breather system...

[XFlow_Fairlane]

ok on my 170 the factory system was a simple road draft tube and breather on the cover. then when I put on a chrome cover I had the closed cap and a PCV on the rear port feeding the carb spacer. this worked fine. when I put the 200 in it had no roaddraft tube so just keep the PCV and closed cap only and now I have so bigtime blowby it seems (dipstick didn't get put in fully when I was timing the motor and after a short drive I had oil spraying out it. so shoudl this stop after my rings seat in? or should I got to a vented cap? I always thought that any blowby would go straight to the PCV system then?

 

[mustang6]

Your problem is the closed system you've created- the PCV is sucking the air and it has to come from somewhere. Since your crankcase is closed (no venting at the valve cover), it is sucking air past the rings out of the combustion chambers. And yes, a configuration like this will cause massive blowby but the good news is it should get better when you install a vented oil filler cap.

 

[Lazy JW]

I find this interesting because I have read about folks actually hooking a vacuum pump to the crankcase in order to evacuate the air and thereby reducing air friction so as to make more power. Wouldn't this also cause more blowby? Or is this not a problem on racing engines? It seems that even if all of the air were removed from the crankcase that this will only increase the relative pressure by about 14 psi. Is that enough to drastically affect ring sealing/blowby? Or maybe I'm missing something here
Joe

 

[BIGREDRASA]

You need a breather cap. When at WOT, the PCV is not opening, and you're generating max combustion pressure (e.g. bypass). The the crankcase system is not meant to be pressurized, and the pressure has to go somewhere. Blown seals and gaskets can be quite messy, not to mention inadequate scavanging of acids/fumes in the crankcase causing internal problems.
As the commercial says, "Breathe my friend, breathe!"

 

[Anonymous]

The PCV valve has a relatively small port and as mentioned, it opens under high vacuum. So without the vented cap, the crankcase is essentially bottled up. The churned up gases can push dipsticks etc out of the hole. The vented caps not only allowed it to suck air in, but push gases out via the oil return passages from the through the heads. When the manufacturers started using closed caps, they tied the caps to the air cleaner with a hose. This allowed the engine to suck air through the PCV into the intake manifold, but also at higher rpms vapors could back flow from the breather cap into the air cleaner to expel gases from the crankcase.

Vacuum pumps on crankcases, actually help the rings to seal better due to the higher differential pressure. The ring not only has to seal against the cylinder wall, but it has to seal against the grooves of the piston itself.