Harley-Davidson Milwaukee-Eight 107 engine: The next-generation Big Twin

Finally! An all-new Big Twin engine from Harley-Davidson! A bit late, but worth the wait! Since Harley doesn’t do this every day (or every decade for that matter) this is a Big Deal Big Twin!

For the purpose of this introductory explanation of what it is and what to expect from it, I thought I’d simply interpret and expand on, as best I’m able, the information H-D has provided on this powerplant, point by point, and explore how The Motor Company goes about delivering on the slogan for the Milwaukee-Eight (M-8): “Stronger, cooler, with more comfort and style.”

Introducing brand new Big Twin engines and associated mechanical entities, like primary drives, clutches, transmissions and such, on the bread-and butter-bikes only, is a bold move by The Motor Company. Face it, Touring models are the backbone of Harley’s model lineup, so to place this new engine exclusively in them—first—is a gauntlet thrown in the face of those naysayers who blather on about never buying the first year of anything. They say it’s risky business until the bugs are worked out. Harley is betting the bugs have been exterminated in advance. It’s a safe bet!

The major engineering decisions that make this engine new (and vastly improved) amount to these: twin-plug four-valve heads, single camshaft, gear-driven counter balancer, and… wait for it… crankcase venting to the transmission. There’s more, but mostly in detailed design, intended to enhance those main features.

The first 4-valve Harley heads since the board-track racers of the old days got those extra valves and those twin plugs because the powerplant has to be emissions compliant world-wide. It meets the toughest of those, “Euro 4,” with no trouble. That’s more than you could say for any previous Big Twin, but only tells half the story. What’s best about the four-valve, twin-plug, pent-roof combustion chamber can be summed up in one word… potential. The level of efficiency in this configuration is such that the engine makes its power in a very low state of tune. Killing a lot of birds with this rock is easy. Lower emissions, improved power delivery everywhere in the rpm range, fuel economy, and lower operating temperatures, to name a few. Those who want to turn up the heat (in both senses of the word) and use some of that latent potential are in luck! (We’ll examine that subject in depth in next month’s Motorhead Memo.)

The decision to use a single cam, regardless of how retro it appears on the surface, is actually the best way forward for this new four-valve, pushrod layout. Quieter because there are fewer parts thrashing around under the cam cover and because of gentler ramps, lobes and lifts… compared to camshafts that came before on Harley Big Twins. Because the flow in the heads is greater (by 50 percent, H-D says), massive amounts of lift are unnecessary. As is radical timing. Might take some adjustment in thinking for folks who intend to hot-rod these engines but it amounts to less wear and tear on the entire valve train as well as better results from milder cams with “specs” worlds apart from the “traditional.”

Smoother V-twins are the reason for counter-balancers in the first place. The Twin Cam “B” got ’em in a chain-driven form to tame vibes in the solid-mount Softail chassis, but it was a piece of add-on engineering…and not without niggling faults. The Motor Company knew they could do better and now they have. This is a definite design improvement, if for no other reason than it cuts reciprocating weight, simplifies and reduces parts count, is quieter and shall not fail or flail at high rpm (although for a Harley that’s 5500–6000 rpm!). Then there’s this: The Motor Company can obviously, easily, change the balance factor to suit different applications.

The thing is, speaking of applications, why put this counter-balanced engine into a rubber-mount chassis? It seems fair to say, that after the M-8 engine has spread to and through the entire Big Twin lineup for a couple of years, we just might see some new chassis to hold it. What if The Motor Company has an improved Touring chassis in the works? Or, maybe the day will come when all “long” primary models go away in favor of short primary redo’s of the Softail and Dyna “platforms?” And, dare I hope, if the Dyna goes away, it might it be replaced with a versatile, smooth, great-handling, light Big Twin model, that combines the best of the Dyna and the FXR… only goes both one better? Change the balance factor to suit a solid mount frame and away we go!

Anyone tired of so-called “oil carryover” can rejoice in the fact that oil no longer goes anywhere near the air cleaner. Instead, it is routed into a cavity in the sump underneath the transmission. May not sound like much, but it’s a kind of quiet revolution in that age-old bugaboo of Big V-twins with small crankcases: oil control. The massive amounts of air and oil churning within is caused by the huffing and puffing of pistons pumping up and down, creating first pressure then vacuum inside. These pulses need to be vented properly. In the past it has been done directly from the cases, then up through the cylinder heads. Never, since 1936, has it been completely tamed. The proof has been found inside air cleaners for decades. After hard running, oil, in theory separated from air in advance, has still managed to make the occasional mess where you don’t want it… all along the right side of the motorcycle. No more… period! An under-appreciated improvement that can not be overestimated.

These features are the foundation upon which Harley-Davidson has erected its latest and greatest Big Twin engine. But there’s more to it than that. More than meets the eye. So much more, we can’t get to all of it in one issue. (So, the second part of this dissection of details… will be in the next issue of THUNDER PRESS.)

OK, next we’ll get to the “bottom” of the new Milwaukee-Eight. In other words—to be continued…

Click here to read part two!