Secret new Harley-Davidson engine revealed

Harley-Davidson’s ‘Big Twin’ timeline stretches back to 1914 and the introduction of the 1000cc F-head. And in the following century and a bit it’s only gone through seven distinct engine ranges. This is the eighth.

That alone might be enough alone to warrant the name ‘Milwaukee-Eight’, but there’s a second, more important reason for the designation, and that’s the number of valves. Insiders say there are eight of them, four in each cylinder. And that’s a big deal on a pushrod, air-cooled Harley engine.

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The name itself dates back to a trademark that Harley applied for back in June 2014, specifically intended for use on an engine. So the Milwaukee-Eight has been in the pipeline for a while, but will finally appear on a production bike in 2017. It will have to live up to a pedigree established by such famous forebears as the Flathead, Knucklehead, Panhead, Shovelhead and Evolution, as well as its immediate predecessor, the Twin Cam, which has been around in various forms since 1999.

While the latest generation of Twin Cam – the ‘Twin Cooled, High Output’ version that accompanied the ‘Project Rushmore’ updates in 2014 – hasn’t been around long, the Milwaukee-Eight is expected to be the basis of the next generation of big-capacity Harleys for years to come.

What we know so far is that the Milwaukee-Eight is an all-new engine design, despite its traditional air-cooled looks and pushrod valve actuation. It’s designed to cope with current and future emissions rules, and is believed to use four valves per cylinder to help do that. Those that have seen the engine say the pushrod tubes on each cylinder are parallel, rather than converging as they do on the current Twin Cam engine. That means there are separate inlet and exhaust camshafts – giving a total of three camshafts or possibly even four, all mounted in the block and gear driven from the camshaft.

Normally, four-valve heads are associated with higher-revving, high-performance engines, usually with dual overhead camshafts, while pushrods are linked to thudding motors more interested in torque, and usually with only two valves per cylinder.

However, as engines become larger, the impetus for more valves also grows since they help get more gas in and out of the cylinder more efficiently and offer a better control of that gas flow, in turn improving the way the air-fuel mix burns. As well as improving performance, that gives better emissions – and that’s a big deal, particularly on large, air-cooled engines. It’s not yet clear whether the water-cooled cylinder head idea of the current ‘Twin Cooled’ twin cam engines is carried over to the Milwaukee-Eight.

The new engine’s capacity is clearly 107 cubic inches – that’s 1753cc – and given that the engine is new there will be scope for much larger versions for future models. There’s already talk of 117ci (1917cc) or 120ci (1966cc) versions for CVO models.

The engine will debut in Harley’s top Touring models, but expect it to spread to more bikes. The official unveiling will come at the end of August, when Harley’s 2017 line-up is launched.

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