There's a lot written out there about keeping a negative pressure in the crankcase. It actually increases horsepower. There is blow-by through the ring gaps.

http://www.dragzine.com/tech-stories/engine/tech-how-external-vacuum-pumps-free-up-horsepower/

The one way valve on the crank vent won't produce negative pressures like those in the article but it won't hurt anything either.

A mildly tuned V-8 (say the family car) crankcase's pressure is pretty stable, sans what blow by there is. They do create constant positive pressure because of it. Normally, it's very little as all the pistons are going up and down are about 50-50, just the motion of the pistons are balanced out.

Back in the '60's I worked as a mechanic on older engines before closed PVC came around, and the breather is where the oil was added and I saw very little fumes coming out of an idling engine unless it had worn rings. Then it was a chimney from all the blow by.

Now, a race engine, as shown in the link, with high pressures constantly being created in the cylinders and making gobs of power could utilize such a vacuum pump to keep the blow by pressures to a minimum.

A narrow V twin is much different that that. Harleys with their separate crankcase creates a lot in the differential pressures in that small case.

Our cases, with the tranny in there as well has a lot more volume to play with. Call it an accumulator.

But, I still think the vent on ours needs to breathe. (Inhale-exhale)

I seem to remember a separate blow by, on early V-8's, besides the oil add tube. You'd see them blowing heat/vapor out, on cold mornings, or when the motor was running, and the temps were near zero. I think the 327, in my 64 Impala, had a tube, that pointed down, next to the water pump. It was about 6 or 8 inches long, IIRC.(that was a while back )

SKWEARpeg wrote:
I seem to remember a separate blow by, on early V-8's, besides the oil add tube. You'd see them blowing heat/vapor out, on cold mornings, or when the motor was running, and the temps were near zero. I think the 327, in my 64 Impala, had a tube, that pointed down, next to the water pump. It was about 6 or 8 inches long, IIRC.(that was a while back )

I remember those, about an inch in diameter with a slash cut tip.........

Wait isn't the transmission separate from the engine crankcase on a road star

brianmac wrote:
Wait isn't the transmission separate from the engine crankcase on a road star

No. The transfer case is, though. The tranny is lubed by the same oil that lubes the motor, on a Roadie.

And here I thought guys paid closer attention to what's going on between their legs.

Flashback wrote:
SKWEARpeg wrote:
I seem to remember a separate blow by, on early V-8's, besides the oil add tube. You'd see them blowing heat/vapor out, on cold mornings, or when the motor was running, and the temps were near zero. I think the 327, in my 64 Impala, had a tube, that pointed down, next to the water pump. It was about 6 or 8 inches long, IIRC.(that was a while back )

I remember those, about an inch in diameter with a slash cut tip.........

That's the one. Sheeeesh!! We're old.

I remember the oil fill caps, with the filter material inside, Ron mentioned also. They were nasty dirty after being used for awhile. They'd get oily from the blow by, and the dust would stick to them. Ahhhh The good old days.

brianmac wrote:
Wait isn't the transmission separate from the engine crankcase on a road star

HD, and the knockoff engine builders like S&S, are about the only ones that does that, anymore. Except for the VRod.

Even Victory and new Indian, as well as all the metric Vtwin builders, use the single cavity cases running with engine, clutch and tranny components in the same oil.

For what it's worth, I had a filter on mine like the ones linked above, on my Roadstar. Gotta get that crap out of the intake! And on my Strat, I put on a KrankVent. Not sure if it's any better, but that's what's on it. http://www.et-performance.com/index.php?main_page=product_info&cPath=4&products_id=87. Makes a little racket you can hear it opening/closing at idle. And I get a drop or two of oil overnight from the end of the hose, especially just after an oil change, and it's really crappy stuff. Glad it's not going into the intake, and gumming things up. Strats are especially susceptible on the ISC valve (in my opinion).