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Centrifugal Superchargers
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Any self-respecting Ford enthusiast will be somewhat familiar with centrifugal superchargers. The
premise is simple, even if the execution is not. Air molecules have enough mass and weight so that
if you build a device to propel them from point A to point B quickly enough, they’ll tend to keep
moving in that direction. If you trap these molecules inside an intake manifold, static pressure will
build. Centrifugal superchargers use an impeller to do the propelling.
If you think about the term “centrifugal” in terms of a supercharger, you might think it must take one
hell of a spin to thrust air into an engine with anything more than atmospheric pressure. The
impellers used on either turbochargers or centrifugal superchargers require high tip speeds to
compress air. Tip speed is relative to the diameter of the impeller, requiring higher RPM for smaller
diameters, or a lower RPM for a larger diameter, to attain a given tip speed.
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Small-block Mustangs and
Shelby’s were available with
a Paxton centrifugal
supercharger in the later
1960s. The entire carburetor
was simply enclosed in a box.
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There are two practical ways to spin an impeller at such high speeds:
1. With a turbine, as used in a turbocharger and described elsewhere in this book.
2. Mechanically, using power from the engine’s crankshaft and a method of compounding it from a
few-thousand RPM to tens-of-thousands RPM at the impeller. This is how it’s done in a centrifugal
supercharger.
An impeller should be a masterpiece, incorporating the results of many sciences. To work well, this
core part of a centrifugal supercharger must be designed around a detailed understanding of
airflow dynamics, material properties, and careful machining techniques. If any of those areas are
overlooked, the supercharger in question will not provide the anticipated performance, or it may just
fail entirely.
Impeller design is very important because of its extremely high operational speed – up to 60,000
rpm in some applications. Another factor is efficiency; the shape and contour of the blades and
voids determine its ability to accelerate air within a specific range of rotating speeds. In this
particular aspect, the shape of the housing that surrounds the impeller, referred to as the volute, is
critically important for the impeller to achieve its intended efficiency.
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Robert Paxton McCulloch’s
1953 release of his new VS-57
found favor on cars such as
the short-lived 1954 Kaiser
Manhattan, the 1956
Studebaker Grand Hawk, the
1957 Packard Clipper, and
1958 Packard Hawk.
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The basic centrifugal supercharger design has been used to supercharge engines almost since the
appearance of the four-stroke engine itself. Innovators like Indy 500 great Harry Miller took impeller
design to its practical limits in automotive and marine racing engines as early as the 1930s. For
Miller’s famed and fabled 750-hp Novi V-8 (originally commissioned by a Novi, Michigan, Ford
dealer), the 10-inch impeller enlisted turned at a speed that caused the outer tips of the impeller
blades to travel at Mach 1.6.
Impossible? Well, because the impeller was moving so much air, the density around those
supersonic tips was high enough to prevent the shock waves that destroy most metal parts moving
at that high a speed.
At the point where air is delivered from the impeller circumference to a diffuser, its mass has been
accelerated and its volume reduced. In other words, the air has been (greatly) compressed. In a
reverse of the venturi principle, a diffuser provides an increasing cross-sectional area for the air,
slowing it down and increasing its static pressure. The compressed air will only move back through
the impeller if the tip speed drops significantly or if the static pressure on the intake tract exceeds
the pressure change across the impeller.
The relationship between impeller speed and how efficiently it will accelerate an incoming air mass
is governed largely by:
1. The shape of the impeller blades and the cavities among them.
2. The shape of the impeller housing and the diffuser cavities.
3. Clearances between the impeller housing and the impeller’s blades.
4. The positioning of the impeller and the housing surface behind it.
Every impeller design has a speed range where it’s highly efficient at moving air. With today’s
computer-aided design (CAD) and automated profile machining, these RPM ranges tend to be fairly
wide, but that wasn’t always the case. Incredible as it may seem, Harry Miller was forced to
experiment with a lengthy series of one-off impellers, each one an investment casting requiring
many hours of finishing and balance work, before he discovered one with the right contours to suit
the available drive speed of the Novi racing engine.
Within the sweet spot of an impeller’s efficiency range, the dynamic sealing between the blades and
housing improves to the point where the system begins to seriously tax the inlet airflow
characteristics and whatever might follow the supercharger in the sequence of intake components.
This means that when the blower starts to get in its performance curve, anything in the rest of the
intake tract that can’t handle the flow will seriously compromise the net effect of the supercharger.
While this is true regardless of the general type of supercharger installed on an engine, it is
particularly true with the centrifugal types because of the remarkable change in airflow that can
take place with a very small change in engine speed.
