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-   -   Sportster Crankcase Pressure / Engine Breathing / Wetsumping and Mods (http://xlforum.net/forums/showthread.php?t=2073932)

60Gunner 5th August 2019 22:23

You know what, tee off your vent hose in the tank.

dieselvette 6th August 2019 00:00

Thinking out loud about that crappy oil cap I had - there is one thing I can say for certain - with the stock, old umbrellas and whatever blowby was occurring - there must have been at least momentary pulses of pressure in the oil tank , enough to push past the spring-loaded o-ring on that temperature stem.

Would a better vent at the heads have fixed it?

Or were the pulses not even getting there and were only in the cam chest/tank? In that case, an umbrella at the cam chest may have worked better.

Get the check valve as close to the source as possible to minimize CC pressure. Shorter line the better.

Get it as far from the source as possible to minimize oil puking. Restricted breather bolts, etc so pulses are dampened.

Hippysmack 6th August 2019 00:26

Using the adage from Dr Dick,
When in doubt, do as the factory did.
If you have oil puking, first replace the umbrellas.
Then if you still have oil puking, do a compression / leak down test to see if you have low compression / excess blowby.
Is so, inspect the top end for ring / valve leaks.

If these don't fix it (especially with engine conversions) research the breathing mods before performing any.

Did you replace the breathers and try that?
The pulses go everywhere in the entire breathing path, CC, cam chest, oil tank, rocker boxes...
When I pulled my oil cap, I could hear the air pulsing inside it.
That's why the breather valve works in top or the bottom.
The air is pulled out at the vent opening no matter where it's at.
The closer you (vent) to the source, the lower overall CC pressure will be.
Too low and you impede scavenging.
Those that have done the Deimus mod effectually lowered CC positive pressure a little.
The vent location doesn't control oil puking, Too high of pressure from blowby and increased intake air does, gasket leaks also.
I don't think the breather bolt holes dampened anything but rather raised overall CC pressure behind it.
The negative pressure on upstroke needs to compensate for the positive pressure on upstroke.
Adding more positive without adding more negative = oil puking.
Negative is also increased with blowby.
But negative could minimize if the pistons pull oil up into the rings while positive is still increased with more cylinder volume = oil puking.
Leaky valves in the heads can cause extra blowby past the rings into the crankcase.
Did you really think there was a short answer to all this? :)

dieselvette 6th August 2019 01:38

Quote:

Originally Posted by Hippysmack (Post 5767941)
The pulses go everywhere in the entire breathing path, CC, cam chest, oil tank, rocker boxes...

I disagree. At least, partly.

Certainly there are pulses everywhere. But they will be most dramatic below the pistons, less so at the cam chest, and even less at the valve covers. This will vary depending on RPM, and how restrictive the paths are.

Consider an open hole at the timing plug. This will definitely breath air in-out pretty dramatically. Connect a 6ft hose to that fitting, and notice a net outflow with much more subtle pulses. Now I would think you have a similar scenario when you get to the heads - even with no umbrellas at all, the pulses here would be a lot less dramatic than at the timing plug.

For this reason, I think the krankvent at the cam chest would net the lowest overall CC pressure - for better or for worse. Because the pressure goes higher there with each pulse than it does at the heads, more air would be pushed out with each pulse. (Of course the difference in venting methods would vary with RPM, in the same way that exhaust systems perform differently at different RPMs)


(Edit: actually a krankvent at the timing plug would net the lowest overall cc pressure, but that is impractical)

Tomcatt 6th August 2019 01:47

Is there some end result you're seeking with this discussion?

Hippysmack 6th August 2019 02:19

I think he's trying to convince himself to do the Deimus mod.
There is nothing wrong with this mod.

But the pretense for doing this mod thinking your solving a crankcase pressure problem takes more diagnosis than speculation.
We need flow rates on the different breather designs before concluding it's necessary.
I'm just saying 'wanting to' is different than 'needing to'.

You are not pushing all the pos pressure out on downstroke but rather cycling the air that's there thru pos and neg directions.

If there was an airborne fly in the crankcase, he'd hit his head on both sides of the cam chest wall as he was pushed and pulled thru the splash ports.

You may have higher pressure in the crankcase due to the splash ports being partially blocked by the squirters.
But that pressure is also being sent up the head drains.
If you tapped into the crankcase wall, you may end relieving pos pressure there (reducing scavenging).
That would also take testing.
The MoCo intentionally bottled up pressure in the lower end.
It wasn't to satisfy the EPA.
So any changes you make, think about what the equal and opposite reaction will be. :geek

60Gunner 6th August 2019 03:12

I noticed no difference in the strength of the pulses in the cam chest or the heads. I do have some additional hose length both before and after the Krankvent in the heads. Also the horseshoe before is 3/8" but the roughly 2.5 feet after is 5/16.
I'm going to reduce the length of the line after tho now that I know I'm not getting any oil out of it. I may even run no hose off it for awhile.

dieselvette 6th August 2019 04:56

I'm not interested in doing any mod, unless there is a reason to. My reasons would either be to reduce oil dripping, or to solve a cc pressure problem. I don't expect either problem because I'm not increasing displacement, and I'm reducing blowby with new rings. But we'll see.

