General learning questions. I see in some of the threads I read trying to learn. Long story short. I'm looking for a high level explanation and differences of top and bottom breathers? I'm guessing my stock 89 FXSTC is a bottom breather...? I'm new and learning so I get it's a question most of you already know. Everyone learns somewhere... I have no issues with mine; just trying to learn. I also have the service manual; but if my reading comprehension was any good I wouldn't be asking someone smarter to paraphrase stuff for me.

 

Bottom breather "vents" the Crankcase out the Bottom end... Originally sent from breather tube to Air Cleaner to avoid Pollution...
Top Breather vents Crankcase up to the heads ... thru Umbrella Valves, then tubes to the Air Cleaner..
Much less likely to observe "sumping" with a head breather... tho can/will still sump without displaying itself... unless severe.or Someone Put Oil into the System before running the engine...

 

Yes it is.

 

I'd much rather have a bottom breather.....

 

You may need therapy for that....

 

It was the EPA that forced the top breather-
Not an engineers .
The answer is simple as to what is better, but might not be doable in the latter configurations.

 

A few posts above mine you can see an animation of the Evo motor in Texas Hill Country's avatar. You see the pistons moving up and down? What happens is that creates positive pressure in the crankcase when the pistons are moving down and negative pressure in the crankcase when the pistons are moving up.

This crankcase pressure needs to equalize somewhere otherwise your engine would leak oil everywhere from the crank pressure forcing oil out wherever it can find a path. Earlier Evo's (and I believe all HD engines before that) were called bottom breathers because the port the crank would breathe out was down somewhere on the crankcase. The top breathers have ports that run thru the cylinders up into the heads so the engine can breathe clean air from the carb side of the air filter.

You get it now?

 

Now I've got a related question. Are the bottom breathers more "oily" when it comes to the mist being vented?

I'd imagine it is simply because oil is heavier than air and the top breathers have less oil transmission up thru the cylinders to the breathers in the heads.

 

I'm gonna say that seems logical... but, the amount of Oil in the Mist, is much more a function of the oil pump condition and breather timing... and ... especially how much restriction is in the oil return line...That's why newer Engines, with head breathers can almost get away with two or more oil coolers jamming up the works... you just don't notice the Engine is being Strangled...
Finally... The Best performance solution is an evacuation pump on the Bottom end... no matter how it was originally designed... Then, all ya need worry about is Controlling the Oil in the bottom end... remembering... as designed there would [theoretically] be None...

 

On a bottom breather you can eliminate most "oil" problems by rerouting the breather hose away from the air cleaner. Use a catch can or just toward the ground. Then make sure the ball check valve in the oil pump is clean and seals well.