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The Rolls-Royce & Packard V-1650 "Merlin" Engine

History of the Rolls Royce Merlin Engine


Aviation enthusiasts are aware of the incredible story of the Rolls Royce Merlin Engine marriage with the North American P51 Mustang. But it is worth nothing that although it is tempting to dismiss in one sentence the entire process akin to a hotroder swapping a Chevy V8 into a 1932 Ford, in reality the whole complex process required over 223,000 engineering hours, almost three times the 78,000 hours to design the original Prototype Mustang! Remember, this was in an era when design work was accomplished without cad cams, e-mail, computers, etc. but with slide rules, T-squares and 2H size pencils. It is, however, perhaps worthwhile to reflect on the circumstances that allowed this marriage to be consummated.

The New York Herald Tribune, commenting on the New Mustang said "Many have long regarded it as the best fighter plane produced in the states, but it remained for the British to discover it." If it had not been for the initial British orders it would not have been developed at all. Its full potentialities were brought out only when the British designed Rolls Royce Merlin was installed.

The gestation of the Merlin began with the famous R type engine of the Supermarine SG Schnieder Cup Seaplane racers in 1929 with the ultimate goal of developing a reliable engine suitable for military application developing 1000 horsepower. Succeed they did, with at least eleven major manifestations designed for various mission applications and/or improvements in power or reliability culminating in a tribute to the genius of the basic design with the Reno Air Race Merlin Mustangs ultimately producing over 3000 h.p. at 3800 rpm!

Besides its use in the Mustang, the Merlin was used extensively in many other British Designs. Most notably, the Spitfire, Hurricane, Mosquito, and Lancaster, as well as the Halifax, Whitely, Wellington, Barracuda, Beaufighter, Lincoln, and Battle. It even had civil application following the war in the Avro York and Douglas De-4m before the jet engine brought an end to further development.

Of the total of 150,000 total Merlin engines produced, 60,000 were built under license in Detroit, Michigan at the Packard Motor Company, a grand old name in the annals of the motor city famous for its slogan "Ask the Man Who Owns One". Packard, of course was a famous builder of V-12 powered classic luxury automobiles that in all probability had not produced 60,000 cars in all of its previous history. It must have been with some trepidation that the proprietary engineering secrets of the Rolls Royce Co would be relegated to one of their primary competitors in the "Glory Days' of the pre-war auto industry. Fortunately, the urgent realities of the war prevailed and under the 24 hours a day 7 days a week regiment that ensued, Merlin engines were soon being produced at an unprecedented rate.

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Packard V-1650 Merlin Engine

Packard V-1650
Merlin Engine

Consider that not only the ability to cast, forge, and machine the hundreds of parts required to assemble the Merlin was rapidly developed but that it was necessary to adapt the basic design to American Standard Fasteners, splines, gearing etc. Further, that both major and minor innovations and improvements were ongoing throughout its production life - items ranging from the two piece engine block to improved alloys and various machining techniques to continually improve the reliability of this magnificent power plant. It is, after all, one thing for an incredibly complex piece of machinery to be developed and assembled by a team of highly skilled engineers and technicians - and pampered by same under controlled conditions. But it is truly one of the great unappreciated feats of the Rolls Royce/Packard Partnership that the Merlin engine could be assembled by "the butcher, the baker, and the candlestick maker" and, not to be forgotten, scores of former housewives who became highly skilled workers in response to their country's call. The result of their efforts was then sent to perform flawlessly under life/death conditions in climate conditions ranging from the numbing cold of the skies over Europe to the blowing sands of the North African Deserts, to the steaming tropics of the South Pacific.

In September 1940, the Packard Co. agreed to build the Merlin engine for both the U.S. and British Governments and adapted it for American mass-production methods. The USAAF used the engine almost exclusively in P-51. It gave greatly improved high-altitude performance over the Allison V-1710 engine.

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The V-1650 liquid-cooled engine was the U.S. version of the famous British Rolls-Royce "Merlin" engine which powered the "Spitfire" and "Hurricane" fighters during the Battle of Britain in 1940. In Sept. 1940, the Packard Co. agreed to build the Merlin engine for both the American and the British Governments, and adapted it for American mass-production methods. The first two Packard-built Merlins to be completed were demonstrated on test stands at a special ceremony at the Packard plant in Detroit on August 2, 1941. Full production began in 1942 and by the end of World War II, 55,873 Merlins had been produced in the U.S.A. The Army Air Forces used the engine almost exclusively in the famed P-51 "Mustang", for it provided greatly improved high-altitude performance over the Allison V-1710 engine used in earlier series of the airplane. The V-1650 Merlin also replaced the V-1710 in the "F" series of the P-40. The British also used Packard-built Merlin's during the last three years of the war in their "Spitfire", "Mosquito", and "Lancaster" airplanes.


