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L. J. COTTON, M.I.R.T.E.
BECAUSE of additional torque, the Dennis Pax 5-6-tonner, fitted with the new overheadvalve petrol engine, climbs hills more easily than its predecessor, and its liveliness on the road is indeed noticeable, Compared with the side-valve-engined Pax, the test ofwhich was reported in "The Comn-iercial Motor " on August 31, 1945,, the new engine provides a performance which can be described as at least "half a gear" better.
a4 The Dennis four-cylindered 3.71litre overhead-valve engine, first described in "The Commercial Motor" on October 21, 1948, incorporates various components which are common to the side-valve unit. It has the same bore and stroke of 100 mm. and 120 mm. respectively, and all the components in the cylinder block are interchangeable. The exhaust and. inlet manifolds are reversible for rightor left-hand drive, and the main casting can be turned, end for end, so that the components can be arranged on either side.
The crankcase and cylinder block form a monobloc iron casting, and a one-piece cast-iron head covers all four bores. All timing gears are arranged at the rear of the engine and the valves are operated by pushrods and rocker gear. The counterbalanced crankshaft is supported in the crankcase on three white-metallined bearings. Other details of the unit include an integral impellor-type water pump and fan, which is beltdriven in tandem with the dynamo, and an external water gallery pipe to return the coolant to the radiator.
A Zenith down-draught, carburetter is provided, overseas models• being fitted with an air cleaner and silencer, whereas the home engines have a plain cover to restrain water
or foreign material from entering the carburetter intake. Ignition is by coil, with a vacuum and mechanical automatic advance timing device embodied in the distributor.
A single dry-plate clutch replaces the former wet unit, which shared the engine lubrication. The engine, clutch and gearbox are retained in the chassis on a three-point rubber mounting, plain rubber pads surrounding the mounting units, which are of blade shape.
Apart from the engine and clutch, no radical change has been made in the Pax specification. A slight modification in the brake linkage has improved the retardation efficiency, and the fittings to the cab now include chromium-plated windscreen frames, draught welts to the doors and polished wood fillets at waist level.
• A representative load of approximately 5 tons 14 cwt. was carried on the trials. This load included the weight of a platform body, which would account for 6-7 cwt. After "weighing-in" at the Dennis works, we left Guildford and followed a course towards the Hog's Back in search of steep gradients. There was ample room for three in the cab, although some operators specify a longer driving compartment to give additional leg room behind the engine cowling, which projects slightly through the front panel.
After starting from cold, the chassis made a good climb in top gear up to the Farnham junction road, which is no mean feat for a well-laden medium-class chassis. This part of Surrey abounds with long gradients, and we were soon climbing to the top of the Devil's Punch Bowl between Milford and Haslemere. Because of the steeper slopes, the use of third gear was demanded occasionally, but, even so, the Dennis put up a remarkable performance and overtook others of its class which appeared to be only part-laden.
Braking efficiency was put to test when descending a sharp incline, which terminated in an acute T junction. After applying light pressure on the brake pedal and finding only slight retardation, I was prepared to overshoot the turn, but increased effort on the pedal brought the Pax abruptly to rest. As no servo is fitted, retardation is proportional to pedal effort. I handed over to the works tester and observed water-temperature rise and gears used when climbing Nutcombe Hill, a long gradient rising to 1 in 41. After taking the radiator-water temperature and the ambient reading, we started the climb.
Low and second gears were engaged, successively, and the engine pulled energetically in the second ratio on a 1-in-4 slope. Low gear was again required when the Tapley meter registered 1 in 10, and the engine speed started to fall off as we
climbed the 1-in-41 section. Altogether, the first and second ratios were employed for 11 mins., during which time the water temperature rose from 164 to 172 degrees F., with an ambient reading of 49 degrees F.
As there was no space to turn, the Pax was reversed to the steepest point to judge the engine and transmission capabilities when starting from rest. Descending from the cab to observe this attempt, I found the acute camber was causing at least an 8-in.. articulation between the front and rear axles.
It appeared to be a formidable task to move away from this point and, to make conditions worse, the angle of the chassis caused enrichment of the mixture, and the engine stalled. It roared into life at the first touch of the switch, and the start from rest was successfully WI accomplished without loss of marks The hand brake held the chassis from reversing during this test.
Several sharp turns in the narrow roads beyond Nutcombe gave opportunity of demonstrating the good steering lock. Although a fair proportion of the load was carried by the front axle, the steering was light and positive. This, combined with the situation of the steering wheel and the controls, made the Pax a lively and easy vehicle to control, especially along the by-ways.
The brake facings were well heated after descending from the Devil's Punch Bowl into Milford, and a reasonably straight and level 'stretch of concrete road invited brake tests to be made while they
were still smoking. The effect of an emergency application from 30 m.p.h. almost provided a challenge to the hypothetical "g," the stopping distance being 30 ft. Repeat trials at 20 m.p.h. and 30 m.p.h. proved that the braking system was fully effective, and subsequent tests when the facings and drums had cooled down showed the efficiency to be little impaired by hot drums.
This test was followed by acceleration trials, in which the second ratio was employed when moving away from rest. As the Pax gearbox incorporates straight spur-toothed wheels, a brief delay is necessary between gear changes. Nevertheless, the Dennis required only 28.5 sees. to reach 30 m.p.h. from rest. Third gear was engaged at 15 mph. and direct drive at 27 m.p.h. This ably demonstrates the flexibility of the engine, especially in the middle and top range of its speed.
Using direct drive, it took 36.1 sees, to reach the maximum legal speed from 10 m.p.h. During the top-gear trials I remarked on the othness of was not the ign of pinking at any throughout the. range of eed. The torque curve shows a steep climb at half-speed, and this is closely followed by the acceleration rate in direct drive, the rate from 10-15 m.p.h. being relatively slow when compared with that in the tipper range.
By comparison with the sidevalve-engined Pax, the improvement in acceleration in the latest model is remarkable. Acceleration through the gears with the side-valve unit required 43 secs. to reach 30 m.p.h. from rest, whereas the latest Pax attained 30 m.p.h. from a rolling start in direct drive in 53 secs. In addition, the• previous running weight was almost half a ton lower.
Then came the consumption trials, the first being a non-stop run. This was made from Guildford towards London, with a sharp turn on the outskirts of Esher to retrace the course. The turn was made "in one," the vehicle having a 49-ft. turning circle. Maintaining a convoy speed with other goods vehicles, the outward run was completed at an average speed of 28 m.p.h. A smarter pace was set for the return journey, and the speed for both runs averaged 29.8 m.p.h. The fuel return worked out to precisely 12 m.p.g., which is slightly more economical than the result obtained with the side-valve-engineil chassis.
The one-stop-per-mite. consumption trial was run over the Guildford-Horsham road, which involved a fair measure of traffic in Bramley. With a 7-to-1 rear-axle
ratio, the Dennis had a liking for local work;.. and. as, Welt. as being quick off 'the . mark, it yielded a decidedly favourable fuel return of 10.6 m.ps. After deducting standing time' on the journey, the average
speed was 24.8 m.p.h. , Satisfied that there was .adequate braking power for any emergency, I indulged in a short burst of speed on the return to. Guildford. The liveliness of the new engine affords a cruising range of 35-45 m.p.h., which can be maintained comfortably with
out overworking the unit. Lubricant temperatures taken on arrival at the works showed the engine oil to be 148 degrees F., gearbox 140 degrees F. and rear axle 128 degrees F. Ambient at the time was 52 degrees F.