Download file Free Book PDF Land Rover Overdrive Manual at Complete Installation Manual Series Land Rovers Options Fairey Overdrive. OVERDRIVE UNIT AND SELECTOR MECHANISM. -. Ye |. 17 Gasket, overdrive to transfer box. Dipstick FITTING INSTRUCTIONS. (in). Fairey Overdrive Fitting Instructions. Page history last edited by Stephen Jarman 10 years ago. Download: Fairey Overdrive Fitting

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Overdrive is a term used to describe the operation of an automobile cruising at sustained speed with reduced engine revolutions per minute RPMleading to better fuel consumption, lower noise, and lower wear. The most fundamental meaning is that of an overall gear ratio between engine and wheels, such that the car is over-gearedand cannot reach its potential top speed, i.

The purpose of such a gear may not be immediately obvious. The power produced by an engine increases with the engine’s RPM to a maximum, then falls away. The point of maximum power is somewhat lower than the absolute maximum RPM to which the engine is limited, the ” redline ” RPM.

A car’s speed is limited by the power required to drive it against air resistance, which increases with speed. At the maximum possible speed, the engine is running at its point of maximum mnaual, or power peakand the car is traveling at the speed where air resistance equals that maximum power.

There is therefore one specific gear ratio at which the car can achieve its maximum speed: Therefore, a car needs one gearing to reach maximum speed but another to reach maximum fuel efficiency at a lower speed.

With the early development of cars and the almost universal rear-wheel drive layout, the final drive i. The gearbox was designed so that, for efficiency, the fastest ratio would be a “direct-drive” or “straight-through” 1: Achieving an overdriven ratio for cruising thus required a gearbox ratio even higher than this, i. The propeller shaft linking gearbox and rear axle is thus overdriven, and a transmission capable of doing this became termed an “overdrive” transmission.

The device for achieving an overdrive transmission was usually a small separate gearbox, attached to the rear of the main gearbox and controlled by its own shift lever. As popular cars became faster relative to legal limits and fuel costs became more important, particularly after the oil crisisthe use of 5-speed gearboxes became more common in mass-market cars.

These had a direct 1: With the popularity of front wheel faigey cars, the separate gearbox manuak final drive have merged into a single transaxle. There is no longer a propeller shaft and so one meaning of “overdrive” can no longer be applied.

However the ocerdrive meaning, that of an overall ratio higher than the ratio for maximum speed, still applies. The power needed to propel a car at any given set of conditions overrive speed is straightforward to calculate, based primarily on the total weight and the vehicle’s speed.

These produce two primary forces slowing the car: The former varies roughly with the speed of the vehicle, while the latter varies with the square of the speed. Calculating these from first principles is generally difficult due to a variety of real-world factors, so this is often measured directly in wind tunnels and similar systems.

This is known as the point of maximum power. Given a curve describing the overall drag on the vehicle, it is simple to find the speed at which the total drag forces are the same as the maximum power of the engine. This defines the maximum speed the vehicle is able to reach. The rotational speed of the wheels for that given forward speed is simple to calculate, it is simply the tire circumference multiplied by the RPM. At even slightly lower speeds than maximum, the total drag on the vehicle is considerably less, and the engine needs to deliver this greatly reduced amount of power.

Clearly this condition calls for a different gear ratio. If one is not supplied, the engine is forced to run at a higher RPM than optimal.

As the engine requires more power to overcome internal friction at higher RPM, this means more fuel is used simply to keep the engine running at this speed. Every cycle of the engine leads to wear, so keeping the engine at higher RPM is also unfavorable for engine life. If one runs the same RPM transmission exercise outlined above for maximum speed, but instead sets the “maximum speed” to that of highway cruising, the output is a higher gear ratio that provides ideal fuel mileage.


In an era when cars were not able to travel very fast, the maximum power point might be near enough to the desired speed that additional gears were not needed.

But as more powerful cars appeared, especially during the s, this disparity between the maximum power point and desired speed grew considerably. This meant that cars were often operating far from their most efficient point.

As the desire for better fuel economy grew, especially after the oil crisisthe need for a “cruising gear” became more pressing. The obvious solution to this problem would be to add more gears to the transmission. Indeed, in modern vehicles this is common. However, due to historical particularities, this was not always practical. In the conventional rear-wheel drive layoutthe transmission system normally contained two sections, the “gearbox” or “transmission” mounted behind the engine, and the “final drive” mounted in the rear axle at the rear of the car.

The reason for this separation of duties between the front and back of the car was to allow the drive shaft to run at lower torque, by using higher RPM. As power is the product of RPM and torquerunning the shaft at higher RPM allowed more power to be transferred at lower torque.

Doing so reduced the torque the driveshaft had to carry, and thus the strength and weight it required.

Land Rover S3C / Fairey Overdrive Fitting Instructions

Although the designer was theoretically free to choose any ratio for the gearbox and final drive, there is one additional consideration which meant that the top gear of most gearboxes was 1: This is chosen for efficiency, as it does not require any gears to transmit power and so reduces the power lost by them. This was particularly important in the early days of cars, as their straight-cut gears were poorly finished, noisy and inefficient. The final drive then took this output and adjusted it in a fixed-ratio transmission arrangement that was much simpler to fairry.

