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Purpose of crankcase breather

Posted: Thu Aug 10, 2017 5:24 am
by Crimson
Hello everyone

Last night I was thinking about engines, when I thought about the crankcase breather hose that goes from the right-side of the XJ600 engine to the air intake box. I thought, what would happen if I just disconnected this hose from the airbox side, and instead just tucked it away so rain wouldn't enter?

From what I understand, air-cooled engines have significantly wider clearances around the piston rings and cylinder wall. This is to tolerate a wider range of differentially heat-expanded and contracted components than a water-cooled engine. For this reason, they generally aren't manufactured to produce the same amount of power as water-cooled engines. Because the thermal expansion of components in them is less, water-cooled engines can run with tighter clearances, which permit a higher state of tune, which allows higher compression, etc.

Also, from what I understand, a crankcase breather system in any engine is designed primarily to allow blow-by gases (from between the piston rings and cylinder) to vent out. These gases contain no free oxygen molecules at all - they're mostly just noxious hydrocarbons. Now, because of the slightly larger clearances in air-cooled engines, there are even more blow-by gases to deal with than in engines with more effective cooling systems.

Given that blow-by gases are a fairly important consideration in the XJ600S/N engine, it makes little sense to me that these gases would be directed straight back up into the air intake box. That air leads nowhere but back into the engine. In effect, because it's not normal atmospheric oxygen-containing air, it must poison the air breathed by the engine. It would contribute to the operation of the venturi effect of the carbs, so the carbs continue to operate properly, but the air-and-fuel mix would be less combustible as it contains less oxygen, which would mean that as time goes on, with more and more blow-by gases by extending running at higher revs, the mixture would get richer. And probably have detrimental ancillary effects on the cylinder head, carbon deposits, etc. (in much the same way as an EGR valve on a modern engine). It probably also raises induction temperature after running for a while like that.

For these reasons, I was thinking of disconnecting it and letting it vent directly to the outside air.

What do you think? Is this oversimplistic?