Pushrod Motor Life Extended with Variable Valve Timing

Unclear Horizon

Recent patents filed by Harley-Davidson suggests that an engine design change is on the horizon. The Motor Company is exploring Variable Valve Timing (VVT), a technology that dates back to the 1980s, and one long adopted by other motorcycle manufacturers. Its application is unclear, but if implemented, it would fundamentally change the look of the MoCo's iconic powerplant.

Increasing Response

Harley-Davidson has a steadfast adherence to traditional styling. While slow to effectuate change, the approach works, instilling confidence, and building loyalty in generations of customers. However, company change has been accelerating in the past few years. Part of the shakeup is in response to a new rider demographic, but pressingly, it is due to increasing environmental regulations.

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Fine Lineage

At the heart of the issue is Harley-Davidson's pushrod motor. Bluntly put, the design is antiquated, dating back to 1936 and the company's first OHV engine—the Knucklehead. Yet, despite its age, the air-cooled, pushrod V-Twin is a solid and effective performer that has stood the test of time. Confirmation of its fine lineage is evident with the Milwaukee-Eight—the design's latest incarnation.

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Catalyst Step

Far from being a geriatric plodder, the Milwaukee-Eight has raised the bar. The Screamin' Eagle 131 version is the “most powerful street-compliant engine that Harley-Davidson has ever created.” With performance, not an issue, emissions is the next likely catalyst for change. The introduction of Variable Valve Timing is a step toward preserving the venerable pushrod motor, but it comes with a contradiction.

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Wide Mark

A recent Cycle World article debates that “future emissions laws, particularly in Europe, will make air-cooled, pushrod-valved engines a thing of the past.” They state that the projection is “wide of the mark,” because “emissions rules are much harder on high-revving engines, compared to low-revving big twins that can achieve surprisingly clean emissions.” As evidence, they point at the new BMW R18.

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Application Timing

Cycle World concluded that unsophisticated, “large-capacity, air-cooled engines will be with us for a long time to come.” Other reviewers of the BMW's 'Berlin-built' behemoth, like Canada MotoGuide, simply urged readers to forget about the “double overhead cam, variable-valve timing mumbo-jumbo.” So, what is going on at Harley-Davidson, and what is the intended application of Variable Valve Timing?

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Cleaner Boost

The Milwaukee-Eight could be ruled out, given that future emissions regulations would have been considered during its planning phase. Cycle World's best guess is that the patent applies to the Sportster's Evolution engine. With little development since its introduction in the late 1980s, the higher-revving motor would benefit from VVT, with cleaner emissions and a low-end torque boost.

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Variable Response

While the Sportster is a good candidate, its logical replacement is pending—armed with the potent Revolution Max. Whatever the application, VVT comes with a bit of a surprise, or concern. The technology forces the pushrods to be located on either side of the cylinder head, irrevocably altering the engine's appearance. Now, that is bound to receive a variable response at best.

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