Final drive ratios of 4: In an era when different models of car with different wheel sizes could be accommodated by simply changing the final drive ratio, it made sense for all transmissions to use direct drive as the highest gear. As noted earlier, however, this would cause the engine to operate at too high an RPM for efficient cruising.

Although adding the cruising gear to the main gearbox was possible, it was generally simpler to add a separate two-gear overdrive system to the existing gearbox. This not only meant that it could be tuned for different vehicles, but had the additional advantage that it could be offered as an easily installed option.

With the fairfy of front-wheel drive layouts, the gearbox and final drive are combined into a single transaxle. There is no longer a drive shaft between them and so the notion of overcrive drive” isn’t applicable. Although “overdrive” is still referred to, this is now mostly a marketing term to refer to any extra-high ratio for efficient cruising, whether it is achieved through the gearbox ratios, or by an unusually high final drive.

Generally speaking, overdrive is the highest overdrlve in the transmission. Overdrive allows the engine to operate at a lower RPM for a given road speed. This allows the vehicle to achieve better fuel efficiency, and often quieter operation on the highway.

When it is off, the automatic transmission shifting is limited to the lower gears.

The automatic transmission automatically shifts from OD to direct drive when more load is present. When less load is present, it shifts back to OD.

Fairey Overdrive Fitting Instructions

Under certain conditions, for example driving uphill, or towing a trailer, the transmission may “hunt” between OD and the next highest gear, shifting back and forth. In this case, switching it off can help the transmission to “decide”.

It may also be advantageous to switch it off if engine braking is desired, for example when driving downhill. The vehicle’s owner’s manual will often contain information and suitable procedures regarding such situations, for each given vehicle. Virtually all vehicles cars and trucks have overdrive today whether manual transmission or automatic. In the automotive aftermarket you can also retrofit overdrive overdribe existing early transmissions.


Overdrive was widely used in European automobiles with manual transmission in the 60s and 70s to improve mileage and sport driving as a bolt-on option but it became increasingly more common for later transmissions to have this gear built in.

If a vehicle is equipped with a bolt-on overdrive e. GKN or Gear Vendors as opposed to having an overdrive built in one will typically have the option to use the overdrive in more gears than just the top gear. In this case gear changing is still possible in all gears, even with overdrive disconnected. Overdrive simply adds effective ranges to the gears, thus overdrive third and fourth become in effect “third-and-a-half” and a fifth gear. In practice this gives the driver more ratios which are closer together providing greater flexibility particularly in performance cars.

An overdrive consists of an electrically or hydraulically operated epicyclic gear train bolted behind the transmission unit.

It can either couple the input driveshaft directly to the output shaft or propeller shaft 1: Thus the output shaft may be “overdriven” relative to the input shaft. Overdrice these cases, there is no separately identifiable “overdrive” unit.

In older vehicles, it is sometimes actuated by a knob or button, often incorporated into the gearshift knob, and does not require operation of the clutch.

Newer vehicles have electronic overdrive in which the computer automatically adjusts to the conditions of power need and load. The vast majority of overdrives in European cars were invented and developed by a man called de Normanville and manufactured by an English company called Laycock Engineering later GKN Laycockat its Little London Road faorey in Sheffield.

Fairfy system was devised by Captain Edgar J de Normanville —[2] and made by Laycock through a chance meeting with a Laycock Products Engineer. Pverdrive British company, the former aircraft builder Faireybuilt a successful all-mechanical unit for the Land Roverwhich is still in production in America today. The first production vehicle to feature the Laycock system was the Standard Vanguard Saloon. The first unit to be created was the A-type overdrive, which was fitted to many sports cars during the s, and into the late s.

Inthe Laycock Engineering Company introduced the D-type overdrive, which was fitted to a variety of motor cars including Volvo and sSunbeam Alpines and RapiersTriumph Spitfiresand also MGBs those with 3-synchro transmissions.

The P-type overdrive marked the last updates and included both a Gear Vendors U. The Volvo version kept the same package size as the J-type but with the updated 18 element freewheel and stronger splines through the planet carrier.

Land Rover Restoration, Tech, Maintenance and Repair – O

The Gear Vendors U. Over a period of 40 years, Laycock Engineering manufactured over three and a overdrige million overdrive Units, and over one million of these were fitted to Volvo motorcars. In the U. The system features an oil pressure operated device attached to the back of the standard gearbox operating on the gearbox output shaft. For instance, the overdrive system applied to overdrve Triumph TR5 operates on 2nd, 3rd and top gear. When engaged, mahual overdrive would drop the revs from by RPM, or from the drop would be RPM to net.

In the days before automatic transmissions were common, especially in the s, many rear-wheel drive American cars were available with an overdrive option. Borg Warner provided the box that was factory-installed between the transmission and a foreshortened driveshaft. Since the overdrive vairey, if enabled, could be shifted by simply easing up on the accelerator without depressing the clutch pedalthe action was much like a semi-automatic.

Also, an electrically operated solenoid would deactivate the unit via a switch under the accelerator pedal providing the equivalent of the kickdown of the automatic. A knob connected to a bowden cablesimilar to some emergency brake applications, was also provided to lock out the unit mechanically.