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Re: supercharger vs. Turbo (very long)
I'll offer a few corrections & clarifications, but I'm no expert; so I'm
sure my corrections will also be corrected or at least elaborated upon. I'm
not sure what level of mechanical knowledge you have; so I apologize in
advance if portions of this are too elementary. I don't mean to insult
anyone's intelligence.
It's almost correct to say that turbos & superchargers are fans that improve
airflow to the engine. However, they are actually miniature air compressors
that force air into the combustion chamber. In a normally aspirated engine
(no turbo or supercharger), air & fuel is sucked into the engine as the
piston travels downward in the cylinder. A turbo or supercharged engine
will actually force air into the cylinder as the piston is traveling
downward. This forced induction results in a great deal more air/fuel
mixture in the combustion chamber which is what make the additional
horsepower.
The difference between a turbo & a supercharger is the method that each uses
to force air into the engine.
You are correct in stating that a supercharger is run off of engine power.
In fact, it is driven off the crankshaft by an accessory belt in the same
manner as your alternator, water pump, etc. You're a little off base in
your reasoning of advantages & disadvantages. It doesn't have to do with
back pressure or lack of. A supercharger's advantage & disadvantage is
that it is driven from the crankshaft. Because it is driven from the
crankshaft, it can provide full boost almost immediately. That is the
reason it yields a more flat power curve than a turbo. However, because it
is driven off of the crankshaft, it cannot spin as fast as a turbo, &
therefore, cannot provide as much boost pressure & high-end power as a
turbo. The fact that it is driven from the crankshaft does cause some
parasitic horsepower loss, but it is mostly a supercharger's relatively low
boost pressure in comparison to a turbo that limits its output.
You are also correct in stating that a turbo runs off of the exhaust. There
is an impeller in the exhaust that is spun by exhaust gas flow. This
impeller, in turn, spins a shaft which spin the equivalent of a miniature
air compressor. Just as in a supercharger, this miniature air compressor
forces air into the engine. You were a little off base in the
advantages/disadvantages here as well. Because a turbo is spun from exhaust
gas flow, it is capable of spinning a great deal faster than a supercharger.
Thus, it can produce a great deal more boost pressure (roughly twice as
much) than a supercharger which results in greater high-end power. A
turbo's disadvantage also has to do with the fact that it is driven from
exhaust gas flow. The engine has to be revving to produce enough exhaust
gas flow to turn the turbo. Therefore, the turbo is not working much if at
all at lower RPM's. It is the time between the point at which you give the
car full throttle to the time that the engine is revving fast enough to
provide the exhaust gas flow necessary to turn the turbo that is referred to
as "turbo lag." There has been a great deal of advancement in turbo
technology in recent years that has minimized the effects of turbo lag (I'm
not completely sure how), but in the end, the laws of physics cannot be
broken.
The turbo itself does not create exhaust back pressure. The back pressure
issue is a result of exhaust system design in a turbo charged car. A more
restrictive exhaust system can be utilized to help spin the turbo to full
boost more quickly & easily. This is because the velocity of exhaust gas
flow is increased as the exhaust becomes more restrictive (up to a point).
If that sounds odd, think of it this way: When you turn on your water hose
without any restriction at the end of the hose, a large volume of water
comes out of the hose, but it is not moving very fast. If you use you thumb
to cover part of the end of the hose, thus making it more restrictive, the
volume of water coming out of the hose is decreased, but the water that is
coming out is moving very fast. It is the same in an exhaust system.
Manufactures of turbo charged cars seek to increase the velocity of exhaust
gas flow so that the turbo can be spun to full boost more easily & quickly.
Thus, while a more restrictive exhaust system has greater back pressure,
turbo lag is reduced.
So, if you want greater horsepower across the RPM range, a supercharger is
the way to go. If, however, you seek maximum high end horsepower, then you
would probably be happier with a turbo.
Michael Keith
'98 Jetta GLX
'85 Golf SCCA ITB/DSP
...both normally aspirated
-----Original Message-----
From: Khan Klatt <kmk@pacificrim.net>
To: jettaglx@igtc.com <jettaglx@igtc.com>
Date: Tuesday, October 06, 1998 7:36 PM
Subject: RE: supercharger vs. Turbo
>At 4:48 PM -0700 10/6/98, Bryan Billings wrote:
>>Let me tell you, a SC on the GLX even stock will put a permanent smile on
>>your face.
>
>Let me see if I can get this right:
>
>Supercharger
>------------
>Technique: Uses power from the engine directly to run a fan to improve air
>flow into the engine.
>Advantages: Doesn't create an increase in back-pressure, which reduces HP.
>Also, power is evenly distributed across the RPM band because of active
>powering.
>Disadvantages: Requires power from the engine, so you don't get a 100%
>increase in power.
>
>Turbo
>-----
>Technique: Uses exhaust pressure to run a fan to improve air flow into the
>engine.
>Advantages: Runs on passive power, so you don't lose any power from the
engine.
>Disadvantages: Restricts free air flow out of the engine, increasing
>back-pressure, and reducing overall power from the engine. Provides power
>only once you start getting air flow in the engine, so it doesn't give you
>much low end power on the rpm band.
>
>Corrections, anyone?
>
>-Khan
>---------------------------------------------------------------------------
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>
>Khan Klatt <kklatt@nw.verio.net> Verio
Northwest
>Network Engineer Bellingham Branch
Office
>Tel 800.591.2757 x 13 725 N. State
Street
>Fax 360.738.8315 Bellingham, WA
98225
>
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