For more than 100 years, Harley-Davidson has been producing its legendary lineup of Big Twin engines. Through the decades, various technological advances have led the company to develop nine different types of basic Big Twin motors.
The original Harley Big Twin engines, the F-Head models were IOE, or intake/inlet over exhaust- type motors. These simple yet powerful engines were available in 61 cubic inch and 74 cubic inch sizes, and were produced from 1914 through 1929.
Named for its flat-topped, vented cylinder heads, the side valve-equipped 45 cubic inch flathead engine debuted in 1929, and the 74 cubic inch V model Big Twin came out in 1930. The Big Twin model was built in part to compete with the 74 cubic inch Indian Chief.
In 1937, the U series of Harley-Davidson flathead Big Twin engines made their debut, replacing the V series. The U and UL models featured 74 cubic inch powerplants, and the UH and the ULH models were outfitted with 80 cubic inch engines. The 80 cubic inch models were produced until 1941, and the 74 cubic inch U and UL models were in production until 1948.
The three-wheeled Harley-Davidson Servi-Cars made from the early 1930s through 1975, were powered by flathead engines during their entire run of production.
Made from 1936 through 1947, the Harley-Davidson Knucklehead motor was Harley's first production bike to have overhead valves. A more-efficient circulating oil lubrication system on the knucklehead replaced the "total-loss" lubrication system that previous models featured. Also known as the EL model, the knucklehead got its name from its rocker boxes that looked like knuckles on a human fist. Both 61 cubic inch and 74 cubic inch models of this Big Twin engine were produced by Harley-Davidson.
This Big Twin engine gets its name from the distinctive cake pan-like appearance of the rocker covers. Available in 61 cubic inch EL and 74 cubic inch FL and FLH models, this engine was produced from 1948 through 1965. The Harley-Davidson Panhead was equipped with aluminum heads and hydraulic valve lifters, and the 1965 Electra Glide model brought about the introduction of electric starters on Harley-Davidsons. The Panhead is widely considered to be the most attractive of all Harley-Davidson Big Twin engines.
Produced from 1966 until 1984, the Harley Shovelhead engine was available in 74 cubic inch and 80 cubic inch models. The Shovelhead was designed in part to produce more power and higher performance to make up for the increased weight of new Harley-Davidson motorcycle models.
This Harley-Davidson Big Twin motor also gained its name because of the appearance of the rocker covers. Somewhat resembling the knucklehead, the slightly rounded Shovelhead rocker box covers are reminiscent of small shovels, with the push rod tubes serving as handles.
With the Evolution engine, Harley-Davidson had an engine that not only evolved from the Shovelhead, but was very different, and technically superior in a number of ways. The Harley Evo was not only more powerful, but it ran cooler and smoother than the Shovelhead. The 80 cubic inch Evolution motor was produced between 1984 and 1999.
In 1998, Harley-Davidson introduced the eighty-eight cubic inch Twin Cam engine for the 1999 model year, and continued production of different Twin Cam models until 2016. A 96 cubic inch Twin Cam model, as well as a 103 cubic inch, and a 110 cubic inch model were also produced. This model was named for the two chain-driven cams it contains. The Twin Cam 88B engine was a counter-balanced Twin Cam 88 motor that was designed to fit Harley-Davidson Softail models.
The Twin Cam engine provided more torque and horsepower than the Evolution model, and was originally available with a choice of carburetor or fuel injection.
Issues with oil circulation on the Evolution engine prompted Harley-Davidson to outfit the Twin Cam with a better-performing internal twin-gerotor oil pump.
The Twin Cam motor offered higher compression than its predecessor, and a dual-coil ignition system that eliminated wasted spark.
In place of the familiar 45-degree V-Twin engines that Harley is known for, the Revolution engine is a liquid-cooled 60-degree V-Twin powerhouse. This engine has been standard equipment on Harley-Davidson's cruiser/muscle bike, the VRSC, or V-Rod since 2001. Originally available as a 69 cubic inch dynamo, the Revolution was beefed-up to 76 cubic inches in 2008.
Designed to provide competition to both imported and domestic popular street/cruiser bikes, the Revolution engine is actually a collaborative effort between Harley-Davidson and Porsche.
In 2016, Harley-Davidson announced that all 2017 touring and trike motorcycle models would be equipped with their new Milwaukee-Eight engines. These new Harley-Davidson Big Twin motors include a 107 cubic inch model, a liquid-cooled version of the 107 cubic inch machine, and a 114 cubic inch, liquid-cooled model.
The Milwaukee-Eight engine features Harley's traditional 45-degree V-Twin design. The engine provides more torque and overall power than previous models, and is counter-balanced to reduce vibration.
Each cylinder utilizes two spark plugs, and each cylinder head contains four valves, that increase the capacity of the intake and exhaust flow. A development objective of providing a cooler-running machine, has been achieved by the inclusion of an improved heat management system.
The various types of Big Twin engines manufactured by Harley-Davidson through the years have revolutionized the motorcycle industry. If history is any indicator, Harley-Davidson will continue to blaze new paths with these amazing engines well into the future.