Last episode… we took up the gauntlet on evacuating air from the lower end of “case breather” Harleys. Fact is, the importance of letting air out efficiently is the easiest part of the equation to understand… and appreciate. My friend Paul has made (pardon the pun) exhaustive studies of it on his 100“ Sportster… to very good effect! Of course, his machine is a head breather, which have their own issues and quirks (to be dealt with in future) but the overriding principle so far has been to vent, vent, vent… and let the air figure out how to get in… on its own. There’s merit in that. After all, you’d have a hard time stopping air from flowing into the lower end… “engine’s a pump”… remember? Lots and lots of folks have come up with very creative approaches to this, and last month’s column on the crankcase exhaust setup for the “lab rat” FXR…was probably very familiar (yawn!) to most readers. But worrying about venting alone is only half the battle when it comes to crankcase tuning. Other factors, like whether to set up for top-to-bottom or bottom-to-top circulation come into it as well. Case breathers that exhale from the crankcase and inhale into the cam case are “top to bottom” and head breathers vice-versa… by nature. But, it can be done either way on either type and which is better depends on variables we’ll discuss further in the next episode.
The “intake” (or inlet) Brit-bike reed valve I originally intended to use turned out to be a non-starter… way too bulky to mount cleanly. As are many other, more conventional valves (although your particular install might be more conducive to hiding these things). However… for a small “flea fart”-sensitive inlet valve which won’t be oiled up… I chose this S&S valve, installed with the “white band” towards the engine. There’s little risk inlet valves like this will be contaminated with oil either.
The engine side of the inlet valve hose… goes here… where the oil tank “vent” hose was connected to the cam cover. Couple things; it may seem counterintuitive to have air enter the cam case and not get out… but it does. The crankshaft creates suction when pistons are on the rise and air/oil mist in the cam case travels through the right main bearing into the crankcase and there’s one other thing that happens, which essentially is a key part of the “workaround” for the S&S reed valve’s inability to allow “normal” oil drawback. We’ll get to that…
The hose running from the other end of the intake valve goes to an air filter. This arrangement is based on a suitable place to put the filter and will be changed. A smaller air filter would work as well and be easier to mount tidily. Mainly… any hose routing done should be double-checked for kinks or restrictions. Can’t allow those!
For now we need to finish what we started, to address the curious and critical “intake” side of the plan. The “plan”… to reiterate… is to manage air flow through the “under the rings” parts of the engine. Few Harley folk have hands-on experience with tuning the air inlet, so some of what you see here might be new and different… even a little nutty. Doesn’t matter. Get this intake business right and the benefits of controlled internal air flow get real… real quick!
Stock… the “vent” line on the oil tank actually serves more as an uncontrolled breather hose between the air space above the oil and the cam case. Stock… this arrangement can easily lead to creating a vacuum in the tank (as evidenced by a change in idle speed when the oil cap is removed)… or more often… a tendency to blow the cap off… especially when the tank is too full. In other words, the oil tank on a dry sump Harley serves as part of the crankcase breathing system… like it or not. Since part of this exercise in bottom breathing involves eliminating that connection the last step is to vent the cap. There are plenty of ways to do this, but the easiest and cheapest… you see here. Simply drill a tiny (about 1/8“) hole that intersects the hole in the hollow rubber center of the cap, but allows air to pass when the cap is in place. (Carry a spare cap with a dipstick with you on rides, if it adds a comfort level.) The idea here is to minimize or neutralize the oil tank as a player in case breathing. Keep oil levels correct and it’s no problem.
All installed and ready to test. 150 miles of 4000-plus rpm riding indicates the motor is happier, more oil tight, but mostly no longer puking oil out the breather or anywhere else (like my dodgy rear rocker box gasket). From here, refinements in looks and function are to follow, but by controlling air flow through the engine I think I dodged the S&S reed valve “bullet.” Barely! All bets are off if this engine, as configured, spends much time at 6000 rpm or more. (Not that it will!) For reasons inherent in V-twins, the only thing that really controls air flow at that engine speed is… the engine. But anywhere below those speeds things are greatly improved, ironically… mostly because of another S&S part… the HVHP oil pump.
A little-pondered fact regarding Harley oil pumps is that they have an unsung but pivotal role in case ventilation! They scavenge air… not just oil! By “feeding” air to it, in a more precise and useful way, the more efficient HVHP pump helps scavenging at lower engine speeds, where the stock pump would fall behind and let oil puke from the breather. As engine speed rises the air-oil ratio through the pump tilts in favor of oil but it’s still pumping air as well. Until… as mentioned… you crowd the rev limit and the engine takes charge of breathing… period. This “air flow through the cases” exercise does in fact help a lot in daily riding. Enough so that I’ve become an advocate. But the “Ah-HA!” in the process was discovering just how much the oil pump contributes to the outcome. Put another way… what excess oil the reed breather valve might leave in the cam case, an HVHP pump and “tuned” crankcase breathing combine to remove.
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