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Roots Superchargers
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It matters little whether you call them blowers, compressors, huffers, windmills, air transfer pumps,
wheezers, Jimmys, or anything else. The supercharger in its various forms is almost as synonymous
with high-performance enthusiasts as the beloved V-8 engine itself.
One of the more familiar superchargers is the Roots blower. It’s considered by most to be the one
that started it all. The Roots blower was first designed as a water pump, but it soon became clear
that it was more effective as an air pump. People used it to solve a coal-mining problem, pumping
fresh air in and methane gases out. One of the prerequisites for operating in such a dangerous
environment was that the blower must not produce any friction or sparks. This turns out to be one
of the reasons why we like the Roots blower so much.
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As a rule of thumb, any
supercharger running a
cog belt means
business. If you want
that classic look with
that great blower whine,
look no further than a
Roots blower.
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In order to build something that will move plenty of air without any possibility of creating a spark, the
design must be simple and hopefully not make any metal-to-metal contact as it does its job. The
Roots blowers originally manufactured for ventilating mineshafts featured iron outer cases, but
typically used wood rotors to actually move the air. When aluminum came into wide use, the non-
ferrous metal was substituted for most of the components in the Roots, with the exception of the
timing gears (manufactured from machined steel), which establish the relative positions of the two
rotors within the case.
One of the major misconceptions about the Roots blower is that the air is moved straight down the
center, between the rotors. That’s not true. In fact, the air is moved between the vanes of each
rotor just inside the outer case of the supercharger. The rotors turn so that their vanes pass along
the inside surface of the case in a downward motion.
The clearance between the rotor tips and the case must be small, but if it’s too small, the tips of the
rotors will rub against the case, especially at higher blower RPM. This isn’t usually a problem on a
properly setup Roots blower, but it will become extremely critical with certain other types of
superchargers detailed later in this book.
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This cutaway of a Roots-type GMC 6-71
supercharger clearly shows the inner
mechanical relationship of all the internal
components. The supercharger snout, drive,
and drive gears are at the front of the unit.
These photos also show the rotors inside the
case and the end plate. The Roots-type GMC
supercharger prevents leaks by using a series
of tip seals and annular seals at each end of the
case. Oil seals on the ends of the rotor shafts
prevent the higher pressure inside the housing
from pressurizing the bearings. The rotors
themselves are a helical design, although there
have been several rotor variations tried on
these blowers over the years.
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There are other seals in a Roots blower that make a difference in its efficiency. In addition to the tip
seals, there are annular or ringed seals located at either end of each rotor to seal them against the
end plates of the blower housing. As you might expect, there are also oil seals on the rotor shafts to
prevent the higher air pressure inside the supercharger case from moving oil away from the
bearings.
There have been several rotor variations tried by the more innovative Roots supercharger builders
through the years. The rotor configuration used in most of the early units, including all of the GMC
types, involved a helical arrangement of the vanes and a tip shape (when viewed from the end of
the rotor) that resembled that little paddle device your eye doctor has to hold in front of one eye as
he tests the other. This vane shape allows one rotor to interact with the vanes of the other rotor,
providing a fairly good seal against air leakage without the vanes actually touching. Without that
unusual vane shape, the two helical rotors couldn’t turn against each other without grinding to a
screeching halt.
Drag racer Larry Bowers (Bowers Blowers) spent many years developing and refining one of the
first practical straight-vane Roots-type superchargers. Although it seemed much simpler than the
cast-aluminum helical rotors, there were several problems that required serious engineering time to
make the straight-vane rotors work. Bowers, motives in pursuing this course included the much
lighter net weight of a CNC-machine-cut billet rotor, higher component strength, less rotor tip seal
strength, and something we haven’t discussed up to this point: adiabatic efficiency.
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Adiabatic Efficiency
On the inlet side of an engine, heat buildup is the all-conquering enemy. Depending on what type
of supercharger setup you select, the air flowing through the inlet system may be heated
significantly when it’s compressed.
Adiabatic efficiency is a number representing the amount of heat added by the supercharger
system as a whole. In engineering terms, we’re talking about the thermodynamic efficiency of the
system. A given supercharger will often come with a manufacturer’s claim about its adiabatic
efficiency, but the numbers for the whole system are a compound result of the individual
components used between the air filter and the intake valve. In general, here is how the various
power adders rank in terms of adiabatic efficiency, from lowest to highest (most efficient to least
efficient):
• Roots helical-rotor blower
• Screw-type variants of the Roots design
• Centrifugal superchargers with external drive
• Turbochargers
The amount of boost employed, and whether or not the compression takes place within the
supercharger, will have a great influence on how much heat is added to the inlet air charge. As a
rule of thumb, the more compact the installation is, the less likely it will be to have a high adiabatic
efficiency number.
With a traditional Roots blower, the amount of heat added by the supercharger itself is very low
compared with most of the other supercharger designs (Roots screw or centrifugal scroll types).
The reason for this is that a Roots blower isn’t built to compress air as it operates – its effect is to
feed the engine with (much) more air than would normally be contained within the intake manifold
under normally aspirated conditions.
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High RPM, small-displacement
superchargers made a comeback in
the late 1970s and early ’80s. Shown
here is a B&M Performance Products
street blower, mounted on Herb
McManken’s highly modified early-
model Mustang convertible. Note that
this application uses a pair of Mikuni
side-draft carburetors.
