breather on oil filler. blow off? or intake

[acaciolo]

I was looking at some engine pictures online and saw this beauty (think it was at DCS this year also) and I had a question on the breather tube that comes off the oil filler and normally connects to the airbox.

is that tube a "blow off" and normally puts air back into the air box? Or does it suck some fresh air from the airbox in? On this stainless steel airbox, there is not fitting for it, so this picture shows a little breather filter on that line rather than connected to the air box. but if it is a blow off, is the filter even needed?

31649

[Delorean Industries]

The crank case configuration pulls from both the cold start tube and intake air at the plenum inlet. It is barely enough for a naturally aspirated motor in good working order let alone one that is 30 years old. While you can pull from atmosphere as pictured it is better to have an actual "pull" present. While it isn't technically vacuum on the intake pre throttle plate it is better than venting. Only time this isn't required is on a turbo application. In this case we modify both valve covers to vent positive crank case pressure created under boost.

[David T]

The crank case configuration pulls from both the cold start tube and intake air at the plenum inlet. It is barely enough for a naturally aspirated motor in good working order let alone one that is 30 years old. While you can pull from atmosphere as pictured it is better to have an actual "pull" present. While it isn't technically vacuum on the intake pre throttle plate it is better than venting. Only time this isn't required is on a turbo application. In this case we modify both valve covers to vent positive crank case pressure created under boost.

It isn't clearly explained in any of the service literature about how the crankcase is ventilated. Starting at the air filter box, filtered air is pulled into the oil filler cap. Also connected to the cap is a hose going to the cold start tube where, through a fixed orifice, (NOT a one-way PCV valve!) vacuum is introduced to remove the crankcase vapors and keep the crankcase below atmospheric pressure. Not the greatest flow, usually air will go in one valve cover and out the other on most engines with 2 heads. That hose going to the air box from the oil filler is necessary so only filtered air gets drawn into the motor. As mentioned this system is barely adequate on a healthy naturally aspirated motor.

[acaciolo]

so that hose that connects to the air filter has flow going from the air filter into the oil filler? It was described to me as a "blow off" and that what comes out of it goes back into the air filter box and burned up in the engine. But it sounds like it really is sucking air into the oil filler, not blowing it out.

thanks for the education!

[David T]

so that hose that connects to the air filter has flow going from the air filter into the oil filler? It was described to me as a "blow off" and that what comes out of it goes back into the air filter box and burned up in the engine. But it sounds like it really is sucking air into the oil filler, not blowing it out.

thanks for the education!


It can be considered a "blow off" when, if the motor creates too much blow-by, air will flow through the hose towards the air filter. You will see that because the inside of the air filter where the hose enters will get oily. That is because the air also has the vapors of the oil in it and it condenses in the air filter box.

[Doogie]

I always thought that Crankcase pressure was created by blowby around the piston rings, but reading this thread and the mention that on 2 headed engines like ours, that one valve cover ususally sucks in air and the other usually expels air, got me to wondering.

What effect does the pumping action of the pistons have on crackcase pressure?

[David T]

I always thought that Crankcase pressure was created by blowby around the piston rings, but reading this thread and the mention that on 2 headed engines like ours, that one valve cover ususally sucks in air and the other usually expels air, got me to wondering.

What effect does the pumping action of the pistons have on crackcase pressure?

The pumping action surges the air back and forth but since one piston is going up as one is going down there is no net increase. Crankcase pressure is raised by blow-by past the piston rings and any leakage through the valve guides. You want to keep the crankcase under negative pressure to
A) reduce hydrocarbon emmissions
B) prevent or reduce any oil leakage from the seals.

[DMCMW Dave]

It also keeps the oil cleaner.


Sent from phone

[SS Spoiler]

Remember reading about an early Bronco owner had rigged up a fan to blow / suck? air through the engine crankcase.
He said his oil never turned black.

[Bitsyncmaster]

Remember reading about an early Bronco owner had rigged up a fan to blow / suck? air through the engine crankcase.
He said his oil never turned black.

I tried to put vacuum on the passenger side valve cover to improve engine venting. It may have worked if I could keep the oil from that vacuum port but it ended up pulling oil and burning it. The exhaust would burn your eyes when you first started the engine.

[DMC5180]

I tried to put vacuum on the passenger side valve cover to improve engine venting. It may have worked if I could keep the oil from that vacuum port but it ended up pulling oil and burning it. The exhaust would burn your eyes when you first started the engine.

This is why there is an anti-splash plate covering the bottom of the filler neck opening. It might work successfully if you installed a protection plate between the port opening and the valve train.

[Bitsyncmaster]

This is why there is an anti-splash plate covering the bottom of the filler neck opening. It might work successfully if you installed a protection plate between the port opening and the valve train.

I figured that may help. I've seen those plates in all the valve covers that have PVC venting. But I did such a good job sealing the valve cover gasket, I don't want to experiment with it now. I put the port at the top corner thinking oil would just drain from gravity.