Model: V-1650-7
Type: 12-cylinder with two-stage mechanically-driven supercharger
Displacement: 1,649
Weight: 1,690 lbs.
Max. RPM: 3,000
Max. HP: 1,695
Cost: $25,000


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Schematic of Rolls Royce Merlin 1650 Engine


The idea of a marriage of the P-51 airframe and the Rolls Royce Merlin engine began long before anyone had ever heard of a Mustang. And, contrary to popular belief, this was not exclusively a British idea. Why the Merlin? The answer to that question is simple expediency! The Rolls Royce Merlin engine was a proven design and available at the time. It was already powering some of the world's best warplanes - the Hurricane, Avro Lancaster and, of course, the superb Supermarine Spitfire (at left). To develop a new engine for the P-51 comparable to the Merlin required time, time the Allies simply did not have. Remember this was mid-1942 and the United States was still reeling from the Pearl Harbor attack, Great Britain was still standing alone opposite an entirely hostile continent and Soviet Russia was still retreating before the Nazi Blitzkrieg. The Allies needed weapons to fight the tide of defeat and they needed them right away! One of those weapons would be the Merlin-powered Mustang.

The Rolls Royce Merlin engine had begun development in 1925. That was the year that Rolls Royce produced the first aluminum engine block. Previous engines had had jacketed cylinder liners. It got its power from a (then) radical overhead valve, four valves per cylinder head. With supercharging, the Kestral V-12 (Rolls Royce had a habit of naming their engines after birds.) developed 690hp at 11,000ft altitude and 745hp at 14,500ft. The basic Kestral design evolved into the Buzzard which produced 955hp. This engine, with the addition of a two-sided impeller in the blower, developed a fantastic 1900hp at 2900rpm. It was called the R engine. This was the engine that powered the Supermarine S-6 racing seaplane to a top speed of 357mph in the 1929 Schneider Cup Race - a new World's Speed Record. Further development, mainly in the supercharger, led to 2530hp and 407.5mph. Remember, this is in 1929 and the aircraft was not a sleek fighter plane, but one with huge floats hanging from the wings. The engine could turn 1000rpm faster than other designs and do it at 100psi more torque pressure.

The Rolls Royce Merlin, named after a small European falcon (not the legendary wizard), began tests in October 1933. The engine incorporated all the latest advances, such as 4:1 boost ratio in the supercharger and the first use of fuel injected directly into the blower. In September 1940, Packard Motor Car Company began license production of the Merlin engine at a Detroit facility. It was known as the Merlin 28 in Great Britain, or V-1650-1 (see below) in US Army terminology. Packard-built Merlins were the first to have engine main bearings of a silver/lead alloy. Intake and exhaust valves were coated with an 80% nickel 20% chromium alloy for much greater heat resistance. These two items made for longer engine life. A new Wright-designed supercharger drive put the finishing touches on the Packard/Merlin V-1650-3, the powerplant destined for the P-51.

The supercharger design was the real key to the  performance. A two-speed/two-stage design with tolerances measured in millionths of an inch. What the supercharger did was keep atmospheric pressure inside the induction system equal to sea level pressure. It did this so much better than the Allison design that a Merlin developed more horsepower at 26,000ft than an Allison did in full power setting for take-offs! The problems with such a system are in cooling the fuel-air mixture, which has been heated by the compression of the supercharger, before it gets to the cylinder. A cooler fuel-air mixture results in a denser mixture in the cylinder, which results in more power. Cooling of the mixture was done through use of an 'intercooler' passage between the first and second stages of the supercharger, and an aftercooler between the blower outlet and the intake manifold.

