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Motorhead Memo: M-8 and beyond

By Kip Woodring

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Elsewhere in this issue, you can see more details about the new Milwaukee-Eight Big Twin. Plenty of insight can be gained from some online searching and magazine reading as well, but the point of it all… the real difference… the big deal… about the new Big Twin is its breathing capabilities via four valves per cylinder. It’s news—even if it isn’t new!

Fact is, Harley has a history with four-valve heads dating back almost exactly 100 years. The Motor Company built an all-conquering racing engine with this notable feature, starting in 1916! The irony is, it wasn’t so much about breathing then, and more about keeping things together for long races—small valves and springs being less likely to melt or explode, primitive metallurgy and lubrication being what it was. Thing is, it sure as hell worked then and it works now.

Now, in addition to all the benefits four-valve heads have always offered, over a century of development and refinement has added many more. All these upsides with virtually no downside have culminated in the Milwaukee-Eight engines, in both oil-cooled and liquid-cooled versions. Only in the future will we know what levels of further development the factory has in mind for this powerplant, but that future is bright, rest assured. For now, however, let’s look at certain advantages and options we have before us as is.

Take a deep breath

H-D touts the fact that the new M-8 heads offer 50 percent more flow. Then they turn right around and say the engines offer 10 percent more power. What’s up with that? Anywhere I’ve ever looked and anything I’ve read or experienced indicates 50 percent more flow meant 50 percent more power, all else equal. Not a mere 10 percent. Where’d all that extra poop go? Well, there are plenty of valid reasons why you won’t see all the extra benefits of higher flow up front. For one thing, there’s that pesky “compliance” the factory has to contend with—various government emissions and noise standards mostly, and nowadays worldwide. There’s also the effort to cool things off, relative to the waffle iron levels of heat the Twin Cam began to radiate as it grew. (More power does equal more heat as the laws of physics constantly remind us.) All the same, Harley-Davidson is also noted for never giving us the best they have all at once on the showroom floor. Meaning, in time-honored tradition, you really don’t need to go much further than the relevant pages of the Screamin’ Eagle section in the new 2017 P&A catalog to see where plenty of that unrealized potential is hiding. If you want “all else equal,” the equalizers are there already—SE Stage II cam kits and Stage III engine kits that will get you much closer to that extra 50 percent! Not at all incidentally, all of it is smog legal and, once installed by your friendly local dealer, covered by the full factory warranty as well.

It gets better. If a stroll through the showroom and a test ride make you feel like committing your hard-earned to the acquisition of a new Touring M-8 model, as you visit with the salesperson at point of sale, ask them this question: “What’s the difference in my payments, if I have a cam kit or Stage III kit installed before I take delivery?” Then, prepare to be surprised! There’s a concept known as “cost-benefit analysis” which applies here for sure. Will all this added strength add to the bottom line? Sure, when hasn’t it? The news is, if the tariff for turning potential into results is stretched out over several years the bump in the monthly nut is negligible. Of course, you could always pay cash or install the goods within the first 60 days, after the engine is broken in, and put it on the credit card. But what’s point? The best deal is to buy the bike with the performance stuff taken care of, up front.

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Left: The additional muscle available from a 107” courtesy of a simple cam change is graphically demonstrated here. We’ve seen these kinds of charts before from The Motor Company, but what makes it compelling in this iteration is that it has to do with what a new Milwaukee-Eight is capable of, legally. It also hints at the potential in this new powerplant. Give the M-8’s four-valve heads what they want… they will not disappoint! Pay attention to the asterisks about exhaust choices and hardware kits and such, though. Because you’ll need more than meets the eye to get this cam kit installed and yielding these kinds of results at this level. Right: Same thing goes for this chart of the 114”. Both the torque cam (SE 447) and power cam (SE 468) are represented and they are the same cams as for the 107”. The difference in results is down to the difference in displacement as derived from two different bore and stroke specifications. The 107” measures 3.937” x 4.375” (99.999mm x 111.125mm) and the 114” 4.0”. x 4.5” (101.6mm x 114.3mm). No matter your specific choice, big improvements in torque and horsepower can be had with a pipe, air cleaner and cam.

