Driving Tips: 3-litre Bentley
Extract from: Chapter 4 (Handling and Starting Instructions - Engine and Chassis Details - General Maintenance and Servicing Instructions for all 3 Litre Models)
Vintage Bentley Manual, J. H. Haynes, c1962

The engine having great torque at low speeds means that not as much gear-changing has to be done as in an ordinary car, but in the Speed Models with high gearing and a close ratio gearbox, the best results are obtained by making full use of the gears.

The Gear Ratios for the Standard Chassis are as follows:     First gear 16.2 to 1 Second gear 8.8 to 1 Third gear 6.5 to 1 Fourth gear 4.23 to 1 Reverse gear 16.2 to 1

As soon as the car is in motion, the change from first to second gear is best made immediately, and to make a silent change an appreciable pause should be made in neutral.

The faster the car is going when the change is made, the longer will have to be the pause. Changing from second into third gear only requires a short pause, but changing from third to top requires a longer one. In every instance, matters are greatly helped if double-declutching with a closed throttle is used. When changing down double-declutching is always necessary, and the engine must be speeded up the requisite amount to make a smooth silent change.

This technique can only be achieved by experience, but once the driver has become accustomed to the feel of the engine, smooth changes can be made with little or no trouble. All gear changes should be made at no higher than 3,000 r.p.m.

Presuming that the car is fitted with a 4.23 to 1 rear axle ratio, the following speeds are achieved.     First gear 18 m.p.h. Second gear 33 m.p.h. Third gear 46 m.p.h.

The ratios of the gears in the "A" box as fitted to the speed models are as follows:     First gear 10.0 to 1 Second gear 6.18 to 1 Third gear 5.05 to 1 Fourth gear 3.78 to 1 Reverse gear 10.0 to 1

With the "A" gearbox the change up depends on whether a clutch stop has been fitted. If a clutch stop has been fitted it is possible to pull the gear lever through the gate very quickly, the rate at which the change is made depending on the distance that the clutch pedal is depressed, which brings the clutch stop into action more fiercely. Without a clutch stop a pause in neutral must be made between first and second gear, but the change from second to third can be done without any pause at all. A pause is again necessary when changing from third to top, but the length of the pauses with the "A" box are much shorter than with the "B" unit. In all cases when changing down it is necessary to double de-clutch except when changing from third to second. Here the gear lever can be pulled through very rapidly, providing the throttle is kept open, and the clutch is only slightly depressed.

The engine revolutions beyond which gear changes should not be delayed are 3,500 r.p.m. and with a 3.75 rear axle ratio corresponds to the following speeds:     First gear 34 m.p.h. Second gear 54 m.p.h. Third gear 67 m.p.h.

If it is wished to achieve considerable economy of fuel consumption on the standard model fitted with the Smith Bentley carburetter this can be achieved by leaving the air control knob on the switch plate in the fully weak position. This practice is quite acceptable under normal touring conditions, but if the car is being driven really hard then the knob should be left in the normal position.

Twin S.U.s are fitted on the speed model, and the air control knob should always be left in the fully weak position, as the rich mixture is only necessary when the car is started, and warmed up.

It is a sound idea to occasionally test the magnetos and plugs in order to make sure that they are working properly by switching off each magneto alternatively. This will immediately show if all the plugs are firing, and if there is any fault in the magneto itself. Under normal running conditions both magnetos should always be used, but if one breaks down it does not hurt the engine to run on one, till the broken unit has been repaired.

To start the Standard Model fitted with the Smith Bentley carburetter from cold, the following procedure should be followed. The throttle should be placed in the closed or slow running position, the ignition lever advanced about three quarters of its travel, and air control knob pushed right in on the plate on which it is mounted. Turn the magnetos on, press the starter button, and at the same time gently dab the accelerator pedal. If the engine does not fire within eight seconds, release the foot from the self-starter button and pause for a minute or two before trying again. If when the engine fires it kicks back, then the ignition is too advanced, and should be retarded to halfway or even further.

As soon as the engine fires and runs correctly, the ignition should be fully advanced and the throttle opened slightly. The air control knob must then be pulled out till it is in the normal running position where it is held lightly by a spring and ball catch. When starting from cold, less strain will be placed on the started motor and the battery, if the clutch pedal is depressed, which frees the motor from also having to turn over the gears in the gearbox.

The engine should be run stationary for a minute or two to allow it to warm up before moving away. If the weather is really cold it will be necessary to both flood the carburetter, and to inject petrol into the inlet manifold via the priming cocks. If too much petrol is introduced though, the mixture will be over-rich and the plugs wetted. Not more than a teaspoonful should be introduced through the priming cocks.

To start the Speed Chassis' model, the engine of which is fitted with twin S.U. carburetters, the throttle should be placed in the slow running or closed position. The mixture control should then be twisted between half and three quarters of a turn towards the rich position. The self starter switch can then be depressed, and at the same time the accelerator should be given a gentle dab. If the engine kicks back the ignition should be retarded until the engine fires in the correct direction. As soon as this occurs open the throttle a little and fully advance the ignition. Turn the mixture control back about half a turn, till the engine has warmed up, when it may be turned to the fully weak position. It is important that it is turned right back, otherwise the plugs will soot up because the mixture is too rich.

Twin S.U. carburetters should never be flooded, as their position if flooded allows neat petrol to run into the cylinders and wash the protective oil film from the cylinder walls. If the car is ever started by the handle, ensure that the ignition is not advanced more than about a quarter of the quadrant so as to ensure that no backfire occurs. Because of the low compression ratio of the engine, it is not necessary to run it on a high octane rating fuel, and indeed it is a waste to do so. Ordinary grades are quite sufficient.

Other points that is as well to bear in mind in connection with running the car are as follows. The engine should never be raced when running free, and should never be revved hard till the oil has had time to circulate properly and the unit has reached its normal working temperature.

The S.U. carburetters should never be flooded and the Standard Model with the Smith Bentley carburetter should never be driven hard with the air control knob in the weak position. No grease should be applied to the rear hubcaps, and only a little to the front ones. Ensure that the rear axle is never overfilled. It is important that the locking rings on the detachable wheels are kept tight and nothing but a copper or hide-faced hammer should be used for tightening them. Never run with one magneto switched off except for testing purposes. If the starting handle is allowed to stay in the down position when running, the dogs on its end will rub against those on the end of the crankshaft, causing an unpleasant knocking noise not unlike a worn big end bearing, and will wear rapidly.

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The eleven-gallon petrol tank is carried at the rear of the chassis, and in the filler orifice is a detachable gauze filter of large dimensions. The filler is fitted with a two-way tap. For normal running the position of the tap control is horizontal, and the engine will then run dry of fuel when there is a reserve of two gallons still in the tank. To use these two gallons the tap must be placed in the vertical position. The feed from the tank to the carburetter is by an Autovac, which is mounted on the front scuttle above the steering column.