[blu67]

What is the difference in breather caps? Emission system cars and non-emission cars? I've seen some that have a hose that connects to bottom of air cleaner and those that have no hose at all. What is the difference?

 

[68_wildfire]

The one's with the hose allow the air and blow by to be sucked into the carb, similar to the pcv valve. The ones without the hose simply allow any blow by to escape out.

 

[jimbo]

To add to what has already been said... there is a third type
with a road tube. the oil fill cap captures the breeze under your hood and directs the noxious crankcase gases to the other valve cover then safely towards the ground.
jimbo

 

[GypsyR]

Actually the hose to the air cleaner is to draw IN filtered air, just like the carb does. On a healthy running motor, you can pull the cap and feel it drawing in air. After changing to an aftermarket air filter a lot of times there is no hose hookup provision. So you would chunk the hose and change the cap for one with a built in air filter.
At some point your engine will wear enough to where there is more blowby than PCV vacuum. Then indeed that hose will blow the blue smoke into the carburetor. If you had the filtered cap, it would emit a blue cloud and soon start clogging up with oil mist.
If your engine is a beater, you'd prefer that mess to be sucked into the carb so you can pretend it's not THAT worn out. At least until it starts blowing blue smoke out the tailpipe.