The overall success of this famous engine is built on one simple fact. By employing technology many years ahead of its time, fueled by the urgency of war, Rolls-Royce produced a powerplant that still has no equal as a piston fighter powerplant.

Ironically, but not untypical of British industrial practice in the 20th Century, the British government, through the Air Ministry, declined to fund a new engine despite the Schneider Trophy success of the early 1930's. In December 1932 the Rolls-Royce board, in the true spirit of British entrepreneurship, elected to finance the engine themselves as a private venture. Thus, early development of the engine were designated P.V.12 No one could have envisioned that this step would contribute so significantly to the allied victory in the air.

Few would dispute the Battle of Britain was won as a result of the Merlin II and III. Another significant development was the brilliant discovery, by a scientist at the Royal Aeronautical Establishment, Miss Shilling, who designed a most simple system - a small metal diaphragm in the float chamber of the carburetor - that enabled the engine to continue running in negative G situations in combat.

1938 saw Stanley Hooker join Rolls-Royce. It was Hooker who concentrated on the development of the Merlin's supercharger. Even modern day Merlin rebuilders will tell you the engine's supercharger is the finest of its type ever produced. Why? It improves the power envelope from 12,000 to 40,000 feet. This unit is at the heart of the engines remarkable performance over a significant altitude range.

At the beginning of World War II and the beginning of the Battle of Britain, every front line fighter aircraft was fitted with the Merlin. One of the main Luftwaffe tactics was low flying and the sea level power of the Merlin had to be enhanced as a result. Rolls-Royce achieved a 40% increase in power at sea level due to the supercharger development and this forced the Luftwaffe to fly higher to combat air superiority at low level. With the success of supercharger development the Merlin XX and Merlin 45 were introduced into the Spitfire and Hurricane and this enabled the Royal Air Force to maintain fighter superiority.

During the war years over 150,000 Merlin engines were manufactured in the U.K. and U.S.A. The Packard Motor Car Company undertook the latter and it was this version that was to power the legendary North American Mustang. However, unrecognized by many are the huge differences in the Packard built version. Packard manufactured some 57,000 Merlin engines, over a third of all Merlin production. From its earliest version the Merlin was clearly a precision, hand built work of engineering art constructed by some of the best British engineers. Even in its infancy the powerplant was built in small lots and had a string of design changes incorporated in an effort to squeeze performance and increase reliability in combat situations. The setback was this was not an engine designed for mass production, so in 1940, when the British Government turned to Packard for help with production of the Merlin some provisions had to be made.

In June 1940, Packard was requested to undertake the production of some 9000 Merlin XX engines. It took just three days for the company to respond positively but on that proviso that some modifications would be made to the powerplant to enable American accessories such as carburetors, fuel and vacuum pumps to be utilized. Packard then began the monumental task of organizing an engineering group to handle drawings and redesign for American production. In parallel Packard organized U.S. sources for carburetors, magnetos, spark plugs, and other accessories. Packard assigned some 200 personnel to the project and this included the draughtsmen who would undertake complete revisions of production drawings. At the outset the Packard engineers ran into problems, generally due to the fact that the engine was not designed for mass production. That the project was completed at all is testament to the determination and skill of those assigned the task.

After the end of hostilities in World War II, the Merlin saw extensive commercial service, particularly in Great Britain and Canada, extending well into the late 1950's. The Merlin 600 and 700 series were the subject of most testing and modification until as late as the 1960's. Since the Merlin emerged there have been three categories of operator in aviation. The use of the engine in battle tanks and boats is beyond the scope of this chapter. Broadly, these can be categorized as the originally Military operators, commercial (such as airlines) and right up to the present day, civilian warbird operators.

When commercial operators began using the Merlin cost became an important consideration. Most airline operators know the profit is in the last 20%. Civilian operators logged more TBO (time between overhaul) on their Merlin engines than military operators by a significant amount and consequently experienced associated problems military engineering teams never witnessed. Importantly, it was not cost effective to replace every component when it was worn past military service limits, so Rolls-Royce developed countless repair schemes and legal modifications to salvage otherwise unserviceable parts. Hence this extended the running life of parts in the engine. The repair schemes and modifications were the result of hours of re-engineering, development, and testing in the field. As these schemes became available for release, they had to have approval from Great Britain's Air Ministry. Many of these repair schemes are still in (legal) use today.

In the late 1950's the Mustang was released for sale to the general public. Most airframes sold had low times and consequently warbird owners enjoyed a period of three decades where engine overhaul posed little problem, and very often a spare engine (or two) had been acquired with the airframe anyway. Overhauls undertaken during this time were similar to military schedules where the engines were disassembled, cleaned, and reassembled. Though new seals and rings were sometimes installed major rework was just not necessary.