The smooth, reliable power of the Merlin was mated to the ultra-smooth, sleek Mustang airframe in the summer of 1942. On both sides of the Atlantic, simultaneous and independent studies had been underway concerning the feasibility of a Merlin/Mustang combination. In Great Britain, a Mustang 1, AL975, was modified to accept a Merlin 65 engine. Entirely new engine mounts were fabricated along with new metal panels to cover the slightly larger Merlin engine. The carburetor air scoop on top of the nose was deleted and a much larger air scoop for the intercooler was added to the underside of the nose. Finally a four-bladed Spitfire prop was added. The project was known as Mustang X. Eventually five Mustang I airframes were so modified. The first flight of the Mustang X took place on October 13, 1942. Take-off was smooth but at 376mph the cowling came loose and the pilot was forced to land. Later that same day, the Mustang X hit a top speed of 390mph. By the sixth test flight on October 19 it had been discovered that an intercooler was not needed when a new Bendix pressure carburetor was used. This meant the scoop could be cut down, making the nose much more aerodynamic. An all new prop of 11'4" diameter was designed for the Mustang/Merlin combination. The end result for the Mustang X was 432mph at 22,000ft.

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At North American Aviation in the US, a similar program was underway. A Packard-built V-1650-3 Merlin was installed in P-51 41-37352, one of two P-51s held back from the original production line specifically for Merlin engine tests. As with the British installation, all new engine mounts and sheet metal had to be fabricated to fit the taller Merlin The finishing touch was a four-blade Hamilton Standard prop with 'cuffs.' Prop'cuffs' had been tested back in 1940 and found to greatly add to propeller efficiency. The Merlin installation in the Mustang required 223,000 engineering hours, compared to the 78,00 engineering hours required to make the NA-73X.

When North American Aviation began production of the P-51D, every modification that had been made to the Merlin engine would be incorporated into the new model. The P-51D used the Packard-Merlin V-1650-7 engine which was the same as late model P-51Bs and Cs. Lower supercharger gear ratios resulted in much better low altitude performance. Maximum power from the -7 was 1750hp in War Emergency Low Blower setting at 3000rpm.

Davis, L.  Carrollton



The Merlin was developed in England in 1936, and used in a prototype Spitfire F39/34. The first production Spitfire in 1939 had a 1030 hp Rolls-Royce Merlin MK II engine.

The Merlin went through continuous development throughout World War Two, ending up with at MK 71. The Merlin series was then superseded by the Griffon series.

In early 1941, Rolls-Royce licensed Packard Motors to build Merlin engines. By 1943, the Mustang P51B & C (RAF Mustang III) had a 1520 hp, V1650-3 Packard Merlin engine. In Canada the Packard Merlin's were designated Merlin 28 and 29.

Standard engine for the P-5ID was the liquid-cooled, l2-cylinder, Packard-built, Rolls-Royce Merlin V-1650-3 or -7 developing 1,400 hp at take-off. The original Mustangs were fitted with the low-altitude rated Allison engine, but as the possibilities of the Mustang as a high-altitude fighter became realized, it was decided to fit a Merlin engine. For this purpose, four Mustang Mark Is were sent to Rolls-Royce for use as development aircraft, AL963, AL975, AM203 and AM208. They had Merlin 61 series engines installed with a frontal radiator, in addition to the normal ventral scoop. The Mustang/Rolls-Royce combination was an instant success and it was adopted as standard for all the Mustang variants. To increase the flow of engines, the Packard Car Company of America built the Merlin under license.

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The Merlin was fitted with an injection-type carburetor and a two-stage supercharger. The the -3 engine supercharger cut-in at 19,000 feet, and on the -7, between 14,500 and 19,000 feet. The supercharger was automatic, but could be manually over ridden. In order to give the engine an extra burst of power during an emergency, the throttle could be pushed past the gate stop by breaking the safety wire. If used longer than five minutes, there was a risk of severe engine damage.

Mustang pilots were left in no doubt when the supercharger cut into the high-blower position, for the aircraft shuddered violently. They had to learn to anticipate the cut-in and reduce throttle. When descending, the change to low-blower took place at about 14,500 feet, and the only indication of the event was a drop in manifold pressure. The Packard Merlin drove either a four-blade Hamilton-Standard Hydromatic or Aeroproducts automatic, constant-speed propeller. Coolant (30/70 ethylene-glycol/water) and oil radiators were installed in the pronounced belly scoop radiator fairing under the fuselage.

One weakness of the Merlin was that it could be put out of action by a single bullet, or piece of shrapnel, but this applied to all liquid-cooled engines, and did not detract from the Mustang's all-round capabilities. The aircraft was a welcome sight to the Fortress crews as they plunged deep into German skies during the daylight offensive against the Nazi armament industries.


