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Motorhead Memo: Beating the heat

By Kip Woodring


So, at long last they’ve gone and done it! Adding “liquid cooling” around the exhaust valve on dressers with lowers was almost a forgone conclusion at this point in Motor Company history. Yet probably not for exactly the reasons you’d first assume. First off, IMHO, mostly they are testing the water (pun intended) for future emissions, and water-cooling might well be the only way to get there… assuming you’d like any horsepower to go along with those cleaner emissions. There’s also a little more competition in the marketplace these days, some of it from right here in the U.S. of A., although I suspect you already knew that. Of course, it’s also entirely possible that what we see of “the new cool” this year, and perhaps next, might be a shakedown, a test of the market, for this new technology as embodied in a new Big Twin engine. After all, the Twinkie is officially 15 years old as of this model year and no Big Twin has lasted more than 18 years. Whoa—check that! Shoulda said no overhead-valve Big Twin! Which is a semi-sly way of working around to a little treatise on the history of heat control (mostly in the form of cylinder head cooling) in motorcycle engines in general and, most specifically, Harleys.

Inlet overhead exhaust V-twin

Photo caption: Ignore the weird actuation system in this early IOE (inlet over exhaust) V-twin and look at the valve layout on the upper left of this cutaway drawing. This was ubiquitous in the first decades of motorcycle engineering… because it worked! More power than most folks would expect and relative reliability because the intake valve helped cool the exhaust valve and compression was about 4:1!

Since day one of internal combustion, controlling and/or shedding heat has been a huge issue. In almost every way you could imagine this has been the major bottleneck to engine development. In fact lots of developments have come and gone over the years to address exactly how to maintain levels of heat required and reject levels that aren’t! Almost every aspect of four-stroke engine design—taken for granted by us—wasn’t a part of engines that existed over a century ago. Some features that we now see as archaic were cutting edge in their day, and for good reasons. Even when technology allowed the occasional breakthrough in engine design, it wasn’t the norm… and still isn’t! Instead we get more of an ongoing evolution (if you’ll excuse the expression) that remains incremental and, ultimately, limiting. Even today—and even on cutting-edge sportbikes (like the Ducati Panigale)—there’s a gap between what’s desirable and what’s possible in terms of temperature… and it’s all in their heads. Hot heads at that!

side-valve V-twin engine

Photo caption: Once the side-valve layout was perfected it took over rapidly because of its many advantages over the IOE engine… not least of those were sheer simplicity and good heat control characteristics. With the hot exhaust valve out of the way of direct combustion heat, these engines proved remarkably durable and sturdy… on a diet of no more than 6:1 compression, that is!

Picture yourself taking delivery of a brand-new Harley. You’ve done the homework, you like the reliability, the reputation for solid engineering and no tricks or fads. You’ve checked around with others who own one and, for you, it’s not about the latest doodads and accouterments. There’s less concern for lifestyle than for knowing you’ll get something you can use, fuss-free and for a long time. You know that the engine design you’re contemplating is proven—not necessarily flawless (nothing ever is)—but built with some integrity by a firm that has over a decade worth of experience and development under their belt with the powerplant. Now… pretend this is 1914!

Motorhead Memo: Beating the heat

Photo caption: Yeah, that head is pretty flat all right. The only thing up there is the spark plug, but because of that, it’s more sophisticated functionally than most of us realize half a century later. The accidental discovery (and subsequent development) of so-called “squish” in the combustion chamber helped keep heat from damaging parts, made more power and kept the side-valve engine competitive in this country for nearly 40 years.

At the time, H-D was still using an engine design descending from the successful French De Dion Bouton single created in the late 1890s. The layout of the valve train was referred to as “inlet-over-exhaust,” which is actually pretty descriptive. Then, as is the case now, the heat the running engine made was concentrated in the exhaust valve, dealing, as it must, with combustion temperatures roaring past it constantly. By stacking the intake valve right on top of the exhaust valve, the cool air coming in during the intake stroke did a reputable job of keeping its partner cool.

Engine speeds weren’t much (more like what we’d call a fast idle today) and power—even from a 61-inch V-Twin—was barely into double digits. But you had that power all the time and for quite a long time between “fixes” 100 years ago. I mean the term “octane” didn’t exist, which says a lot about the fuel available. Metallurgy was more about cast iron than cast aluminum, and to keep things from breaking you had to make ’em bigger, thicker or stouter because of it. Very true with early steel valves, by the way! Vanadium steel had just come about in the U.S.A. because Henry Ford wanted it for Model Ts and exhaust valves flat out couldn’t take much abuse. In Harley street bikes they didn’t have to. Thing is, Harley also built some pretty gnarly race bikes starting in 1913 and the ones the pros raced had nothing in common with the one you wanted to buy. The factory racers had eight-valve top ends (four per cylinder head) but not because it helped breathing. Hell… almost nobody understood “breathing” in those days, at least not like we do today. For Harley the mission on the racetrack was a lot like the mission on the street—to win you have to finish. To finish, a century ago, meant keeping the valves cool, and four smaller valves not only ran cooler, they weighed less, so heat-fatigued valve springs were less of a problem as well. If more power came along with the package, that was fine, but power to finish long races without any mechanical mayhem was the objective. All this worked all right till about 1920.

Early overhead valve engine diagram

Photo caption: As you can see here, early OHV engines (like this John Alfred Prestwich V-twin from the ’20s) let the valve springs bounce in the breeze. It kept the heat at bay as long as airflow was sufficient… and compression wasn’t too high. (Actually, these engines, in competition, used alcohol rather than pump gas, ran way cooler and you could bump compression up to what we now consider “normal.”)

