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Updated 7-01-2010
The Leaning Tower of Power
Mopar Slant six:
170, 198, 225
The scraper for this engine has gone through
several generations of design over the last six years. I am
fond of the slant six because I owned a 1966 Plymouth Valiant and
extensively modified it and the engine. I will expand more
than I normally do in descriptions because of this. Please
feel free to copy this information if you feel it is useful.
The slant six has a well
deserved reputation for durability under normal operating
conditions. However, there are some very real issues with the
oil system that should be considered if you really "push" your
engine or race it.
In most automotive
applications the slant six pan design (and there are numerous
pans) has a very shallow sump that allows the sump oil to migrate
backwards and/or collect there if the car is under steady
acceleration forward -- at even fairly modest levels. This can
happen under braking as well in the front of the pan. The oil
is then struck by the rotating assembly and whipped full of air.
This "entrainment of air" is different from simple oil foaming.
It takes much longer for this air to be released than for foam to be
dispersed and broken.
This air entrainment can also
happen in competition when the car enters high speed left turns and
the oil collects in the rear corner of the pan. The clockwise
moving rotating assembly tries to push the oil up the side of the
block and in the process churns it full of air. This is simply
an inherent issue when using an engine tilted to its side. BMW
racers are very familiar with this issue and have dealt with it for
at least 50 years now.
Above: typical pan (1973 Dart
pan shown)
If enough air is entrained
into the oil this can lead to rod bearing failure and other issues.
When highly aerated and pressurized oil passes through the
lubrication circuit any abrupt changes in the flow path can cause
localized pressure drops. The localized pressure drops allow
air that is dissolved in the oil to come out of solution and collect
into bubbles. Yes, air really does dissolve into your oil - it
is a normal thing. The higher the oil pressure, the more air that
can be dissolved.
It is extremely important to
realize that this high aeration or entrainment can occur at
relatively low to medium engine rpms. Try to imagine the big
end of a rod whipping through a puddle of oil at ten times a second
-- that is merely idling speed (600 rpms). People testing
engines alone on stands or in cars on roller dynos can sometimes
forget what happens when significant movement of the engine/car is
introduced.
These entrained bubbles can
cause damage in the bearing shells. In fact, the slant six has
been known over its production history for losing the number 5 rod
bearing and now you know why. There are fixes to the oil
circuit that have been developed by very talented tuners and
engineers, notably Doug Dutra.
When the slant six had its
crankshaft redesigned for lighter weight and higher efficiency one
of the major changes was to dramatically reduce the swept path of
the counterweights. At the same time, this helped to reduce
the amount of air churned into the oil. Why?
Hydraulic lifter circuits
were being developed for the engine. Hydraulic lifters will
not tolerate extremely aerated oil: by the time oil reaches the
lifters in a dynamic hydraulic circuit the pressure has lowered and
air that was dissolved under higher pressure at the pump would be
coming out of solution. A fluid with air bubbles in it is
compressible and will cause erratic lifter operation.
Another more esoteric problem
that emerges at high engine speeds (approximately 6000 rpms or that
neighborhood) is vibration from cavitation at the oil pump -- from
the aerated oil. This chaotic vibration and shock loading was
not contemplated when the small "point contact" oil pump pinion
drive gear was designed. Racers have reported loosing pumps in
this rpm range. Then too, six cylinder inline crankshafts have
a third order harmonic that emerges around this rpm. Overall,
a difficult situation. Click here for a possible partial
solution -- performance oil pump modifications.
All in all, the best solution
to the entrainment problem is to try to prevent the excess air from
being churned into the oil in the first place and this is why crank
scrapers are offered.
Note: if you are running a
reverse rotation marine engine there are scraper designs for
this available.
These parts should all be
compatible with the windage tray designed by Larry Shepherd during
the Direct Connection days -- many thanks to Slant Six guru, Doug
Dutra for that information. Here is a picture of an earlier
generation of the scraper with the tray installed:
Upstroke scraper with trap
doors: trap doors have been added to help prevent migrating oil in
the sump from being blasted up the side of the block and churned
during hard left turns. This is a problem that is inherent in
the engine design and can be seen in BMW and Porsche engines with
the same sort of slant. $89.95 standard steel version; $189.95
with a Teflon blade. This scraper comes with our standard
downstroke scraper (seen below).
Downstroke scraper standard:
The downstroke scraper helps strip out oil prior to it reaching the
floor of the pan. The earlier you can remove entrained oil the
better. [170 version shown]
Racing
scraper set: This version is pan and
stroke specific. It includes the standard steel upstroke
scraper with trap doors. Teflon bladed version optional.
If your pan is
similar in design to the 1973 Dart pan then it can be adjusted by the owner using basic sheet
metal hand tools. Designs for additional pans will be done
with owner's pan being shipped to the shop for fitting. This
design reaches into the 1st quadrant ATDC and directs oil into a
shielded area. This area has directional screening within it
to slow the oil and then protect it as it travels to the sump floor.
Normally, the angle of the pan wall would have this oil dropping
onto the rotating assembly. The louvers are adjustable.
When used with a Shepherd design windage tray oil is directed away
prior to the cloud entering the tray. This helps to minimize
oil bouncing off the interior surface of the tray and being struck
again by the rotating assembly. Again, the design seen below is for
a pan from a 1973 Dodge Dart (225). $249.95; $349.95 Teflon
bladed upstroke version.
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Updated 1-14-2010 Here are some
remarks on the Slant Six distributor gear.
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Several years ago we were asked to design an exhaust
manifold flange for the Slant Six community. This design is
shareware; please double check to ensure that the measurements are
accurate. You can have this pattern cut out at a local water
jet jobshop. Please click here.
Here is a jpg:
We were recently asked to design a halo type
girdle for a dedicated slant six engine. A custom pan was to
be made by the customer with scavenge pumps. Please note the
design will probably NOT fit into any OEM slant six pan and will
probably have frame clearance issues. It also does not have a
provision for a stock pickup tube. Please double check all
measurements and clearances. It is designed to come very close
on the pressure or downstroke side of a stock 225 rotating assembly.
You may have to make small notches for rod bolt/nut clearance there.
It is suggested that the part be cut by water abrasive jet from cold
rolled plate steel. Any warp in the plate will be bad for main
bearing alignment.
Here is a jpeg:
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