So many changes
have occurred in the development of engines and cars
during the last quarter of a century that mere consideration
of the famous 8-litre Bentley car must inevitably excite
the query as to why a model with such an enormous engine
capacity should have been designed for sale as a standard
product.
During those years, and especially since the last war,
the drive with British manufacturers has been steadily
towards smaller engine capacities, higher power and
torque, lower fuel consumption and general all-round
economy; engine capacities now are seldom more than
4 1/2 litres in the larger cars, while 3,000 c.c. or
much less has been ample for many high-performance cars.
Although engine capacities have tended to become very
much smaller over the years, the outstanding feature
in car design has been, nevertheless, the virtual abandonment
by the majority of manufacturers of the production and
sale of separate chassis in favour of the integral construction
vehicle, enabling the makers to retain full control
of the design of the car and engine and its ultimate
performance, with cost-cutting applied in every possible
direction.
In the 1930 period, when there was a much larger number
of big cars on the market, chassis were built, as a
matter of course, to be fitted with standard bodywork
by the manufacturers, or with coachwork produced by
specialized firms. Within the performance requirement
of the average buyer, this system of chassis-plus-bodywork
was perfectly satisfactory, but when one came to a luxury
car such as the Bentley, the situation was markedly
different; to produce the best results, the tout
ensemble in excellence had to be matched to a much
closer degree, especially with closed bodywork.
The 8-litre, six-cylinder Bentley chassis was the last
of a series to be produced by W. O. Bentley before the
reorganization into the present company, and it was
designed to take almost any type of passenger bodywork
that could be fitted to the chassis, irrespective of
its bulk, height and weight, and to maintain a speed
of a genuine 100 m.p.h. with the maximum of reliability
and roadworthiness and at the same time the manners
of a good town carriage. Whereas, with other Bentley
chassis, there were always some limitations as to bodywork
weight and so on, none applied to the 8-litre model,
such was the capacity of this giant.
Specification
To cater for what might occur in the matter of all-up
weight, air drag, passenger and luggage weight, fuel
for long-distance travel and other imponderables, W.
O. Bentley designed a big engine on very much the same
lines as the previous 6i-litre six-cylinder, but with
a capacity increased from 6,597 c.c. to 7,982 c.c, and
fitted it into a long wheelbase chassis. The prime characteristics
of engine behaviour had to be: Maximum r.p.m. not much
higher than 3,000, sustained and smooth pulling power
over a wide speed range, freedom from major servicing
over long periods and complete reliability for high-speed
motoring. Petrol in those days varied from 1s 2 1/2d
to 1s 4 1/2d a gallon, so that fuel consumption at 10-15
miles per gallon represented a total cost no higher
than would be the case with a much smaller car at the
present time.
In the range of Bentley engines, from the early 3-litre
four-cylinder unit to the big 8-litre six-cylinder,
W. O. Bentley had always shown a strong individuality
and taste in design. This was exemplified in the adoption
of four valves per cylinder to obtain ample cooling
of valve seats and other locally hot areas; non-detachable
cylinder heads; built-up cylinder water jackets for
internal access to cylinder and head castings; lightweight
connecting rods and crankshaft; multiple crankshaft
bearings; neatly arranged and very accessible auxiliaries;
and a superb finish inside and outside the engine. Externally,
every Bentley engine had to be faultless to the connoisseur-
none more so than the vast 8-litre.
With a bore and stroke of 110 by 140 mm, the 8-litre
engine pulls standard top gear ratios of 3.533, 3.785
or 4.071 to 1, according to the rear axle chosen; on
seven special chassis a ratio as high as 3.3 to 1 was
fitted. This, with a tyre size of 7.00 x 21in, resulted
in a speed of 32 m.p.h. per 1,000 r.p.m. in top gear.
At 5.3 to 1 compression ratio, 225 b.h.p. was developed
at 3,500 r.p.m.; at 6.1 to 1, 230 b.h.p. at 3,300 r.p.m.
Getting Inside
The cast-iron cylinder block has side plates of stainless
steel secured by a multitude of 2 B.A. studs and nuts,
to facilitate full inspection and cleaning of the jacketing
and also the cylinder casting generally. A detachable
plate on top also enables the interior of the cylinder
head to be examined when necessary. The block is secured
to the Elektron crankcase by pairs of long bolts extending
to the main crankshaft bearing caps, the upper nuts
of which are concealed behind the lower extension of
the water jacket plates.
As was common in the days when it was designed, the
crankshaft, of marine design, has no balance weights,
but is supported by seven main bearings, and has, at
the front, a large multi-plate friction damper, which
is assisted in its work by the driving of a large dynamo.
An eighth main bearing, just ahead of the flywheel,
acts as a rear support for the shaft. The connecting
rods, which are machined all over, are light in weight
and small of mass, and both crankshaft and rod bearings
are of white metal and steel backed. The alloy split
skirt pistons, each with two compression and two scraper
rings, have fully floating gudgeon pins provided with
soft metal end caps. Lubrication of the main and other
bearings is effected in the normal manner from an oil
pump in the rear part of the Elektron five-gallon sump,
the drilled crankshaft being fed from a fore and aft
gallery pipe below the main bearing caps.
