Motorhead Memo: Air apparent

Harley-Davidson’s original Big Twin oiling system is mystical and mysterious. It came down to us, courtesy of Bill Harley, from 1936 to the end of Evolution production in 2000. In fact, what we refer to as a “breather” turns out to be the key to the recirculating lubrication system… which he patented. US patent #2111242 (*GTS) makes for a good read BTW… not least because Bill never refers to a rotating breather. Instead, he called it a “trap!” The key concept was to time engine air pressure and vacuum, via the trap, to help the mechanical pump move oil. Make a note; we’ll be getting back to that.

Some 70 years later, the good folks at S&S are developing a new engine with a 4 1/8” cylinder bore. Making a motor roughly twice the displacement of Harley’s original 1000cc Big Twin, using the same size crankcases and a version of the original oiling system… proved to be a trap alright! (Makes you wonder if Harley was right when they decided never to produce a Knucklehead-descendant Big Twin displacing more than 1340cc… don’t it?) I digress.

S&S found themselves forced to rethink oil supply and air circulation to make their engine work. That’s why the HVHP pump… to provide for piston oilers and heat control. Trying to get the oil to move through those dinky cases also lead to another breakthrough… the reed breather valve!

The S&S patent ( # 7395790*) clearly shows, in its wording, the connection to 4 1/8” bore engine development as the genesis of the idea, but mostly it reveals an almost gushy enthusiasm for what they wrought. The reed simply shot down the rotary… completely! To quote their succinct description of function, “Breather valves for motorcycle engines have been used for many years. The purpose of a breather valve is generally twofold. First, it expels air and oil from the crank region, on the down stroke of the pistons, to the gear case where the oil lubricates various gears and other parts. Second, it draws oil from the bottom of the oil separating pocket (also sometimes referred to as the “settling pocket”) in the bottom of the gear case.” Then they get into it: “Many known valve assemblies, such as rotary breather gears, are typically designed for engines operating at specific revolution per minute (rpm) ranges. Such a configuration, however, is disadvantageous for engines typically operating at lower rpm ranges, at higher rpm ranges, or wide rpm ranges—i.e., ranges different from the optimized ranges of known valve assemblies. Incorrect timing can cause conventional breather assemblies to become flooded with oil due to severe pressure fluctuations. In contrast, the pressure wave actuated reed valve assembly disclosed in various embodiments of the present invention operates whenever crankcase pressure exceeds cam chest pressure, regardless of crank position.

Thus, a need exists for an improved breather valve assembly.

As disclosed in the aforementioned embodiments, the presently disclosed reed valve assembly can operate over a wider range of rpm (e.g., about 500 rpm to about 8,000 rpm) than conventional breather valves, does not require timing to crank position/orientation as do many conventional breather valves, and generally has improved performance over conventional breather valve assemblies.” Bam! No wonder I wanted one for my FiXeR!

An early (circa 2006) catalog description of the new reed breather shows the euphoria had not passed. The copy goes like this: “Installation is fast and simple. Remove the original breather gear and insert the new S&S reed valve assembly into the breather valve cavity.” Then, “The difference between the way these breather valves function is important because the ideal breather timing actually changes with rpm and the condition of the engine. By opening and closing “on demand” the reed valves automatically compensate for the lag time associated with accelerating the mass of air as it is forced out of the crankcase. This is most significant at high rpm where the time available to get the air out is the shortest.” And finally… (wait for it!): “Installation of the S&S crankcase breather reed valve has actually stopped high rpm oil carryover in test engines with marginal ring seal.” Wow, music to the ears of the average Evo rider! The copy, in a little “thought bubble” sidebar, goes on to note, if you close off the primary on a Shovelhead, it works on those too.

Only apparently it did not work on Shovels and case-breather Evos, because S&S changed their mind. Today the reed breather is recommended for ’93-on “head breather” engines… only! WTF?

The only pseudo-official explanation for this boils down to an Internet post by an S&S employee in 2010. “In a bottom-breathing engine (like a Shovel or early Evo) the oil mist works its way around in the cam chest and through the passages of the cam cover into the breather cavity in the bottom corner of the cam chest. Throughout this process the oil gathers in the walls of the chamber and settles to the bottom. There is a drain hole in the back of the cavity that leads back up to the breather gear bore. A rotating breather gear has a small hole at the back that applies vacuum to this hole and draws oil out of the settling pocket. This keeps the oil from building up in the pocket. If it did not happen, the oil would be pushed out the breather tube and onto the ground. A reed valve does not have this ability so it should not be used in a bottom breather.” (For those in the dark about this… see figure 2… and follow the yellow string.)

So, the reed blows… because it doesn’t suck? Now what do ya do? Admit a mistake and go back to the troublesome rotary breather? Ah… not me! Not yet! Call me a sucker then, because there just might be a way to keep the benefits and reduce or eliminate the hassle.

I wrote a Motorhead Memo “The Air And Oil You Breathe*” in 2012 and espoused the idea that reed breathers used with HVHP pumps would be alright, because the pump’s massive scavenging ability would help with any excess oil build up in the cam case (OK, “gear case”) settling pocket. It was a case (pun unintended) of focusing on oiling rather than breathing.

When it comes to engine breathing, we tend to deal with Harley’s arcane oiling in terms of pressure. Whether it’s oil puking from the air cleaner or air blowing off the occasional oil tank filler cap… oil or air… it’s pressure to us. But in the context of sucking oil out of the gear case without a valve to help… you really need another source of vacuum. That “search for suck” led to a quick study of the one and (so far) only guy who’s done his homework on the true nature of motorcycle engine breathing. There are only two books on the subject and he wrote ’em both. Result?

Near to here, you’ll see a drawing (figure 3, crudely done by yours truly) intended to show one possible way to implement his concepts into a practical method of solving the reed problem (among others) for bottom-breather Big Twins. Until next time, when you’ll get a detailed dissertation on how and why this should work, I hope you’ll take a close look and think about the thinking behind it. The goal is to “tune” engine breathing.

(* Google This Stuff)