RE: SuperChargers vs. TurboChargers

["Charles W. Barber" <cbarber@xxxxxxxxxxxxxxxx>]

Years ago I looked into superchargers/turbochargers pretty carefully.
That wasn't for a BMW and equipment has changed a lot since then, so I
can't offer any specific advise. But I supply a little history and
theory.

Supercharging is the process of forcing air into the engine at higher
than ambient air pressure. It was first used on piston airplane
engines to avoid a drop off in performance as they flew higher (where
ambient air pressure is less). A supercharger is basically an air
pump.

There are two common sources of power for the air pump: crankshaft and
engine exhaust. Crankshaft takeoffs -- usually via a belt and pulley
arrangement was the first system tried. Later the exhaust scheme was
invented and was called a "turbocharger" to distinguish it from the
original scheme. Now when people refer to a "supercharger" they
usually mean one driven from the crankshaft, but occasionally the term
is used generically to refer to both approaches.

Both designs put out more "boost" with higher engine speeds. By their
nature, superchargers tend to be linear while turbochargers are
peaky -- they go from little to near max output over a small input
power range. Turbocharger designers have to pick where in the power
curve the turbocharger peaks. Usually they are designed to kick in at
the upper power levels and have little effect on normal cruising. That
way you have a dual character engine -- good mileage under cruising
conditions and good power when pushed. Superchargers, on the other
hand, can contribute boost over a wider power range (at the expense of
mileage, of course, power ultimately comes from the fuel).

Turbochargers are hard to build. The energy in the exhaust drops
quickly the further you get from the exhaust valves. For that reason,
the turbine that drives the air pump in a turbocharger is placed close
to the cylinders. Close means hot, so the turbine that is stuck in the
engine exhaust is subject to very high temperatures. Close also means
small (the exhaust path starts out the size of the valve and gets
bigger. To get a lot of energy from a small turbine means spinning it
very fast (think of difference between a small fan and ceiling fan and
the difference in blade speed for the same amount of air circulation).
I seem to recall numbers like 70,000 RPM. These requirements can only
be met with exotic materials and bearings. The turbine is usually made
with ceramics instead of metal.

Superchargers are much easier to build but can be much less efficient.
The belts, pulleys and air pump are always running and take a
significant percentage of engine power to run at low power range.
Result is poor cruising mileage. Turbochargers, on the other hand, are
almost a free lunch. Their non-linear characteristics mean that can be
designed to idle when your are puttering around the neighborhood or
cruising the interstate, then switch on when you really want to go.
The downside is that the switch isn't instantaneous -- stomp on the
accelerator and the engine RPMs rise but the turbo isn't spinning yet.
As engine output rises, the turbo starts to spin, which increases
engine input, which increases engine output, which increases engine
output which .... Result is non-linear engine power build up and a
delay -- "turbolag" -- between pedal movement and power.

Power train reliability will definitely suffer with the addition of a
turbocharger. Anything you do to increase engine power will put more
strain on engine components -- bearings, rings, connecting rods, et.
al -- and the rest of the power train -- clutch/torque converter,
transmission, differential, et al. More power also means more waste
heat and a hotter engine compartment is bad news for lots of parts.
Engines equipped at the factory with turbos often have beefier drive
train parts (at least some). That lets the factory go with a bigger
boost than would be prudent with an aftermarket turbo.

The big question is how much will reliability suffer. It is not
uncommon for the drive train to outlast the rest of car. Shortening
the life a few percent might be worth it for the extra fun.

Your mileage will definitely vary.

- -----Original Message-----
Date: Wed, 06 Oct 1999 02:05:09 -0700
From: "Gary D." <the_mod@domain.elided>
Subject: SuperChargers vs. TurboChargers

I have a 318TI black which is currently stock.  I'm thinking of
investing
in a SuperCharger or a TurboCharger.  I'm not sure what is the
difference
between the two.  And exactly what they do.  I don't want to get it
installed and find out a few months later that my engine life has been
reduced by 50%.  I want to know all the consequences and advantages of
it.
I also want to know with what other components (ie. intake, filters,
exhaust, spark plugs, Performance Cards/Chips) do i have to use it
with in
order to get the BEST performance out of my 318TI.

If anyone out there could give me some advice on what i can do to get
the
best performance out of my car, please let me know.  I'm planning on
being
able to out race GTI V6's, MR2's and Preludes / Accord Coupes.  (Of
course
they are all stock. )

Thankx,
Gary D.
318TI 1995 Black

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