From: davet@loowit.wr.tek.com (Dave Tharp CDS)
Newsgroups: rec.motorcycles
Subject: Re: Dry Sump Lubrication
Date: 26 Aug 92 21:12:26 GMT
Distribution: usa
Organization: Tektronix - Colorado Data Systems, Englewood, CO

OK, enough of this hooey.

Dry sump systems have TWO oil pumps.  One is fed by oil from the tank,
and forces oil into the engine under pressure.  Oil circulates around
the engine through oil passages and eventually winds up in the bottom of
the crankcase.  It is then picked up by another pump, called the
scavenge pump, and is returned to the tank.  The scavenge pump is
designed to pump a higher volume of oil than the main pump, therefore,
oil is prevented from accumulating in any quantity in the crankcase.
Note that volume is different than pressure.  The main pump is designed
to produce the proper oil pressure in the engine, but pump a smaller
volume than the scavenge pump, which only produces enough pressure to
return the oil to the tank.

Advantages:
  It works in almost any attitude.  With wet sump lubrication, oil can
slosh away from the pump pickup (under heavy braking or acceleration,
for instance) and starve the engine for oil.  A dry sump tank can be
placed and shaped so that a reserve is always available for the engine.

  It provides additional cooling because the tank is in a cool place,
rather than inside the engine.

  It allows the volume of the crankcase to be reduced, which improves
performance on single and twin cylinder engines, especially when
combined with a timed or reed-valve breather.

  It greatly reduces internal engine friction from oil splash drag, and
it prevents oil "foaming" which occurs because of the splashing around
and atomization of the oil by impact.

Disadvantages:
  It is mechanically complex, requiring a second pump, separate oil
tank, and oil routing arrangements.

  Additional oil connections greatly increase the tendancy to leak.

  It requires check valves to prevent oil from the tank from filling up
the crankcase when the engine is not running.  These check valves can
fail, either preventing the engine from getting oil, or allowing the
cases to be filled up (a condition known as "wet sumping").

Historical:
  Motorcycle engine lubrication systems were originally "total loss",
that is that oil went from the tank, through the oil pump, through the
engine, and out onto the ground.  In the late 1920's, the idea came
about that spraying oil out of the engine on purpose was undesirable.
The simplest way to prevent this from occuring, without major redesign
of the engine, was to install a second oil pump at the former oil
outlet, and return the excess back to the tank.  What a good idea! 
-----------------------------------------------------------------------------
| Dave Tharp                      | DoD #0751   | "You can't wear out       |
| davet@loowit.WR.TEK.COM         | MRA #151    |   an Indian Scout,        |
| '88 K75S  '48 Indian Chief      | AHRMA #751  |  Or its brother the Chief.|
| '75 R90S(#151) '68 CB450(#751)  | AMA #524737 |  They're built like rocks |
| '65 R50/2/Velorex  '57 NSU Max  |             |   to take the knocks,     |
|       1938 BMW R12              | (Compulsive | It's the Harleys that     |
| My employer has no idea.        |   Joiner)   |   give you grief."        |
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