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How to do the boosted car crankcase breathing right

 
Old 12-15-2013, 11:03 AM
  #46  
ptuomov
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Originally Posted by Strosek Ultra View Post
"One thing that has been shown to work with the 928s is the drainback covers that Mike Simard has made".

Tuomo, any pictures? ┼ke
You'll have to ask Mr. Simard himself.
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Old 12-15-2013, 07:41 PM
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What is the purpose of introducing fresh air into the system?
Is this because of the gases eventually being sucked by the intake?
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Old 12-15-2013, 08:08 PM
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Not really that - it helps to flush out the contaminants of which there are many in the typical blow-by combustion products including water vapor & likely some unburned fuel too, many of the combustion components are corrosive and best not just left to settle in the sump (the water vapor can contribute to sludge in the pan if not evacuated.

Normal breathing into the intake evacuates this mixture fairly sucessfully, esp. when the oil is hot - and the stock system flushes air in under many conditions. Evacuating only out of an AOS set-up does run the risk of separating out both oil and water vapor and returning both back to the sump... not ideal.

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Old 12-15-2013, 08:10 PM
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since I'm not using knock sensors, I could pull a vacuum from all 3 of the V plugs..... it can't hurt.... if you increase the vacuum on the lower block that will always help oil drain back... for sure...
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Old 12-15-2013, 08:26 PM
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Sterling,

How do you overcome any potential for knocks with no knock sensors?
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Old 12-15-2013, 08:48 PM
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Originally Posted by namasgt View Post
What is the purpose of introducing fresh air into the system?
Is this because of the gases eventually being sucked by the intake?
To get rid of substances that vaporize at low temperatures. Gasoline, water, etc.
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Old 12-15-2013, 08:51 PM
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Originally Posted by 928sg View Post
since I'm not using knock sensors, I could pull a vacuum from all 3 of the V plugs..... it can't hurt.... if you increase the vacuum on the lower block that will always help oil drain back... for sure...
In my opinion, it's more important to let those points breathe with the valve covers than to pull vacuum from those points specifically. My understanding is that with a vacuum pump you're lowering the overall cycle average density in the crankcase. It doesn't matter where you pull that from (as long as you don't pull it only from the valve covers, and even then it might not matter.). I think that it's just fine to pull the vacuum from the oil filler neck, and it's more important to connect those three knock sensor ports (and the oil filler neck) to the valve covers. The end effect is that the gas moves from those inside channels and oil drains down from the outside channels.
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Old 12-15-2013, 08:53 PM
  #53  
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Sterling,
No need to pull vacuum from those locations.
Just from the filler neck is needed.

The discussion for using those is when one is using open atmosphere breathing.
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Old 12-15-2013, 08:55 PM
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Originally Posted by Alan View Post
Normal breathing into the intake evacuates this mixture fairly sucessfully, esp. when the oil is hot - and the stock system flushes air in under many conditions. Evacuating only out of an AOS set-up does run the risk of separating out both oil and water vapor and returning both back to the sump... not ideal.
Water and gasoline will stay in gas form much longer, so you probably don't return those to the sump as liquids. However, if you can get fresh (non-saturated) air into the crankcase every now and then, that's of course ideal. If you have a vacuum relief valve in the heads, you'll probably get a decent vacuum spike in the crankcase when you close the throttle at high rpm, which will flush the crankcase with fresh air. I am thinking that at high rpm, throttle closed condition, there's almost no blowby and the vacuum pump is still moving a lot of air. You might want to calibrate the vacuum limiting valves to open up in those conditions. (All conjecture, haven't tried this.)
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Old 12-15-2013, 09:07 PM
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Originally Posted by Lizard928 View Post
Sterling, No need to pull vacuum from those locations. Just from the filler neck is needed. The discussion for using those is when one is using open atmosphere breathing.
Running the inside breather channels will help with or without a vacuum pump. All that the vacuum pump from the filler neck does is lower the average density. That will certainly help, and may be enough. But it might not be enough for a high-rpm stroker, so if someone has an ITB engine without knock sensors, using connecting the valley to valve covers is a no-brainer.
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Old 12-16-2013, 12:53 AM
  #56  
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Here's a visual:

