The Modern Motorcycle Engine

By David Tong

It was not long ago, the 1960s, when overhead-valve, pushrod actuated, two-valve, air-cooled, single and twin cylinder engines ruled the worldwide motorcycle roost. While Honda racing engines debuted the double-overhead camshaft head and over-square (larger bore diameter than crankshaft throw length) design, the preponderance of motorcycle engines in those days were essentially updated versions of 1930s ideas.

British manufacturers, such as BSA, Norton, Triumph and Royal Enfield, produced a series of roadsters from 200cc to 750cc, the Italians had Moto-Guzzi 750 Eldorados and V7 Sports. The Germans had their OHV BMW flat-twins, while in the US we had Harley-Davidson 61cu in Sportsters and 74 cu. in. Electra-Glides. All of these were powered by four-cycle engines built to the pattern described above. (Most Japanese motorcycles, except for Hondas, were powered by small displacement two-cycle engines in the 1960s.)

These engines were generally not made to turn lots of revolutions, due to their design. Their stroke length, the distance the piston travels up and down, was a larger dimension than the piston diameter (under-square). Usually, these engines were originally designed for relatively low compression and mild camshaft timing, due to low octane petrol, and many of them lacked a center main bearing, so there was some crankshaft flex.

They also had two rather large valves at or nearly at a 90-degree angle (the included angle). This means the flame travel was not optimal for building horsepower, as there was no room at the center top of the head for the spark plug for good flame propagation and combustion. The large valves also precluded high domed pistons for very high compression, because of the danger of pistons hitting valves at top dead center.

Finally, all engines of the era were air-cooled, which also limited the amount of power an engine could produce, as well as shortened their reliable lifespan. None of the mentioned bikes would run much beyond 25,000-50,000 miles before requiring a top-end rebuild. This would usually consist of new valve guides, regrinding valves and seats and the fitting of the next oversized pistons and honing out the iron cylinders or liners to accept them.

Honda, in the mid-1960s, fired the first salvo of technology with their CB175, 250 Dream, 305 Super Hawk, CB350 and CB450 air-cooled overhead cam twins. All of a sudden you had engines that revved to nearly 9,000 RPM in some of these bikes, versus no more than 6,000 safe RPM on any of the other older bikes.

As we all know, the once dominant British motorcycle industry was crippled by a lack of R&D money, failure to adopt manufacturing improvements, bad management, poor quality control and labor strikes throughout the late 1960s through the mid-1970s. Around the same time, Harley nearly went bankrupt for similar reasons.

Old engines blew gaskets, loosened their fasteners, leaked oil and even the new Hondas, as well as the older designs, vibrated like mad. I have ridden a CB175 and a Super Hawk and the former was sweet running with a good gearbox for the era, while the 305 vibrated and had a clunky transmission, but revved and ran great in the upper RPM band.

I have also ridden many of the 1960s and 1970s BMW, BSA, Triumph and Norton motorcycles. Only the BMWs could be trusted to run without constant fiddling and other than routine maintenance and repair.

More than any other, the bike that put paid to the Europeans was the 1969 Honda CB750. This was the first four-cylinder, mainstream production motorcycle with an overhead camshaft (SOHC) and over-square engine design. It also had a hydraulic front disc brake. It represented a technological breakthrough for road bikes.

While the British lingered on for about the next 8-10 years with vertical twins and OHV triples, by 1982 they were finished. Moto Guzzi managed to stay afloat, because they have continuously improved their 90-degree, transverse V-twin, even adding overhead cams and four valve heads in the late 1990s.

Harley-Davidson's new management saved their company by introducing the Evolution engine in 1984. Quality improvements, better design and tighter manufacturing tolerances made the Evo motors oil tight, horsepower was increased and the engines ran cooler, due to their aluminum cylinders. Engine life without major rebuild was increased to 100,000 miles, or more.

In 1999, H-D replaced the Evo big twin with the Twin Cam big twin, which incorporated many improvements, including another increase in horsepower and reliability. As always with H-D big twins, the center of gravity of the Twin Cam engine is down low and the overhead valves are self-adjusting to minimize maintenance and expense.

What has changed the dynamic since the mid-1980s has been one of the dominant themes in engines: the liquid-cooled, double-overhead cam (DOHC), multi-cylinder design with electronic fuel injection. First developed for racing, they quickly gained market acceptance by all four major Japanese makers and many of the Europeans, despite their increased bulk and top weight.

Liquid-cooling keeps temperatures more consistent throughout the cylinders and heads, allowing more power to be produced. It also dampens top end noise to appease government regulators. Some of these liquid-cooled engines were cleaner environmentally, more fuel efficient and ran longer without rebuilds. Liquid cooling systems nearly always include a large, ungainly looking radiator located immediately aft of the front tire. (Of course, liquid cooling can freeze, leak, the radiator is vulnerable to damage and it adds a lot of undesirable weight to the motorcycle. -Editor.)

The newest Harley-Davidson touring bikes now feature liquid cooled cylinder heads with air cooled cylinders. BMW was about a year ahead of H-D in introducing this technology to their 1200cc twins. It is a tacit admission of the things to come for all bikes that are still air-cooled, in order to meet the emissions and noise requirements imposed by government regulators with nothing better to do.

What is interesting is that traditionalists are also catered to. Modern motorcycle engines include single cylinder, parallel twin, V-twin and opposed twin cylinder layouts. V-twins, long the province of Harley-Davidson and the smaller American manufacturers (Indian and Victory), are being offered by all four of the major Japanese manufacturers, as well as by the Italians (Ducati, Aprilia and Moto-Guzzi).

The advent of electronic engine management, electronic fuel injection, synthetic oils and lighter materials have all advanced the state of the motorcycle engine art. It is now nearly an aberration if a motorcycle cannot run 100,000 miles without anything besides routine maintenance.

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Copyright 2015 by David Tong and/or All rights reserved.