Re: [alfa] Supercharged Alfas

[bearbvd@xxxxxxxxxxxxxx (Greg Hermann)]

At 10:55 PM 1/30/04, Bob McKeown wrote:
>
>I had one live encounter with a supercharged Alfa. At the 1993 Baltimore
>convention a fellow pulled up in front of the hotel with his '30s
>supercharged Alfa on an open trailer. He fired it up to unload, and damn -
>what a sound! "Brutal" is the perfect description.
>
>Does anyone know what it is about these supercharged cars that makes the
>sound so fierce?
>
>Bob McKeown
>Nashville

There are a couple of factors, Bob. The crackle in an exhaust comes from
the sonic (choked) flow in the exhaust port when the exhaust valve first
opens, during the 'blowdown' part of the stroke. It takes an absolute
pressure ratio of not quite 1.9 to 1 from the cylinder to the port (or in
any other situation) to produce choked flow.

There are a couple of factors in a supercharged engine that  cause this
condition to exist sooner than it does with an NA engine:

The inlet gas is at higher pressure than the exhaust because of the
supercharger--thus leaving a higher residual pressure in the cylinders
after the expansion (power) stroke. when the exhaust valves open.

The supercharger produces a false load on the engine, thus keeping it in an
operating state where there is higher residual pressure in the cylinders
more of the time.

Supercharged engines are generally built with a lower static compression
ratio than equivalent NA engines. Lower compression ratio goes hand in hand
with a lower expansion ratio, which translates to higher exhaust gas
temperatures. A supercharger with no intercooler also leads to higher
EGT's. The speed of sound increases with the square root of
temperature--and the consequently higher sonic velocity in the exhaust of a
supercharged engine also has an effect on the quality of the sound.

Greg
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