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From: (Andy Dingley)
Subject: Re: Dry Sump
Date: Fri, 28 Jun 1996 14:15:56 GMT

The moving finger of (Mike Flynn) having

>I have encountered the fact
>that the 911 uses a dry sump lubrication system.  

Congratulations on your acquisition !  I don't know if all 911s use
dry sumps, or if it's just the more highly-tweaked models. A "sump" is
the UK term for a US "oil pan", BTW.

Most cars have a "wet sump". The engine is mounted above a bucket of
oil. Oil is sucked up from the bucket, pumped to where it's needed,
then drips back into the bucket. It's simple, it works, it keeps
almost everyone happy.

A dry sump is a solution to two problems, sloshing and churning.
Sloshing is a problem for cars that corner rapidly, particularly
transverse engines, and less commonly for those that can accelerate
really quickly. All the oil sloshes to one end of the oil pan, and if
these leaves the pump's suction pipe exposed, the engine sucks air
instead of oil (A Bad Idea). You can improve matters by fitting sheet
steel baffles inside the oil pan. Small holes, or hinged flaps, in
these baffles let the oil percolate through slowly, but don't allow it
to slosh around in waves.

Churning is what happens if the oil level is high enough that the
crankshaft dips beneath the surface. There's some extra noise, some
wasted power in churning the oil up, but most importantly it tends to
make the oil into a meringue of air bubbles. Pumping air bubbles
around an engine isn't as effective as pumping oil. The solution to
churning is to mount the crankshaft high above the level of the oil,
or to make the sump wider so that the oil sits in a shallower, wider,
pool. Unfortunately we'd also like to mount our heavy engines as low
as possible, so that the car handles better.  This is particularly a
problem with horizontal, or horizontally opposed, engines like your
Porsche - the crankshaft is much higher up in the overall engine black
than a vertical or V engine, so all the problems are magnified.

A dry sump avoids these problems almost entirely by separating the
function of the oil pan into two separate devices. An oil pan
_catches_ the oil and a separate tank _stores_ the oil. As the oil pan
or sump doesn't hold the reservoir of oil, it's "dry"  (although still
rather oily, as oil is still passing through it). The oil pan can now
be much smaller and lower, and the engine may be mounted closer to the
ground. The separate tank also acts as a de-aerator, and is better
than a wet sump at removing air bubbles. It's usual a vertical
cylinder, often with a sight glass in the side instead of a dipstick.
You'll now need two oil pumps and a bunch of hoses. A new "scavenge"
pump sucks oil from the sump into the storage tank and a "pressure" or
"delivery" pump pumps oil from this tank into the engine. It's common
to have both of these pumps mounted together on the outside of the
block, and belt driven.

Dry sumps are better than wet sumps for performance, but they're more
complex. The idea of mass production cars running around with their
lubricating oil flowing through poorly inspected hoses should have an
obvious problem.

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