It found its most notable application in the North American P-51 Mustang fighter, improving the aircraft's performance so it could escort Allied heavy bombers from Britain to Germany and back.

An agreement was reached between Rolls-Royce and the Packard Motor Car Company in September 1940 to manufacture the Merlin under license, with a $130,000,000 order being placed.

This engine used a single-stage, two-speed supercharger, the gear changing mechanism of which originally came from a French Farman patent license.

The initial Packard modifications to this engine changed the main crankshaft bearings from a copper-lead alloy to a silver-lead combination and featured indium plating.

A hydraulic gear change arrangement of oil-operated clutches could be engaged by an electric solenoid to increase this ratio to 8.095:1 in high speed blower position.

The double staging of the compressed fuel/air mixture provided the boost pressure through a diffuser to the intake manifolds that increased the critical altitude of the power plant.

[citation needed] The ability of the supercharger to maintain a sea level pressure in the induction system to the cylinders allowed the Packard Merlin to develop more than 1,270 horsepower (950 kW) above 30,000 feet (9,100 m).

As air density decreases with increased altitude, the throttle valves were progressively opened in response to the reducing atmospheric pressure.

Innovations, such as the use of Allison V-1710 connecting rods and the replacement of the intercooler with ADI (Anti-Detonation Injection) (50% Distilled Water and 50% Methanol), nearly identical in chemical composition to the Luftwaffe's wartime MW 50 system, and similar to the water injection system used on Pratt & Whitney engines during World War II, have allowed great increases in power output.

[13][14] Many of the fastest Unlimited racers increase Merlin manifold pressures as high as 145 inHg (56.6 psi, 4.8 atm) to obtain up to 2,835 kW (3,800 horsepower), achieving Mustang speeds up to 490 mph.