An outstanding attraction of this great engine is the
layout of the drives to the overhead camshaft and auxiliaries,
there being possibly no other engine in the world which
could provoke such interest in these respects. In his
early days, W. O. Bentley was a locomotive engineer
apprentice in the old G.N.R. shops at Doncaster, and
readers who may be conversant with earlier G.N.R. designs
will recall the classic beauty and neatness of the locomotives
built in these works. Something of this same regard
for design was exhibited when W. O. Bentley produced
die 8-litre engine, with a predilection for finely finished
parts and excellence of detail arrangement.
To ensure a sweet, smooth and completely silent drive
to the overhead camshaft, Bentley introduced a coupled
rod mechanism which was a counterpart of the driving
wheel side rods on a locomotive, except that three coupling
rods were used instead of two.
In the Bentley, a miniature three-throw crankshaft located
in bearings in the after part of the crankcase is driven
by helical gearwheels at a 2 to 1 ratio from the main
crankshaft, the throws being, set at 120 deg to each
other. At the rear end of the overhead camshaft is a
duplicate of the three-throw crankshaft in the crankcase,
and the pair of shafts are coupled together by three
vertical rods, each having bearings at top and bottom.
Locomotive-type
A pair of shafts, each with two throws set at 90 deg
(as the driving wheel crankpins are set on a locomotive),
would transmit the drive to the camshaft perfectly well,
but owing to the fact that the crankpins on the three-throw
shafts of the Bentley engine are so large in relation
to their throw that they are, in effect, eccentrics,
a triple rod drive to the camshaft was substituted to
ensure a very low-friction transmissions, with a completely
even torque, combined with almost perfect silence in
running.
The large bearings at the upper and lower ends of the
coupling rods are split for assembly with the one-piece
crankshafts, but in order to compensate for slight variations
of the cylinder block height under temperature variation,
a series of ground washers are fitted on the shafts
of the coupling rods, above and below the halves of
the upper bearings, the fractionally small clearances
between the washers being filled with oil from the engine
lubrication system. In effect, the washers can breathe
oil and pulsate to allow the upper bearing to move vertically
in sympathy with the upward expansion of the cylinder
block, with no appreciable play in the bearing itself.
In normal practice, the system of drive to the camshaft
requires no periodical attention or adjustment whatever,
over mileages of as much as 50,000 to 70,000 miles,
while, of course, the take-off point for the drive from
the rear end of the crankshaft is ideal. A disadvantage
of the system, seen through modern eyes, is the substantial
addition to the overall length of the engine, which
now could no doubt be materially reduced by suitable
design and manufacturing methods.
As was standard on all previous Bentley engines, the
main casing above the cylinder block of the 8-litre
contains an internal casing carrying the camshaft and
rocker assembly, from which one cam operates a pair
of inlet valves, while two cams are used for the two
exhaust valves for each cylinder. Long detachable side
plates permit feeler adjustment of the rockers and valve
stems, but adjustment is seldom needed in view of the
size and reliability of the engine.
With such a large -capacity, the fitting of two sparking
plugs per cylinder was considered a necessity, the plugs
on the exhaust side of the engine being energized by
a coil, while the others (on the induction side) are
operated by a magneto. A cross shaft, skew gear driven
from the camshaft and mounted in the rear of the camshaft
outer casing, drives the distributor and contact breaker
on the left side, and also the magneto on the right
side, giving perfect accessibility to these electrical
components and their wiring and controls.
The setting of the camshaft is distinguished by the
unusual location of two units; a friction damper at
the rear end and the centrifugal water pump at the front
end, the latter delivering water by a short, curved
pipe into the front end of the cylinder jacket and thence
to an internal gallery pipe and jets which direct the
coolant to the exhaust ports. The only auxiliary not
operated from the camshaft is the fan, which is belt
driven from the front end of the crank- shaft. The whole
setting of the auxiliaries and their method of drive
is a masterly piece of design, enabling the engine to
have an exceptionally clean and orderly appearance seldom
seen on modern power units.
The carburation system comprises two very large bore
S.U. carburettors, connected to a common induction manifold
and leading to the twelve inlet valves, and a miniature
idling carburettor mounted midway between the two main
instruments and feeding mixture through small bore pipes
to a point near each main throttle. This small carburettor,
which draws fuel from one of the main float chambers,
was fitted to obtain an economical mixture for slow
idling, which is essential for a big engine used for
town work.
Two other prominent features of the 8-litre unit, which
put W. O. Bentley far ahead of his time, are the three-point
rubber suspension of the engine, and the positive engagement
of the starter pinion before the motor is energized.
The separately located gear box, it may be noted, is
also mounted on rubber, again at three points.
Much has been written on the general performance of
the 8-litre model and of its capacity to eat up the
miles, but unquestionably its appeal to the present-day
motorist would be its very size and capacity to pull
like a giant with just a whiff of throttle, if only
one could afford nowadays to motor with such a car.
W. O. Bentley has said that he used regularly to leave
his London office at 9 a.m. in his 8-litre, have an
early lunch at Catterick, visit Carlisle, and then be
back in London the same evening-some 600 miles in not
much more than twelve hours. Then there was W. O.'s
famous run between Dieppe and Cannes during daylight
hours, among other feats of very fast touring in the
'thirties, while an 8-litre Bentley was the first to
lap Brooklands at over 100 m.p.h. with closed bodywork.
The average weight of an 8-litre saloon is 48 to 50
cwt, the chassis being very strongly built to cope with
high speeds. A number of the cars are still in very
good running condition, and certain engines have been
converted to diree carburettors, still further to increase
performance, the maximum horsepower being raised in
one case to 268.
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