Arguably one of the best engines ever to be widely produced during World War II, the Rolls-Royce Merlin engine was without a doubt one of the most successfully employed aircraft engines in history.  During World War II, the Merlin could be found on any number of different types of allied aircraft, from fighters to bombers, but it was when the engine was introduced to the P-51 Mustang that it changed the face of the war.

The Merlin is a 12 cylinder, 60º “V”, 27 liter, liquid cooled piston aircraft engine capable of mustering 1700 horsepower.  The first Merlin was built in 1936 by Rolls-Royce to fill the 700 and 1,500 hp gap in their aero engine line.  Originally designed as the 1,100 hp PV-12 (PV for Private Venture), the first Merlin only saw a production run of 172 before the adoption of the Merlin II.  The difference between the two was the angle of the inlet valves to the cylinders; the Merlin I had the valves at a 45º angle to the cylinder where as the Merlin II had a conventional flat head arrangement with the valves being parallel to the cylinder.  These early Merlin engines were considered to be relatively unreliable, but thanks in part to a quality control program developed by Rolls-Royce, the Merlin matured and was able to run at full power for eight hours at a time with no problems.

With the introduction of the Merlin in 1937, Rolls-Royce employed fewer than 7,000 people.  By the end of World War II, the company had more than 55,000 employees and produced more than 100,000 Merlin engines (an additional 60,000 Merlins were built under license by Packard in the United States).  19 different types of aircraft were outfitted with Merlin engines including the Spitfire, P-51, Mosquito, Lancaster, and the P-40L with power outputs ranging from 1,000 hp to over 2,000 hp in the Merlin 66.

In 1940 and agreement was reached between the Packard company in Detroit and Rolls-Royce that would allow Packard to produce Merlin engines.  These American built engines were produced by an assembly line, rather than the Europeans method of meticulously hand building each engine, but surprisingly enough were just as well made and actually improved the maintainability of the engine by allowing easier use of interchangeable parts (as opposed to custom finished ones).

Because of the Merlin’s dynamic two-speed, two-stage (in later models) engine driven supercharger, it was capable of delivering high power at altitudes in excess of 30,000 feet.  This made the engine the ideal choice for one of the newest American and British fighters, the P-51 Mustang.  The first of the Mustangs, the P-51A’s, were outfitted with Allison V-1710 engines.  These engines, due to GE’s inability to produce a sufficient quantity, lacked the General Electric turbochargers that would enable the P-51 to have suitable high altitude performance.  The P-51A was forced to stay relatively low because of this (below 20,000 feet) and was not able to fly escort for the allies’ high altitude bombers.  In 1943, the next model of the Mustang took to the skies with a Packard Merlin V-1560 and outperformed the P-51A in all aspects.  The rate of climb had been doubled at all altitudes, and the P-51B’s level flight speed at 29,800 feet was 441 mph, a full 100 mph faster than the P-51A at that altitude.

The Mustang went on to strike fear into the heart of the Axis powers with its newfound ability to fight just as well up high as it could down low.  According to the pilots who flew them, the new Merlin-powered Mustangs were some of the most honest and well-performing aircraft they had ever flown. These P-51s were also some of the only piston powered airplanes that would ever shoot down enemy jets.  When the Germans introduced the ME-262 jet fighter, it was the P-51 Mustang with a Merlin under the hood that kept the plane in the fight and led to more than several ME-262 (as well as other jets) losses to the P-51’s six .50 caliber machine guns.  The Mustang had become the United States’ premier air superiority fighter in the European Theater of Operation in the last few years of the war, and thusly securing the Allied victory over Germany.

After World War II, the Merlin found a niche in boat racing when the boat Slo-mo-shun V took first place at the Seattle Gold Cup Race in 1954.  Merlins continued to be used in boat racing all the way through the 1980s with great success.  The Merlin engine also saw installation on a very few amount of automobiles.  John Dodds, with assistance from Paul Jameson, fitted the engine and chassis to a Ford Capri body and called it “The Beast.”  At one point, “The Beast” was listed in the Guinness Book of World Records as the most powerful road car.  A small handful of other Merlin powered cars were developed, but no matter what other vehicles the Merlin was installed in, it was the Merlin-engined P-51 Mustang that made the engine one of the most well-known, beautifully engineered pieces of equipment ever designed by Rolls-Royce.




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