This effect is similar to – but not nearly as dramatic as – the radical shift in performance that occurs
when the impeller of a turbocharger enters its sweet spot. During these transitional periods, the
airflow through the entire intake tract changes drastically, which is why something as simple as a
loose air filter element or an intermittent leak in ducting can wreak havoc with the engine’s air-fuel
metering, especially with EFI or SEFI systems.
During the mid-1950s, Robert Paxton McCulloch first sold small centrifugal superchargers that
could be installed on automotive engines without too much difficulty. Paxton’s reciprocating-ball
drive blowers were homologated for use in NASCAR by Ford Motor Company, who offered a similar
application for their 312-ci engines and made them available in the two-seat, ’57 Ford Thunderbird
and a limited number of full-size Ford models.
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Shown in this cutaway of the McCulloch VS-57 is: The
variable ratio blower pulley, the manifold pressure-
activated solenoid air valve used to control air-piston
movement, the compressed coil spring pack, air
supply passageway, supercharger air outlet, impeller
shaft pilot bushing, planetary drive balls, the units
impeller shaft which is 4.4 times the pulley speed,
labyrinth oil seal, small diameter magnesium impeller,
the air inlet, the units series of five ball driver cups
that rotate the five drive balls within the fixed outer
race, the pulley shaft ball driver, the split outer race
bearing which pre-loads the drive balls, the units 14
coiled springs which set the pre-load split on the
outer bearing race, the unit’s piston type oil pump,
and the VS-57’s lubricating oil reservoir.
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These early units differed in one specific sense, according to Craig Conley of Paradise Paxton, a
division of Paradise Wheels Inc. “Although both units incorporated a ball-type drive mechanism, the
NASCAR units had what amounted to a direct drive for the impeller, while the street versions used
oil pressure to exert variable tension on the outer races of the drive, allowing engine speed to
somewhat regulate how effectively the impeller was driven. Higher oil pressure brought about a
change in drive ratio in the ball drive, and the impeller speed went up accordingly. It was the ball
drive that eventually became the obstacle for those seeking higher boost pressures. The ball-drive
units are perfect for anyone looking to generate 7 lbs of boost or less – they’ll do that quietly and
reliably. Above that manifold pressure, the impeller should be driven by a more direct method, such
as the modern gear drive units."
Using a gear system, like the one used in Paxton Novi superchargers, to drive the impeller is one
way to eliminate slippage and impeller speed loss within the supercharger itself. While both designs
(reciprocating-ball and gear drive) are actually belt driven from the crankshaft (so there’s one
over/underdrive ratio there), the extreme overdrive ratio required to attain the proper impeller
speed happens within the supercharger housing itself.
The precise origin of the gear-driven centrifugal supercharger remains a hotly contested point. Of
course, Harry Miller’s Novi supercharger was turned by a beautifully crafted gear drive that linked
one of the camshafts to the impeller.
Robert Paxton McCulloch also built a gear-driven centrifugal supercharger around 1937, which pre-
dates his reciprocating ball design (originally developed in 1943 as an air pump for the U.S. Navy)
by almost half a decade. However, Paxton’s crankcase lubricated, gear-driven model for the Ford
Flathead V8-60 was noisy and deemed impracticable from a mass-production standpoint.
Others also tried it with limited success, and it wouldn’t be until the early 1990s when Vortech
Engineering LLC popularized the gear-driven centrifugal supercharger, changing the face of the
street supercharger industry.
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The Paxton Reciprocating-Ball Drive
In the past, Paxton superchargers used a unique ball drive to overdrive the impeller relative to the
size input pulley and shaft speed. The impeller turned approximately 4.4 times faster than the input
shaft speed. The drive arrangement is nearly silent, which is a significant difference from the gear-
drive units. It’s a relatively simple drive system, but its operation is often confusing to grasp. SA
Paxton’s ball drive was an important feature of the units built by the company to circulate air inside
America’s submarines during World War II and later in nuclear submarines. In that application, even
very small mechanical noises are telegraphed through the hull of a ship into the surrounding water,
immediately revealing the boat’s position to a potential enemy.
The input shaft on a silent-drive Paxton supercharger drives a precision-cast separator with drive
lugs positioned between the balls. The balls are contained in a stationary two-piece outer race. The
race is split along the circumference, and the two sections are spring-loaded together with
considerable force. The spring loading between the two sections of the outer race creates pressure
between the ball surfaces and the impeller shaft to spin the impeller efficiently without using drive
gears.
To accomplish this, the impeller shaft features a concave section to accommodate the balls, which
exert considerable pressure against the shaft, thanks to the spring-loaded outer ball race. Because
the balls capture the impeller shaft, it rotates at their surface speed – resulting in an impeller speed
that is about 4.4 times the speed of the supercharger’s input pulley.
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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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