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.
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!