Buck up for a big bang bumpstick

So, what might you want (or get) for your financial commitment to a 2017 M-8? Hereabouts, you’ll find some images of power gains to be had from simply adding an SE cam. There are three. Bear in mind, for baggers, torque is the best medicine. When the Dyna/FXR/Softail/Whatever bikes hit the streets, then a so-called “power” cam could come in handy. Mostly because those other Big Twin “platforms” are substantially lighter and would respond better to high-RPM shenanigans without losing any notable giddy-up from low speeds. Not so with a 900-pound dresser! That means, the SE8-447 (included in Stage II kit #92500047) promises the best results for anyone wanting massive midrange torque. Over 100 ft/lbs from 3000–5000 rpm means never having to worry about passing power, or pulling a trailer or sidecar. More importantly (and true of any new M-8 cam) the torque does not drop off early (see the chart), as is the case with the stock cam. As delivered, I believe this characteristic is the reason the high-flow, deep-breathing nature of the M-8 is “restrained” to only a couple of bike lengths’ advantage over the 103 Twin Cam. The original equipment cam has timing specs that strangle the engine as revs build, just as it should be demonstrating that superior increase in flow of 4-valve technology. Aside from the bulge in torque, installation of the “torque” cam offers a bonus of about 15 extra horses at peak and a notable stable of extra ponies all across the rev range. A usable, wondrous increase… period!

I’m going out on a limb here, to say that this torque cam installation (including all the peripherals required: exhaust, air cleaner, install hardware, etc.) adds about all the power and torque to a Milwaukee-Eight that the vast majority of us can really use or ever will. I’m going further out to add that I doubt anyone gives a rodent’s rectum about that reality. They want more… the whole Magilla… the inches… the maximum punch… the bragging rights. OK!

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Left: This represents expectations for the 107” once made into a 114”. The SE Stage III kits are pretty much the same for both 107” and 114” engines… a big bore kit. Where things go their own way relative to the Stage II cam kits is the cam! All Stage III kits use the third cam in the Screamin’ Eagle lineup for M-8 engines, namely the SE 498. Right: Plain to see in this chart is the fact that the SE 498 cam is engineered to offer the broadest spread of both torque and horsepower complementing large-displacement applications. It does this extremely well! The nature of the “curve” when this cam (in the kit) as applied to the 117” is a factor of stroke-limited rpm. In other words the cam quits before engine speed in a 4 ½” stroke engine can do any damage. Because the breathing of the heads won’t quit! A bit too soon to tell for sure, but I doubt the results would be quite this spectacular if one chose to use this cam in a standard 107” and there’s a reason. It’s part of the learning curve for four-valve Harley engines. Gotta go with flow, once you know. Well done, H-D!

Three-stage rocket

Those who’ve faithfully followed my meanderings in this rag for going on a quarter century know I don’t like to sound (or feel) like a salesman, especially for The Motor Company. I prefer to call it when I see it. Well, surprise! This time, I see it as Harley-Davidson executing a near-perfect “end run” on the aftermarket. Sure, given a year or two, the aftermarket is capable of building a better monument from the foundations of the M-8 engine. They always have. It’s just that this time Harley has kneecapped those efforts, not just by offering complete, competent performance “packages” that can be installed, warranted and financed, but by setting up a whole new learning curve for the aftermarket competition. A learning curve the company has already mastered. Witness the Stage III kits!

If, as alluded to a couple paragraphs ago, folks want (and can afford) their M-8 to “be all it can be,” Harley has provided—available today! The factory has covered these contingencies (as well as future new Big Twin models) complete with extra inches. Look closely at the charts for these kits. The 114” kit for 107” engines picks up a solid 30 horsepower and the torque curve takes off where the stock curve collapses to the “tune” of 110-plus ft/lbs from 3500–5000 rpm! That, gentle reader, is an ape of an engine… an 800-pound gorilla to be precise. (Cynics might note you get even more of this good stuff from a Twin Cam 120 crate engine, but not for 100,000 miles or more, you won’t!) That’s the magic in this new engine—it’s not even breaking a sweat at this “stage.”

Then there’s the 117” Stage III kit for the 114”, which somewhat surprisingly doesn’t make any more peak horsepower than the 114/107, but does make it at more accessible engine speeds. It also gives more power “under the curve,” which is techno-slang for “feels a hell of a lot stronger all the time” to the rider. Not least because the 117/114 engine makes over 110 ft/lbs of torque from 3000–5000 rpm and damn near 120 ft/lbs from 3500–4500, right where you need it! From a good cam (SE 498) and three more inches!

What’s left to say—for now—except maybe, “What are you waiting for?”

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