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How to do the boosted car crankcase breathing right

 
Old 12-16-2013, 02:25 PM
  #61  
ptuomov
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Volume flow is constant in a positive displacement pump. Mass flow is not, if you are pumping gas and you have an inlet restriction.

I wouldn't try welding into the block. However, cutting a hole, and epoxying a pipe into it might not be out of question.
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Old 12-16-2013, 03:28 PM
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Pressure differences between crankcase bays


Hey Guys - am I missing missing something here or what?

While the bays are all separate, they're not sealed off from one another: there's the gap under each crankcase web and above the sump oil (or windage tray), so why the focus on external breather/equaliser passages?

As I see things, there will be a lot of swooshing and whooshing along this path as the engine runs. (great for keeping any loose oil up in the air as well).

In fact the more I look at it the more horrifying the situation appears.

The narrowest gap is below the bearing caps (green). Next widest is at each end of the bearing caps. (green)

But the widest (and which will have the greatest gas movement is along each edge (light blue), right underneath the oil return from the heads.

Meaning that at higher rpm's the return oil from the heads is going to be whipped into froth as soon as it drops from the galleries. Huh !!!

So, back my original proposal, why is it NOT a good idea to return oil from the heads via a different path to the floor of the sump?

Since that post I read some detail on BM's N-55 engine (mentioned in the first post) and found BMW does EXACTLY THAT.

"Oil Pan: ... the oil return ducts are designed so that they extend over the oil deflector. Consequently, the oil return ducts end in the oil sump."

Not a bad recomendation I would have thought.

So. Waiting to to hear those reasons.




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Old 12-16-2013, 04:00 PM
  #63  
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N55 doesn't by my information return the oil drains below the oil level. It does return them below the windage tray (which the Germans seem to be calling "oil deflector"), but not below the sump level.

If what you intended to suggest was to shield the oil drains from the piston pumping pulses but not run sealed extensions below the oil level, then I am all for that solution. Livernois sells that kind of extensions for some Modular Ford engines, by the way. While it doesn't mean they help, at least it means that someone is thinking the same thing as you are.



I wouldn't want to return the drains with sealed pipes all the way under the oil level, because it's just about a foot of height and you need less than half a psi of pressure differential between the crankcase and valve covers to push oil out of the sump into the valve covers thru a submerged pipe. In fact, one has to encourage some air flow in those drains if one doesn't have separate inside-bank gas flow channels.

The reason why everyone is focusing on the external breathers is the following: There's simply enough flow area in the stock 928 sump. The sump is too shallow, especially for displacements over 5.0L. If you could fit a regular Hot Rod American V8 oil pan in the 928 chassis, we wouldn't be having this conversation. In my opinion, there wouldn't be much of a problem to solve. Pan spacer is no brainer for the 928, but one can only fit a narrow one and even then it's a pain in the *** to change many components at the rear of the engine.
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Old 12-17-2013, 04:06 AM
  #64  
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Old 12-17-2013, 06:30 AM
  #65  
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Hello gents,

I have been thinking of using similar oil rails as in the following kit to prevent possible high pressure to push oil back into heads and allow the returning oil to flow directly to sump.
The sump supposed to be in low pressure area and therefore the oil should flow there more easily without excessive aeration.
The high pressure area is generated to passanger side of pan by fast spinning cranksaft, me thinks.

http://www.928motorsports.com/instal...stallation.pdf

The rest of the windage tray may be little too restrictive causing some pumping losses, but it may still be good for minimizing oil aeration..
Oil aeration together with low oil level in sump will result to situation where air doesn't have time to separate from oil, before oil is again recirculated.
https://www.highpowermedia.com/RET-M...3/oil-aeration


Note, talking about 5L NA stuff rather than pressurized intake. the blowby perhaps isn't that big issue with 5L NA if having good piston rings and low end cap..
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Old 03-24-2014, 03:09 PM
  #66  
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Default A note on the 928 shroud tubes

Here's a quick note about the 928 shroud tube that protects on the valve cover ports in the stock configuration. In stock configuration, one is used. Two can be installed without modifications. Up to four can be installed with two shroud tubes modified.

