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Turbos and Superchargers Explained
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Supercharged
(roots) 3.8L Ford T-bird/SC engine. |
Turbochargers and Superchargers are all about
making power, but most don't know how this
occurs. The key to comprehension lies in
grasping the most basic thing about any internal
combustion engine: the fact that it is basically
an air pump. The pistons create NEGATIVE
pressure, or vacuum, as they go down--which
sucks in air at atmospheric pressure. The air is
combined with gasoline inside the cylinders and
ignited by the spark plugs. The resultant
explosion forces the piston down--and it is that
motion, ultimately, that turns the wheels that
drive the car. |
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If you can
get a mental handle on that, you'll see that
getting more air and fuel into the engine would
make for a bigger, more powerful explosion. The
car would go faster when you floored it. Here's
where turbos and superchargers come in. Both are
mechanical compressors that force additional air
into the engine under POSITIVE pressure--which
is called "boost." They make small engines
produce big engine power--and make big engines
more fun. |
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Functionally, the difference between a
turbo and a supercharger is in the means by
which they are driven. A turbocharger
("hair-dryer" to gearheads) is powered by spent
exhaust gases, which are used to spin a
paddle-wheel-like part called an impeller. The
impeller is connected on a shaft to the turbine,
which has vanes on it (picture a paddle wheel
and you'll get the idea) to compress the
incoming air, thereby creating boost, or
pressurized induction. A wastegate is built into
The system to prevent over-pressurization by
venting the exhaust gases away from the
impeller. Usually, the turbocharger will be
lubricated by pressurized oil--critically
important as the impeller/turbine assembly can
spin at speeds in excess of 100,000 rotations
per minute (RPM). |
Basic
Turbocharged engine diagram.
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A supercharger, in contrast, is driven by
an accessory belt--much like the alternator, air
conditioner compressor, and water pump.
Internally, there are two (sometimes three)
lobes that rotate, sucking in the air and
force-feeding it to the engine. The supercharger
makes a very distinctive noise, which can be
music to the ears of performance
enthusiasts. |
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Supercharger
pulley assembly diagram. |
Turbos and
superchargers are not recent developments; both have
been around since at least the 1930s, and both have
their strong and weak points. The No. 1
advantage of the supercharger is that it does not suffer
"lag"--a term used to describe the moment's
hesitation between the time the driver floors the gas
pedal and the time the engine responds. Turbocharged
engines often suffer from lag because it takes a second
or so for the exhaust gases that drive the
impeller/turbine to reach sufficient velocity before
boost is created. For a second or so, the engine feels
sluggish and weak--then it comes alive like a scalded
dog as the turbo "spools up." This is not a problem with
a supercharger because the "blower," as superchargers
have been nicknamed, is belt-driven and therefore
responds immediately as the driver revs the
engine--creating boost whenever the gas pedal is mashed.
Modern turbocharged engines are much better than the
turbocharged engines of the past, such as the old Chevy
Corvair Spyder, because of the switch from carburetors
to fuel injection--and the adoption of electronic engine
controls. Lag is not nearly the problem it once
was--though it hasn't been eliminated entirely. But the
driveability problems of older turbocharged engines
(hard starting, poor cold-weather performance, etc.) are
mostly gone. Some manufacturers have sought to address
the problem of turbo lag by using two smaller,
sequential turbochargers rather than one large unit. The
smaller turbos spool-up faster because there is less
inertia to overcome, yet they deliver the same punch,
together, at full-boost, as a single large turbo. The
1999 Volvo S80 T6 uses twin-turbos in this way, as does
the Mitsubishi 3000GT VR-4.
Superchargers tend
to have one other advantage over turbos. In addition to
not having a problem with "turbo lag," superchargers, as
a system, are less complex than turbos, which require
extensive modification of the exhaust system, among
other things. The supercharger, in contrast, is
simply bolted to the top of the engine (conventional
Roots-style) or mounted off to the side (Whipple and
Vortec). They are thus cheaper to install and generally
cost less to service and maintain. A pulley on the
supercharger's snout is driven by an accessory belt and
sometimes the hood must be modified for clearance--also
true with turbocharged engines.
General Motors has made extensive and successful use of
superchargers as a power booster for the 3.8 liter V-6
engine, which now develops 240-horsepower in the Buick
Regal and Park Avenue Ultra, as well as the Pontiac
Grand Prix and Bonneville.
A "negative" shared by both types of
forced-induction systems (turbos and superchargers) is
the higher operating temperatures they create--and the
added strain placed on the engine itself from these
higher temperatures and from the stress of being
operated under "boost." Usually, these factors
are taken into account when the manufacturers design an
engine intended for supercharged or turbocharged
use--and special, heavy-duty components are specified.
Nonetheless, regular oil changes and extra vigilance
keeping an eye on the temperature gauge are definitely
called for.
With turbocharged cars, it is a very good idea
to let the engine idle for a moment or two (30 seconds)
before turning the key to the "off" position and
shutting the engine down, especially after the
car has been driven hard. By following this
recommendation, you are letting vital lubricating oil
cool down the white-hot turbo. Sludge deposits will be
kept at bay and your turbo will live a long and happy
life.
No
such "shut-down" procedure is necessary with a
supercharged engine because it is not lubricated by
engine oil; typically, there are sealed, pre-lubed
bearings that are designed to last for the life of the
unit. What IS important with a supercharged engine (and
a turbocharged one, too) if you want it to last, is to
be gentle with it during warm-up. That means no
full-throttle starts for at least the first 10-15
minutes of operation. Blasting the gas with a cold
engine is bad for any car, but the pressure generated by
turbochargers and superchargers only exacerbates the
potential for premature failure if you dog it before the
oil gets warm and the bearings and other vital innards
reach normal operating temperature expect failure.
That's it for now. Hopefully this article has helped
somewhat in your decision on getting a turbo or a
blower, or has at least taught you something that you
did not know before hand.
Note: The information in this article
was compiled from various sources on the internet and
have been deemed to be as correct as the documents it
was pulled from.
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