SPICA VS MB 300SL

["John Dohrmann" <jdohrmann@domain.elided>]

Now that you mentioned it, my first sports car was a (used) 1963 Mercedes
300SL roadster.  That was the last year, they only made about 23.  So the
design was really mid-50's.  Three liter straight six, using mechanical fuel
injection.  The injector pump was SPICA-like in having a separate piston for
each cylinder.  The pistons had a constant stroke, mechanically driven from
the crankshaft.  And the injection was directly into the cylinder, timed to
just allow mixing before the spark.  Fuel volume was controlled by twisting
the pistons, which have helical relief grooves that connect with a side
relief port.  So as the piston went up, it pumped gas into the cylinder
until its groove hit the relief port.  The pistons had linear grooves
(teeth) that engaged a metal rod with teeth.  So moving the rod forward and
back (the rack) twisted the pistons to control fuel volume.  The crazy part
was that the rod was attached to a big leather/rubber diaphragm.  And the
amount of vacuum on the outside of the diaphragm was controlled by turning
on and off various ports to different parts of the intake system.  The
intake itself was a beautiful manifold with separate pipes for each
cylinder.  To start the car, you had to pull a starting lever in the dash to
tell the pump to provide gas until the engine starting pulling vacuum.  I
forget all the other details but it got an incredible number of horses out
of 3 liters.  Makes my spider seem weak by comparison but much more
affordible.

John D

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