The purpose of the shroud tube is, in my opinion, to separate oil from air such that only air will exit from the breather elbow. Whether the part is successful in that is a separate question, I am just stating the belief that I think this is the intended purpose of the part.

As a separator, the cylinder shaped shroud is a so called volume separator. Based on my information, a volume separator has to slow down the gas flow to less than 1m/s to be effective.

If the shroud tube ID is about 2" (hypothetical, not posting this from my garage), then at 1 m/s it will flow about 4.3 SCFM or about 120 l/min. That's the maximum one can vent out thru a single shrouded port and expect the separator to work.

At 1% blowby rate, the single shroud tube is capable of venting the blowby from 430 engine SCFM which amounts to about 215 hp. For a stock GTS, the engine SCFM is about 770, which at 1% blowby rate is 7.7 SCFM of blowby and at 3% blowby rate 23 SCFM of blowby. Because of this, if for some reason the main "chimney" breathing is not functioning as intended, a single separator will be easily overwhelmed. Since there are four ports available, I think that anyone who's thinking about using stock-style valve cover breathers with the stock shroud should plan on using multiple ports and multiple shrouds.

What many of us have done, including my self, is to drill out the elbows to minimize restriction to flow and then expect the shroud to still work as intended. It may work, but there's one thing that needs to be verified. As the engine runs, an individual elbow should not exceed 4.3 SCFM flow rate. If unrestricting the flow thru the elbow increases the flow to the point that the flow velocity inside the shroud tube exceeds 1 m/s, that is, above 4.3 SCFM, the shroud may stop functioning as a separator.

The actual flow rate thru the shroud tube will depend mainly on two things. First, the pressure differential between the valve cover and the breather hose. Second, the elbow orifice size. The critical points at which the oil separation probably doesn't work anymore are:

Pressure differential, orifice size
0.5 PSI, 7.5mm
1 PSI, 6.3mm
2 PSI, 5.2mm

For example, if the valve cover is pressurized at 15.7 psi absolute pressure and the other side of the breather hose sees 14.7 psi, then elbow orifice sizes less than 6.3mm in diameter should still allow the separator to work. Elbow orifice sizes greater than 6.3mm in diameter may lead to oil being ejected from the elbow. By measuring the pressure differential and by knowing the elbow orifice size, one can verify whether the separator should be expected to work and the flow per elbow is below 4.3 SCFM.

Assumption in the above analysis is that the breather shroud entry isn't completely submerged in oil, in which case it would just flow out pure oil. Another interesting point is that one can't necessarily infer from a lot of oil being ejected thru the port that the shroud opening is completely submerged. It might simply be that the flow velocity inside the shroud exceeds 1 m/s and the breather flow 4.3 SCFM, which could lead to oil and air not being separated and a mixture being ejected from the port.

Last edited by ptuomov; 03-24-2014 at 11:42 PM.
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Old 03-24-2014, 03:47 PM
  #67  
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I really wish I had been brave/dumb enough to have been right next to Andy G's motor on the dyno stand, with the videocamera running back on 1/28/11, instead of standing in the control room watching the monitor. Had I done so, there would be video content showing the four plumes of liquid oil that ejected simultaneously out of these four crossed valve cover breather hoses just as this video ended:





https://rennlist.com/forums/928-foru...o-results.html



There were 4 breather shrouds, one attached to each elbow, they quickly became drinking straws at 6000 rpm.


-
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Old 03-24-2014, 03:59 PM
  #68  
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Rob --

Thanks for that note.

It's interesting that all four ports started ejecting oil simultaneously. I would guess (not to be confused with knowledge) that if the problem would be _just_ the heads filling with oil, then upstroke head on the passenger side would have started ejecting oil first. Instead, it was both sides ejecting oil simultaneously.

One could see this pointing towards the following hypothesis: The problem wasn't _just_ the heads filling with oil, but also the pressure in the heads exceeding the level that resulted in more than 4.3 SCFM flow thru each breather port.

