas, in moles (that is, a certain weight of the gas, or a certain
number of molecules of the gas), then both n and R in the Gas Law would be constant, and we
could write the equation thus:
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PV
____ = Constant
T
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What this tells us is that for a given amount (weight) of gas, increasing the pressure and keeping
the volume the same will increase the temperature proportionally; increasing the temperature and
keeping the volume the same will increase the pressure proportionally; increasing the volume and
keeping the pressure the same increases the temperature, and so on.
In relation to supercharging, the significant things the Gas Law tells us is that increasing the
pressure increases the temperature, and vice versa. It also shows us that a temperature rise at a
given pressure will increase the volume, which means a decrease in density; and that both the
pressure and the temperature could be increased without increasing the density.
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If the inlet on your blower draws air from inside a
closed engine compartment, it can be drawing
excessively heated — that is, less dense — air into
the blower, which counteracts the supercharger’s
purpose (to make the air denser). The
supercharger can only work with what you give it.
The same relationship holds true for intercoolers;
i.e., they only reduce temperatures a specific
amount, so any way you look at it, starting with cool,
dense air will offer the potential to make more
power.
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The thorn of supercharging is heat. Heat is a very common and easily manifested form of energy –
a very easily wasted form of energy – that is a typical byproduct of work being done. In
supercharging, heat is a multiple detriment. In the first place, the act of compressing air heats it, as
we saw above. Second, much of the engine horsepower used to turn the supercharger and
compress the air eventually is converted to more heat, which is transferred to the air. Most blowers,
especially Roots types, literally beat the air, which heats it considerably. In addition, friction between
the air and stationary and moving surfaces, as well as the friction of air turbulence, all combine to
heat the air further. The heat tends to expand the air (increase its volume), which raises the
pressure, but does not increase the density.
To give you a practical example of what could be taking place in a supercharger, imagine a closed
box full of air at room temperature and atmospheric pressure. Let’s say the box measures one foot
per side, so that its volume is one cubic foot. The volume of air in the box, one cubic foot, cannot
change, since no air can get in or out. Likewise, the density of that air – the amount of weight of air,
or the number of molecules of air, per cubic foot – cannot change. If we insert a pressure gauge
through one of the walls of the box, it will read zero psi, since both the pressure and the
temperature inside the box are the same as that outside. Most pressure gauges read in pounds per
square inch above atmospheric. Thus, a 5-psi gauge reading is actually approximately 19.7 psi
absolute, since atmospheric pressure is about 14.7 psi at sea level.
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This compressor flow map for a Paxton/McCullough
centrifugal supercharger shows adiabatic efficiency
and horsepower required to drive the blower
(mechanical efficiency). The ring near the center of
the map — known as an “island” — indicates the
range of this blower’s maximum adiabatic efficiency;
in this case, it’s 62 percent.
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Now let’s take our box of air and put it on the stove. As the air inside is heated it tries to expand, but
it cannot because it is contained by the box. The result? The pressure increases. We’ll actually see
“boost” on the pressure gauge, even though the air isn’t being compressed, and the density
remains exactly the same. In other words, we see a boost reading on the gauge even though the
amount of air in the box has not increased at all. The same thing can happen, in extreme cases, in
the manifold of a supercharged engine. If a blower heats the air drastically, instead of pumping it
efficiently, it could show a boost reading on the gauge – simply by expanding the air, without
causing any increase in the volumetric efficiency – or power – of the engine.
To see how this could happen, we can use the Gas Law in its simplified form:
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P1 V1 P2 V2
______ = _______ = Constant
T1 T2
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Knowing that the volume is constant in both cases, we can figure the pressure rise by using the
proportion
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P1 P2
____=_____
T1 T2
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In this formula, the temperature must be figured in degrees Rankine (absolute temperature, which
equals degrees Fahrenheit plus 460), and pressure in psi absolute. Using this simple equation, we
find that if our box of air starts out at typical atmospheric temperature and pressure of 60° F and 0
psi (520R and 14.7 psi) and is then heated to 240° F, we will see a boost reading on the pressure
gauge of 5.1 psi. Remember, we have neither compressed the air nor increased its density – we
have simply heated it – and we have five pounds of boost on our gauge!
Now, although a blower manifold is not a closed box, nor is real air an ideal gas, the above example
still gives a good approximation of what could happen in a supercharged engine. If you were getting
a 200° F temperature rise in the manifold at 5 pounds of boost, your supercharger would be giving
you no increase in charge density at all. Further, it would be robbing your engine of horsepower at
the same time, since increasing intake temperature increases the likelihood of detonation. Most
superchargers work considerably better than this example, but all sacrifice a good percentage of
their potential benefit to heat.
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Previous | Next
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This has been a sample page from
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Sport Compact Turbos & Blowers
by Joe Pettitt
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Lightweight and high-revving, sport compacts are today’s most
popular cars. They have developed a cult following among today’s
youth and are fueling a multi-million dollar industry in modification
parts and equipment.
While most owners of sport compacts can afford the simple bolt-
ons available, some owners want to take their modifications a step
further. There is intense competition to be the fastest, and quite
often the only way to win is to go to the next level – by installing a
supercharger/blower or turbocharger on your engine.
This book is an enthusiast’s guide to understanding and using
turbochargers and superchargers on sport compact cars. It
covers the basics of each system and compares their pros and
cons. Building and tuning small-displacement 4- and 6-cylinder
engines to maximize performance and reliability with forced
induction is also covered.
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Click below to view sample
pages from each chapter!
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Chap. 1 - Exotic or Practical
Chap. 2 - Supercharging
Chap. 3 - Roots Blowers
Chap. 4 - Centrifugal Blowers
Chap. 5 - Turbocharging
Chap. 6 - Turbos & Compacts
Chap. 7 - Tuning for Boost
Chap. 8 - Building Engines
Chap. 9 - History
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Softbound
8-1/2 x 11
128 pages
300 black & white photos
Item: SA89
Price: $18.95
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Other items you might be interested in
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Turbochargers
How to select and install the correct turbo for big or small
horsepower gains. Discusses turbocharger design, sizing,
matching, controls, carburetion, exhaust, ignition,
intercooling, marine and high altitude applications. The most
comprehensive book available. Turbo suppliers and kit
maker addresses are included. “Everything you could possibly
need to know about turbochargers for automotive applications
is in this book.
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Price:
$18.95
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Engine Management: Advanced Tuning
Engine Management: Advanced Tuning explains how the EFI system
determines engine operation and how to change the controlling
parameters to optimize actual engine performance. This book takes
engine-tuning techniques to the next level. It is a must-have for tuners
and a valuable resource for anyone who wants to make horsepower
with a fuel-injected, electronically controlled engine.
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Price:
$22.95
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