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Technical
Pages - ENGINE sourced
from Morris Minor Car
Club Victoria The
A Series Engine
Part
I When
one considers the history of this engine, it is hard to believe that after 50
plus years it is still being produced. Amazingly, the -engine originated back
in the late 1930's for powering Austin trucks. Austin had decided that they should
attack the truck market with more vigor, but found themselves without a suitable
engine. To avoid the lengthy and expensive process of developing a new engine,
Austin blatantly copied the GM/Bedford 'Stove Bolt' overhead valve truck engine
and somehow avoided the legal entanglements that could have resulted from such
an exercise. The Austin engineers modified the GM engine by shifting the camshaft
from the left hand to the right hand side of the engine, and designing the plugs
and distributor to be on the opposite side to the manifolds. This necessitated
the location of the push rods, inlet and exhaust ports and some of the head studs
on one side of the head. Room was made by siamesing some of the ports. During
the war the truck engine was cut down to four cylinders from the original six
and used in the Austin 16, the Hampshire/Hereford and Austin Healy. It was scaled
down to 1200cc for use in the A40 and still further to 803cc for the A-series
engine.
The A-series engine was first released as the AS3 engine and was
used to power the Austin A30. It was originally planned to produce an ultra-lightweight
aluminum engine but underwent many changes before the final production engine
was produced. The first prototype engine was completed in March 1950 but underwent
a number modifications before reaching the final production form. The cylinder
head was designed by Harry Weslake, an independent consultant who had long worked
for Austin and whose design was instrumental in making the engine combustion chambers
so efficient. Early testing of the prototype showed design weaknesses in the crankshaft
and bearings and it was subsequently decided to increase the size of the main
bearings and the big end bearings. At the same time a new and stiffer crankshaft
was fitted to the engine. Further testing of the prototype engine again showed
weaknesses and the main bearings were again increased in size and the gudgeon
pins were also increased in diameter. The new engine was first shown at the Earl
Court Motor Show in October 1951 with the new car it was to power, the Austin
A30. Not long after the Motor Show the Austin and Nuffield companies merged.
At the time of the merger, Nuffield was working on a new overhead valve engine
to replace the old 'lump', the Morris eight engine. The Morris eight, copied from
the ancient Ford 8 engine, was too old fashioned and under-powered to continue
to be used in the Morris Minor. So one of the earliest results of the merger was
the adoption of the A series engine for the Morris Minor. The new engine only
marginally increased the power and torque compared with the old side valve engine
(see table below). It can be argued that the Series II Minor fitted with the A-series
engine still lacked power, particularly for the export markets, and was fitted
with a gear box with poor ratios between the gears and of dubious quality. The
engine and gearbox was found to be quite acceptable in England, but on the high
speed autobahns and freeways found in Europe and America was found to be underpowered
and unreliable. The Series II was not a significant improvement compared with
the side-valve MM.
Finally in 1956, the Minor was marketed with a larger
and stronger A-series engine with an improved gearbox to match. Fortunately, it
was decided to improve and strengthen the engine generally. The block was a new
casting with bores 1 - 2 and 3 - 4 siamesed in pairs (there was no longer a water
jacket between the pairs of bores). The crankshaft was redesigned with larger
stiffer con rods, and harder wearing lead-indium bearings were substituted for
the 803cc's white metal. Oil circulation and filtration was changed to a full
flow system to prevent particles of the harder lead-indium bearing material damaging
bearing surfaces. Valve size was increased, the combustion chamber was increased
and the SU carburetor was increased from one and an eighth to one an a quarter
inch. An improved gearbox was fitted with ratios to match the increased power
and better synchromesh and a remote gear change giving a sensibly short and positive
gear stick in place of the long pudding stirrer previously used.
The following
table summarises the performance of the engines fitted to the Morris Minors:-
CAPACITY
918 cc (S.V.)
MAX. BHP 27.5 @ 4400 rpm
MAX. TORQUE 39 @ 2400 rpm
MAX.
SPEED 62 mph
CAPACITY
803 cc
MAX. BHP 30.0 @ 4800 rpm
MAX. TORQUE 40 @ 2400 rpm
MAX. SPEED
62 mph CAPACITY
948cc
MAX. BHP 37.0 @ 4750 rpm
MAX. TORQUE 50 @ 2500 rpm
MAX. SPEED75
mph CAPACITY1098
cc
MAX. BHP 48.0 @ 5100 rpm
MAX. TORQUE 60 @ 2500 rpm
MAX. SPEED 78 mph Finally,
one can only wonder what the history of the Morris Minor may have been if it had
been fitted with a reliable engine of adequate power. It is interesting to compare
the Morris Minor with the VW Beetle; one achieved a production run of 1.6 million
and the other 20+ million. Why the difference in the success of the two cars?