How could have this happened? One possible way would be if the block valley breather somehow become overwhelmed. This would have simultaneously allowed the lower crankcase to pressurize, which in turn would have stopped the oil from draining and force the entire venting to happen thru the valve covers. At 600 hp and 1.5% (assumed out of ***) blow by rate, this would have meant 20 SCFM out the valve covers or 5 SCFM per port, which based on my above guesstimates would have been enough to overwhelm the shroud tubes' separation capabilities.

At minimum, food for thought.
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Old 03-24-2014, 04:28 PM
  #69  
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Originally Posted by Rob Edwards View Post
I really wish I had been brave/dumb enough to have been right next to Andy G's motor on the dyno stand, with the videocamera running back on 1/28/11, instead of standing in the control room watching the monitor. Had I done so, there would be video content showing the four plumes of liquid oil that ejected simultaneously out of these four crossed valve cover breather hoses just as this....


-
Was there any oil ejected from the 5th line? (Filler neck block off plate to air box)
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Old 03-24-2014, 04:30 PM
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If the shroud tube ID is about 2" (hypothetical, not posting this from my garage), then at 1 m/s it will flow about 4.3 SCFM or about 120 l/min. That's the maximum one can vent out thru a single shrouded port and expect the separator to work.
[Dumb Q] Isn't the ID of the breather elbow the limiting factor to flow, rather than the diameter of the shroud itself? [/Dumb Q]
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Old 03-24-2014, 04:33 PM
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In the stock configuration the crankcase breather in the valley is really quite restricted with a tiny intake orifice (few mm) and ~6mm orifice in the port to the throttle body... so it wouldn't take much... Given the other mods to this engine as depicted (all valve cover vents plumbed, all open) I wonder how that was configured (blocked off even?).

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Old 03-24-2014, 04:38 PM
  #72  
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Originally Posted by Rob Edwards View Post
[Dumb Q] Isn't the ID of the breather elbow the limiting factor to flow, rather than the diameter of the shroud itself? [/Dumb Q]
The elbow is very much the limiting factor for the quantity of flow, regardless of what percent oil and gasses is in the fluid.

However, as long as the flow in the shroud tube has a velocity less than 1 m/s, oil doesn't want to flow up there and (in theory) the elbow should only see gasses. If the flow velocity inside the shroud tube goes over 1 m/s, then oil does want to flow up there with the gasses and the elbow will see a mixture of oil and gasses. 1 m/s is an experimental result from a source I read some time ago, below that velocity gravity usually dominates the flow of gasses in determining the oil flow.

2" diameter pipe at 1 m/s is about 120 l/m or 4.3 SCFM.

Last edited by ptuomov; 03-24-2014 at 05:12 PM.
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Old 03-24-2014, 04:40 PM
  #73  
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Originally Posted by Alan View Post
In the stock configuration the crankcase breather in the valley is really quite restricted with a tiny intake orifice (few mm) and ~6mm orifice in the port to the throttle body... so it wouldn't take much... Given the other mods to this engine as depicted (all valve cover vents plumbed, all open) I wonder how that was configured (blocked off even?). Alan
I think they said they had a single -10AN breather line from the block valley. That's over a 0.5" ID for the hose by my understanding. It should flow significant amounts of gas, unless it get's filled with an oil plug that is suspended by the pressure differential.
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Old 03-24-2014, 04:46 PM
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Was there any oil ejected from the 5th line? (Filler neck block off plate to air box)
Nope. The motor ran at ~6000 for just under 50 seconds, at which point the valve cover breathers puked, but not the crankcase vent.

If you watch the video closely, the oil pressure (1st bar graph and the first gauge on the left) was >100 until 0:51, then it drops to 90 by 1:03, then to 80 by 1:12, at which time the oil ejection occurred. So the bottom end was slowly running out of oil to pump for at least 10 seconds there. I remember sitting there thinking I was going to say something when it got to 80, but the engine said something to us instead.

Heres' the vent hose off the filler neck:



And its routing:

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Old 03-24-2014, 05:26 PM
  #75  
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Originally Posted by Rob Edwards View Post
snip . . . So the bottom end was slowly running out of oil to pump for at least 10 seconds there. I remember sitting there thinking I was going to say something when it got to 80, but the engine said something to us instead.

...snip
Rob
What did the engine say?
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