No Oil Filter???

On the power stroke, the hot gasses are forced behind the rings and the pressure expands them outward to the cylinder surface. This is how the rings seal. On the intake stroke, the rings have only internal tension holding them outward which means they seal far less.

When the vacuum spikes on deceleration, this is felt in the combustion chamber during the intake stroke. Especially since you no longer have the charge velocity to compensate. And, with very little happening on the power stroke to offset this, the net result is vacuum (or suction) drawing crankcase air around the rings and through the ring gaps. The resulting vacuum in the case draws air in from your vent.

RT

I run a sand-seal. I've also given it some thought as to the need for extra ventilation. I finally came to the conclusion that diesel engines work with a simple vent tube to handle the blow-by. They usually have a small mesh pad or some other baffle to trap some of the oil droplets but are otherwise just a vent - no other inlet into the crankcase. Yes, some could argue that a diesel has many differences, but the environ- ment inside the crankcase is very similiar.

And I also use Chevron Delo 400 - in a straight 30 weight which is recommended for gas and diesel engines and is designed for "closed-crankcase" engines.

RT

*knocks chess pieces all over the board with frustrated look* OK, where's Bob or Jan when you need them!?!? ;-) I dunno, dude. I was just feeling left out and trying to be a part of things. *sniffle* Wow, Dan Rather almost cried during the closing of the news just now. I'm kinda shook-up over that. :-/

I understand that much 100%...

"this" = what? I think you mean "this" = "vacuum on intake/combustion side."

Suction on intake/combustion side of things on the power stroke, right?

So you're saying all of the vacuum in the crankcase is a result of vacuum in the intake/combustion and is from gases escaping past the piston rings? Like osmosis or something, right? Stuff moving through a semi-permeable membrane (around the rings) from an area of higher concentration to an area of lesser concentration or something to move towards forming a happy balance in pressure? If that's the case I'd hope that at least the vacuum in the crankcaseseen on deceleration is several orders of magnitude less than what is seen on the intake/combustion side of the piston or I'd think you've got some serious ring wear. Unless I didn't understand your explanation. Thanks for at least trying to explain it to me. I'd love to know if I followed your explanation ok or if I missed something.

Jeez, Shaggie -- I'm sorry! Didn't mean to overlook you! But now that you've made a scene about it and everyone in class is looking at you, come up to the front of the classroom and tell everyone what you know about dished cams.

..............This is going to be good.

[BIG ol' discussion snipped]

Keep askin' questions, Shaggie -- I'm totally drawing a blank on this whole vacuum in case issue. Raymond: you took a good shot at explaining it, and for someone sharper than me, a light bulb might have gone off, indicating enlightenment. But as it stands, I am benighted in my inability to understand how the case can have any vacuum at all. All I picture is this big hot magnesium box with pistons going in and out, those going out matching the swept volume of those going it, and bitty bursts of hot gases leaking into the case on every power stroke. The very concept of the case developing negative pressure w/r/t the outside world boggles me.

And:

in the same post. Just found that interesting. :)

Kidd

"We're friends. You smile, I smile. You hurt, I hurt. You cry, I cry. You jump off a bridge.... I'm gonna miss your dumb ass."

How can it get pressurized Mike? Could it be that pressure in the combustion chamber makes it's way past the rings and pressurizes the crankcase?

Under decelleration, you have an engine that's trying to turn fast, and a closed throttle, resulting in a high vacuum. Now air in the crankcase is sucked into the chamber (again, past the rings).

you can't assume the rings are 100% effective, then it makes some sense. The entire reason you need a crankcase vent in the first place is because of combustion gasses making their way past the rings. The same issue is present under high vacuum.

John Aircooled.Net Inc.

Wow. And I thought that /I/ had a dirty little mind. But Kidd: "Mayonnaise-like"? What's Jan been eating?

Hang on -- when a piston is pulling against a vacuum there certainly is a pressure differential across it. And the most extreme case would be if there was a total vacuum in the combustion chamber. The inside of the case is pretty close to atmospheric pressure -- 16psi. So the maximum pressure differential across the piston would be 16psi and not an ounce more. That's several orders of magnitude less than the pressure on the outside of the piston when the air/fuel mixture is burning during the power stroke -- anyone got the figures for peak combustion chamber pressure for these engines?

(Of course, if the rings leak a lot when there isn't much pressure across the piston, and very little when the pressure is great, or if they seal better in one direction than another, my whole argument collapses like a bunch of broccoli.)

you got it, but despite the fact that the pressure differential is smaller it's still there.

like me hitting you in the head with a small hammer instead of a big one, LOL. :-D

John Aircooled.Net Inc.

LOL...no lets don't...please....LOL

------------------- Chris Perdue "I'm ever so thankful for the Internet; it has allowed me to keep a finger in the pie and to make some small contribution to those younger who will carry the air-cooled legend forward" Jim Mais Feb. 2004

don't know how what i posted would be a "wrong guess", but since you agree maybe it is wrong?

------------------- Chris Perdue "I'm ever so thankful for the Internet; it has allowed me to keep a finger in the pie and to make some small contribution to those younger who will carry the air-cooled legend forward" Jim Mais Feb. 2004

Cool, I'll keep all that in mind. :)

I don't think it's very much. You can run your own test even if you don't have a sand seal. Simply plug off the vent hose opening and run it up to 5 grand with the oil fill cap off. Drop the throttle and put the palm of your hand over the opening. Of course, with engine braking, the effect lasts longer but you should be able to feel it.

RT

(Question was about reasons why VW went from 2 stud flat cam to 4 stuc dished cam....)

1) the dished cam head allows room for a larger oil pump. (The pump did get larger in later years) 2) the 4 studs spaced further out from the center provide stronger hold for the cam gear to cam face mating point. Nice bonus.

Jan

Diesel engines don't develop intake vacuum. One reason they must rely on a vacuum pump to provide vacuum for the brake booster and other vacuum operated gadgets.

Jan