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.