I am interested in simplifying the diagnostics, for posterity. I think it's simple enough -replace this, replace that, etc as previously mentioned. But it seems like Hippy is looking for more than that, so i'll indulge.

Quote:

Originally Posted by 60Gunner (Post 5767967)
I noticed no difference in the strength of the pulses in the cam chest or the heads.

In earlier discussion there was some speculation (from Hippy I think) that the pathway from cam chest to heads *can be* inadequate. From what you're saying, it's plenty open. So that's worth noting.

I'm an idiot, and I still can't wrap my head around, or can't remember, why higher CC pressure leads to wetsumping, except that if it's what i'll call "really high" then windage causes more oil to be suspended?

Four Speed 6th August 2019 10:38

Quote:

Originally Posted by Tomcatt (Post 5767957)
Is there some end result you're seeking with this discussion?

What would be the fun in that?:p

Hippysmack 6th August 2019 14:18

Quote:

Originally Posted by dieselvette (Post 5767975)
I'm not interested in doing any mod, unless there is a reason to.
I am interested in simplifying the diagnostics, for posterity. But it seems like Hippy is looking for more than that, so i'll indulge.

In earlier discussion there was some speculation (from Hippy I think) that the pathway from cam chest to heads *can be* inadequate. From what you're saying, it's plenty open. So that's worth noting.

I'm an idiot, and I still can't wrap my head around, or can't remember, why higher CC pressure leads to wetsumping, except that if it's what i'll call "really high" then windage causes more oil to be suspended?

My apology's, I assumed that's where you were thinking.
Where I'm going with this discussion is simply understanding.
edit:
I've been compiling this information in the pedia already.
But it isn't helpful if the information is not clear or is not viewed.
This thread was started in the interest of exposing some of these intricacies to people that want to understand Sportster Crankcase Pressure / Engine Breathing / Wetsumping and Mods.
Also to help me understand it too.

I've learned a lot so far here and that's what it's all about.
Thanks again to bustert, for introducing the slack tube.

Oil tank:
Into the oil tank, oil is sent by the scavenge pump. This is not gravity doing it.
It is pressurized by the scavenge gerotor action.
As we know, the displacement pump doesn't make pressure, only flow.
Just as in the feed side, the scavenge side outlet from the pump is restricted.
This restriction is what builds the pressure. Pressure has energy.
The energy is present as long as the restriction remains (up the return line).
When the oil reaches the tank, the oil is dropped down into it.
The energy it took to get the oil to the tank is released into the air above the oil and flows back to the cam chest through the vent line.
At the same time CC pressure is pulsing the vent line but the released energy is stronger than the pulse so it flows back to the cam chest.
The scavenge gerotors create more power than the air below the pistons.

CC pressure:
The air below the pistons initially is charged by the beginning of the downstroke.
This initial surge is what gets bottled up temporarily until the restriction from the splash ports is overcome.
This creates a little more initial energy in the CC but that is only temporary until the surge gets to the splash ports
(where that energy is released into the cam chest.
That pressure is further restricted in the cam chest by the small holes at the lifter blocks it has to go thru.
When the pressure gets into the pushrod tubes, that energy is released into the rocker boxes.
Then restricted again by the umbrella path and released at the head vents.

I'm not sure inadequate is the right wording.
The head breathers manipulate CC pressure (create more restriction to make higher pressure inside on purpose.)
Newer (OEM) engines don't have the actual problem of wet sumping like older engines did (possibly because CC pressure was raised) upon normal riding conditions.
Higher CC pressure actually helps scavenging (pushes the oil to the scavenge port for pump pickup).
But higher CC pressure also picks up more oil in suspension (creates more dense air/oil mix).
Too high density oil is more difficult to separate from the air and pushes it's way past the umbrellas into the head vents.
The air/oil density can be increased with excess blowby (think 5000+ RPM) and engine upgrades.

And yes, higher windage especially from excess blowby or bigger pistons, does create more oil in suspension (higher density air/oil mix).
....wait for it.......
When doing engine upgrades, the Deimus mod will lower the higher pressure created from bigger pistons, more compression, bigger valves etc.
But, before calling the dogs and peeing on the fire, testing should be done before and afterwards for flow rates of crankcase pressure to know for sure if a mod is needed and if it was successful.
Willy Nilly doing mods can be a recipe for adding more problems than you had to begin with.

We are working at this point toward ways of testing CC pressure to be able to address this in a more controlled way.

edit:
Ill add that we mis-name wet sumping as oil puking out the vents.
That is a CC pressure problem, not wet sumping.
Wet sumping is simply too much oil left in the sump due to not being scavenged fast enough (several different reasons why and some do not involve the oil pump).
But wet sumping can lead to too high of oil density that pukes oil.

Also, anybody that has questions or refutes, please chime in.
As stated, this is all about understanding... for all of us.


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