Is this a stupid question? Compression effects in the crankcase

Copious blowby may have eliminated the vacuum

That’s what I was thinking.

Don’t you breath in through the carb/intake?

Actually thanks for the question and explanations. I really never did understand what those tubes were actually for on small engines. I had engines older than the 60’s but can’t remember if they had any kind of ventilation at all. They were pretty old-like late 40’s.

I do remember on our 58 Chevy 6 cyl, seeing that tube running down the side of the engine and the smoke coming out. At tune up time, the mechanic would light the cap on fire to burn out the oil from the fibers. It was a Chevy garage too so guess it was SOP.

Nope, the crankcase does not breath in through the carburetor.

In a normal engine, vacuum from the crankcase isn’t a problem, as blowby tends to pressurize the crankcase, and there is ability to breath in if it were to ever become necessary via the PCV system and the oil return passages, but in the engine in the diagram vacuum would be created in the crankcase. Both pistons are headed for TDC at the same time (not likely in a two-banger) and the crankcase vent closes while that’s happening.

I think they tried too hard to simplify the diagram and screwed it up.

texases: Another thing great about the GIF is how it demonstrates visually the compression created in the large volume crankcase is miniscule compared to compression created in the much smaller volume combustion chamber.

W/a V8 it must be very windy in the crankcase, what with counteracting pistons moving up and down all over the place, and the air rushing from the high pressure spots to the low pressure spots.

Yeah, but air moves pretty freely.
In my car, it does get windy in the passenger cabin at times, however… especially after I’ve had pizza.

Sorry. Couldn’t resist.

lol …