What made the ’20s roar for American motorcycle fans was the so-called Flathead, or more appropriately, “side-valve” engine. Harley was a little late to the party and had no real experience building this type, so the laurels went to Indian, specifically the oh-so-significant Scout. We tend to equate Flatheads in our era with Briggs and Stratton lawn mowers, but in a way the fact the design exists today in any form says a lot about its inherent advantages. One real biggie, quite overlooked today, is the sheer simplicity of the layout. Remember that cooked valve issue, along with the crap gas and shitty metals? Well, they hadn’t been resolved when Flatheads took over, but owners who didn’t have a dealer nearby (most of ’em) had to D.I.Y. a lot and Flatheads were easier to deal with. I mean, suppose you fried an exhaust valve—or even broke it—on your Flattie, in the middle of nowhere (most places)… what then? In the day, you pulled over got your tool roll out, as well as the spare valves you carried (everyone did), and a few head bolts later you had the head in your lap and were lookin’ at the valves. Extract the dead soldier in a jiffy and you simply dropped a new one in, buttoned the head back on and away you went—20 minutes tops! No heads destroyed, no piston damage to worry about, no special tools or rocket-science valve adjustments to mess with… simple! Try that with your fancy OHV rig! The bonus was that “swirl” was discovered at that time and that meant good cooling, relative indifference to bad gas… and more power! Hard to imagine now, but Flatheads had the legs on those old eight-valvers and it stayed that way for the next 40 years. As late as 1969, Harley-Davidson race bikes had valves alongside the barrels and still ran off and hid from the competition—no matter where they put “their” valves!

By then, everyone else (mostly the Brits) had long since adopted overhead valves, if not overhead cams, and to be honest, had most of the bugs worked out of the layout. All the same, OHV (and OHC) engines really didn’t come into their own in any hurry, and not without problems. (Never mind that the Flathead’s real advantages had finally been canceled out, not by valves in different locations, but by high-octane fuel and space-age metallurgy.) For one thing, most folks who messed with early overhead-valve motorcycle engines coulda/woulda/shoulda wondered WTF at the “wisdom” of all those 90-degree valve angles and pop-up pistons. Even flat top pistons and parallel valves in the head gave no great advantage over side valves… initially. Besides, as you might imagine by now, having the valves and springs directly over the pistons exposed them to combustion heat as never before. At first, to keep those parts alive meant that OHV engines couldn’t exploit their biggest advantage… high compression. See, nothing that came before had the pressure of more than six atmospheres to work with and, more crucially, didn’t need and couldn’t use it on account of heat! In the meantime, OHV engine designers, who desperately wanted to exploit that prime advantage, had their plates full just keeping the valves and pistons from grenading at speed. Interestingly, the OHV crowd used to run the valve stems and springs (sometimes coil and oftentimes “hairpin” design) exposed to the cooling air. Only later did they figure out that by enclosing valve gear in a “cover” and using engine oil to cool it they could control heat even better. (Oil-cooled heads, indeed! Talk about history repeating itself.) Then, along about the late ’20s, gasoline was offered for the first time with a new additive called tetra-ethyl… somethin’-somethin’… known to us simply as “ethyl.” Total game changer! Suddenly engine “knock” could not only be managed, but it was also virtually eliminated! About that same time, tech trickle-down from advances in aircraft design and metallurgy during World War I finally came to motorcycles. By the early ’30s it was possible to build OHV engines with (relatively) high compression that would stay together reliably and make more power than a Flathead.

Drake water-cooled Knucklehead conversion

Photo caption: Funny how what goes around comes around… in this case a water-cooled Knucklehead conversion, known as a Drake. These 90-plus inch engines were developed for light aircraft use in the ’30’s and ’40’s. The Knuckle was the first OHV Harley of consequence… and what consequences they were! These engines had enclosed valve trains and re-circulating oil systems, so you had to be completely oblivious to ignore their potential—and in various subsequent incarnations—longevity. As in this version of the great granddaddy of the engines we know today, Twinkies are about to reap the benefits of a cooling “upgrade,” the origins of which date back three quarters of a century. All to beat the heat!

Perhaps the strangest part of the story is how slowly OHV engines came to the party in America. In Europe, where expensive gas and high taxes ensured small, efficient engines, the changeover was blindingly sudden, but here neither of those things was a factor. We had big, lazy engines, wide-open spaces and cheap gas… and we were fine with that. In fact, those “furrin” things seemed like gutless toys to most American bikers, right up until none other than Harley-Davidson went OHV with the immortal Knucklehead. That one had it all; not only big inches and overhead valves, but more importantly also taking advantage of new technologies to avoid overheating in its “high-compression” engine, giving it power and speed to burn. Once The Motor Company realized what the British OHV stuff was doing in the U.S. market and built a “sporting” middleweight to compete in 1957, four decades of Flatties were slowly phased out (mostly to meter maids) by 1973 and the company never looked back. Overhead valves were and are here to stay. So is government regulation! Those regulations have more to do with engine design (and fuel quality) today than market demands, because (with no space for enumeration and details) the simplified net result has put excessive heat right back in the game. Barring a total redesign of our traditional air-cooled engines—and yet another layer of complexity in our simple powerplants—a hidden “liquid” circulating around the hottest spot overhead, sure seems like a cool idea to me. Kinda wish I’d thought of it first.


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