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If you want to use a pump analogy here, the traditional Roots design is a simple transfer pump that
isn’t designed to raise pressure from its inlet to the outlet. Other supercharger types almost
invariably are, indeed, built to elevate the inlet air pressure before the air intake charge leaves and
goes on to the next part of the air inlet system. In engineering terms, a Roots is a pure blower, while
almost all of the other supercharger types are actually compressors.
Screw-type variants of the Roots design actually compress air within their cases, and are usually
followed in the inlet tract by an intercooler of some type. The intercooler counteracts the added
heat from compression to keep the air dense.
Centrifugal superchargers also compress the air within their housings, but if the boost level is kept
relatively low, they can still be used without an intercooler. For example, the Shelby GT-350
Mustangs that came equipped with the optional Paxton ball-drive, centrifugal supercharger had no
intercoolers, and worked very well.
Turbochargers have gotten a lot more efficient recently because of highly sophisticated materials,
but they still operate with the complication of bringing the inlet air within close proximity of the
exhaust gases. The compressor side of a turbo raises the inlet air pressure and passes it along to
the next component in the tract – usually an intercooler.
An intercooler is often required to maintain thermal-efficiency in a compressor, the payoff in
measurable airflow at the intake valve almost always outweighs the added complexity in the system.
As a total system, an intercooled compressor-type setup has a similar adiabatic efficiency to the
Roots design, but with the advantage of higher net airflow.
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Operational Levels
Up to a certain point, Roots-type blowers like the GMC and all aftermarket units patterned after it
move air in a linear relationship to their rotor RPM or tip speed. For this reason, a Roots blower will
tend to maintain a more constant boost level at the intake manifold than certain other supercharger
types because the rotor RPM and crankshaft speed are directly linked via the drive belt.
The drive ratio of a Roots blower is critical for establishing a given intake manifold boost level.
When a Roots blower turns at the same RPM as the crankshaft, it is said to have a 1:1 drive ratio. If
the blower turns faster than the crankshaft, it is over-driven at a rate governed by the size of the
blower pulley versus the size of the crank pulley. Conversely, if it is under-driven (again governed
by the size of the blower pulley), it turns slower than the crankshaft. An example of an over-driven
ratio would be if the blower turns at 1.2:1 times the crank speed.
This linear relationship does not hold true for superchargers that compress the air as it passes
through (i.e., an Eaton), neither does it always hold true for Roots types once they’re installed onto
an intake manifold. The reasons for this are many, but the most obvious is that the engine will
eventually form a restriction to the blower’s output, regardless of its rotor RPM. At this point,
additional rotor RPM will make no difference and the boost pressure will not continue to rise,
because its maximum point is limited by leakage back through the supercharger, around and along
its twisted rotors.
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Previous | Next
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This has been a sample page from
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How to Build Supercharged and Turbocharged
Small Block Fords
by Bob McClurg
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The supercharger and turbocharger in their various forms and
applications have both been around for well over a century.
What makes them so popular? Looks, power, performance,
sound, and status. And how do they relate to, and improve
upon, the performance level of a small-block Ford pushrod V-
8 engine like a 289-302, a 351-Windsor, a Ford 351-
Cleveland, or even the latest generation 4.6L / 5.4L “modular”
small-block V-8 engines? That’s EXACTLY what this book is
all about!
While Ford dabbled in supercharging and turbocharging on
production cars all the way back in 1957 with the legendary
Thunderbird, and then again with Shelbys and over-the-
counter kits, and then again in the late ‘70s and early ‘80s
with turbocharging 4- cylinder applications in Mustangs the
real revolution in supercharging and turbocharging Ford
products has come through the aftermarket in more recent
times. The Fox Mustang, created in 1979, and the platform
that would eventually feature fuel injection in 1986, allowing
much more boost, created a genre of lightning-quick and
affordable performance cars.
Featuring legendary supercharger and turbocharger
manufacturers like Paxton, Vortech, Pro-Charger, Garret-
AirResearch and Power Dyne, as well as traditional Roots-
style systems, this book covers everything you need to know
about supercharging and turbocharging your small-block
Ford. Read the sample pages to learn more!
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Click below to view sample
pages from each chapter
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Chap. 1 - Considerations
Chap. 2 - Roots Superchargers
Chap. 3 - Centrifugal Blowers
Chap. 4 - Eaton / Magnuson
Chap. 5 - Twin-Screw Blowers
Chap. 6 - Tuning for Boost
Chap. 7 - Turbocharging
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8-1/2 x 11"
Soft bound.
128 pages.
Approximately 425 b/w photos
Item # SA95
Price: $Discontinued
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Click Here to buy now!
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This is a great book that anyone considering the
installation of a supercharger on a Ford should own!
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Other items you might be interested in
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Sport Compact Turbos & Blowers
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.
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Price:
$18.95
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How to Build Big-Inch Ford Small Blocks
Thoroughly explains how to build a stroker, with information that will
help you to better tailor your heads, cam, intake manifold, carburetor &
exhaust system to get the most of the extra cubes. Also included is a
complete guide to head and block castings so you can choose exactly
the right parts for your project.
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Price:
$18.95
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