The Morris Minor had excellent road holding and steering and good styling.
But
it was first released with an old fashioned under-powered side valve engine and
then saddled with the feeble and fragile A-series unit from the Austin A30 with
its terrible gear ratios. If only................. PART
II Bore/Stroke
Ratios The
design of Morry engines has always puzzled me. For example, take the MM side valve
engine which I consider to be the last 'true' Morris engine to be fitted to the
Morris Minor. It has a capacity of 918cc, a tiny bore of 57mm and a very long
stroke of 90mm resulting in a bore/stroke ratio of 1.579:1. Those pistons must
really race up and down the cylinders at high speed when the engine is producing
it's maximum claimed BHP of 27.5 at 4,400 rpm! And the stresses imposed on the
bottom-end compared with a modern big bore and short stroke engine must be considerable.
I had thought the bore/stroke ratio was chosen because of the antiquated British
registration tax based on the bore size of the engine. However, the A series engine
developed by Austin and fitted to the Series 11 Morris Minor, was designed and
manufactured after the tax was repealed. This engine also has a long stroke of
76.2mm compared with a small bore of 57.92mm (bore/stroke ratio of 1.316:1). A
small bore meant that only small valves could be accommodated in the cylinder
head. This, coupled with the 5 port siamese twin design of the head, lead to poor
'breathing' of the engine. Another restriction of the Series 11 engine was the
weak design of the crankshaft, which was often described as "a bent piece
of wire with some journals ground onto it".
As a matter of interest,
the Morris Minors main rival launched on the world market at about the same time,
the VW Beetle, had a bore and stroke of 75mm and 64mm resulting in a bore/stroke
ratio of 0.853:1. It was hardly surprising that the VW could cover considerably
more miles without a major engine overhaul!
The next engine, the 948cc fitted
to the Morris Minor 1000, had the bore size increased to 62.94mm resulting in
a bore/stroke ratio of 1.211:1. The increase in the bore size was accompanied
by a toughening-up of the bottom end resulting in the first reliable A Series
engine.
I can only make the following guesses as to the reasoning behind
the design of the A Series engine:
• They were designed to produce minimum
power and maximum fuel economy.
• The design was cheap to produce.
•
The design was chosen for production simplicity using the available boring machines.
•
"We know best and we are going to stick to this design"!!
I
guess it is understandable that the A-series engine was designed with a small
bore and large stroke in the 1950s when engine technology was not as highly developed
as now. However, it is surprising that BMC were still persisting with this design
in the early 1960 when the American and Japanese manufactures were bringing out
a range of engines with big bores, small strokes and large valves.
Now what
is the importance of bores, strokes and breathing?
Starting from basics,
the engine has a carburetor so as to mix fuel and air in the correct ratio. The
mixture is drawn into the cylinders and burnt, causing the gases to heat up and
expand. The increased pressure pushes the pistons down the bore on the power stroke.
The greater the amount of air/fuel drawn into the cylinder, the greater the pressures
generated in the cylinders. Greater pressures mean higher horsepower and higher
torque. The more air the engine can inhale the more power that can be extracted.
Breathing efficiency boils down to designing an engine that will allow
air to flow into the engine as efficiently as possible. A manifold with sharp
bends, as in the 918cc side valve engine, will constrict the entry of air/fuel
mixture into the engine. Click on photo for more detail.
Air is not "sucked"
into an engine - it is "pushed" in by atmospheric pressure. A small
bore and a long stroke, coupled with an inefficient inlet manifold, is not the
ideal method to attain good "breathing"! When this is coupled with a
head that has inadequate flow due to: (a) small valves, (b) having those small
valves shrouded and (c) not particularly good porting; the result is an engine
with a poor power output for its size.
A pair of inlet manifolds connected
to siamesed inlet ports that branch to a pair of cylinders is not an ideal arrangement
to ensure good "breathing" and maximum engine efficiency.
It
should be noted that the engine was first produced during the 1950's when the
demand for cars far exceeded their production rate. The UK was still experiencing
shortages of materials and fuel which created a demand for small economical cars.
However, by the 1960's BMC seemed to have fallen behind in engine designing when
more powerful cars were beginning to dominate the market. | 
