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Superchargers

To: mgs@Autox.Team.Net,
Subject: Superchargers
From: William Eastman <william.eastman@medtronic.com>
Date: Tue, 06 Jan 1998 21:52:00 -0600
Sick of me yet?  I am between projects at home so, once again, the list get
punished ;-)

Supercharger is the generic term for any device that forces air into an
engine above atmospheric pressure.  Superchargers can be driven directly
from the engine or they can be driven by exhaust gasses.  Exhaust driven
superchargers are called turbosuperchargers or turbochargers for short.

There are many types of crank driven superchargers.  Roots type
superchargers work like a big oil pump to push air into the engine.  The
best known example of these are the GMC 71 series including the 6-71 blower
beloved of hot rodders.  The GMC blower were originally designed to force
air into two cycle diesel GMC truck engines (small two stroke engines use
the crankcase for scavenge pressure).  6-71 stands for 6 cylinders, 71
cubic inches per cylinder for 426 cubic inches total engine displacement. 
Roots superchargers develop a lot of pressure at low rpm and are less
efficient at high rpm.  Roots blowers are popular with drag racers because
they need low end grunt for launching.  They are also used to add low end
grunt to small displacement engines such as the Thunderbird V-6.  The Eaton
superchargers are roots type compressors.

Other types of compressors include the centrifugal compressor.  Where roots
superchargers develop a lot of pressure at low rpm and are less efficient
at high rpm, centrifugal pumps don't work well at low rpm but build
pressure quickly at high rpm.  Turbochargers use a centrifugal compressor
section and a turbine in the exhaust to drive the turbine.  Centrifugal
crank driven compressors are popular on American V-8's such as the Mustang
and the Corvette today because these engines have enough low rpm torque
stock but need help on top.  Paxtons were centrifugal pumps.  These pumps
turn at extremely high rpm- over 100,000 in some cases.

Other types of superchargers exist such as the vane type and Volkswagen's
G-loader.

The advantage of crank driven superchargers is that the power is
instantaneous.  Turbo's operate by reclaiming heat energy from the exhaust
and are considered to be almost a "free lunch."  Problems with turbo's
include the dreaded turbo lag, heat issues, and a problem unique to
catalyst equipped cars, turbos can suck too much heat out of the exhaust
and delay the catalyst firing up from cold or even shut it down in certain
conditions.  Catalysts need heat.

In modern cars, designer that are looking to improve the driveability of a
small engine in a heavy car are turning to superchargers.  Designers who
are looking to claim the highest horsepower use turbos- usually two small
ones rather than one big one to reduce lag.  

Most compressor installations run dry or "blow through" now.  That means
that fuel is added after the compressor.  This is easier with fuel
injection since carburetors are more difficult to pressurize- you have to
pressurize the float bowl too.  Wet or "suck through" systems have the fuel
delivery before the blower.  This makes using a carb easier.  Remember that
compressing air heats the air so high levels of boost often require heat
exchangers (called intercoolers) to cool the intake charge.  Anything over
8 psi would probably benefit from an intercooler.

How much power does a blower add?  Atmospheric pressure is about 14.7 psi
so 15 psi boost would double horsepower, all thing being equal.  Of course
the blower takes some horsepower and hot air is less dense so the actual
number is lower.  6 psi gives about 30 percent more power.

Which would work best on an LBC?  If the car can take the torque, a roots
type compressor would most improve daily driveability.  I would think that
a small Paxton type would probably be the best over all because you could
use a very conservative cam and still get good top end performance without
developing so much torque that you will break things.  A turbo would also
be Ok but it is too seventies to fit with the LBC character, in my opinion.

Regards,
Bill Eastman
61 MGA still sucking air with no mechanical aid.

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