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Old 12-16-2013, 01:05 AM
  #57  
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Originally Posted by ptuomov View Post
I am thinking that at high rpm, throttle closed condition, there's almost no blowby and the vacuum pump is still moving a lot of air. You might want to calibrate the vacuum limiting valves to open up in those conditions. (All conjecture, haven't tried this.)
I think in general its very different from that. Percentage blowby is worse at low load due to poor ring sealing @ lower pressure - so idle % is bad and on most vehicles overun % is bad.

Vacuum in the crank helps with lower ring sealing to some extent depending on vacuum level.

Total flow clearly is ~linear with rpm and (blowby is fraction of this) so no major blowby flow volume issues at low rpm. High rpm overrun can be particularly bad - except that on a 928 overrun fuel cut off helps for closed throttle cases (although even then doesn't eliminate some pure compression blowby).

So I think worse case is slow decelleration from high rpm - with throttle still open.

BTW the vacuum pump doesn't necessarily move much air... it can only move whats available to it. That is crank (etc) leakage, deliberately introduced fresh air flush flow, recirculation flow that you allow in the system design, actual blowby and a tiny amount of evacuated air volume to achieve vacuum.

Unless you are deliberately flushing with air (or have lousy sealing) the net evacuation flow out the AOS vent will always be ~the actual blow-by flow. Due to improved ring sealing its also likely to be a lower flow under consistent vacuum than it would have been without vacuum.

Alan

Last edited by Alan; 12-17-2013 at 10:19 AM.
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Old 12-16-2013, 02:37 AM
  #58  
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Adam,

With that picture, the rear knock sensor location is not the best place the take the pulse from.
It would be best to machine a port aft of the rear sensor where it appears to have the square with the lines.
For best pulsation cancelling effects, I think that a 2" pipe for the 6.5L engine would be best.
I would also machine the filler neck boss at the rear and put the remainder of the 2" pipe there.

This however again brings on a ton of questions as cutting a hole will bring in stress risers, and how do you attach to it? NPT threads will be hard to cut, and will induce lots of areas for possible stress risers and then fractures. ORB thread would be better than NPT, but still will be creating stress risers. You could weld the tube in, and this would likely be best, but welding aluminium to the reynolds 390 you would need either a spare block for the welder to test on, or a very experienced welder. And getting all the way around.... That would be near impossible.......
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Old 12-16-2013, 11:41 AM
  #59  
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If I understand correctly what you're saying, I don't think you are thinking about this correctly.

The pump volume flow scales about linearly in rpm as you say.

Percentage blowby grows somewhat when the compression stroke cylinder-crankcase pressure differential shrinks. This however is not terribly relevant to what I suggested, since when the throttle is closed at high rpm the cylinder density is extremely low. The mass flow of blowby will be very low when the throttle is closed.

So the net balance of gas out with the pump vs. gas in from blowby is going reduce the crankcase pressure when the throttle is closed at high rpm.

This is especially true if some of the stock breather system arrangements are also left in that use manifold vacuum to evacuate the crankcase (specifically the driver side connection from oil filler neck to the intake manifold plenum).

Yes, mass is conserved, so the vacuum pump can only move what is "available." The pump will move displacement times density in mass per revolution. Since gasses are compressible, this however means that the vacuum pump will cause the crankcase gas pressure (and density) to drop when the throttle is closed at high rpm. You can leave it there, or you can use vacuum limiting valves that will allow flushing the crankcase with fresh air from the valve covers in those conditions, increasing the crankcase pressure, density, and the vacuum pump mass flow.

Originally Posted by Alan View Post
I think in general its very different from that. Percentage blowby is worse at low load due to poor ring sealing @ lower pressure - so idle % is bad and on most vehicles overun % is bad.

Vacuum in the crank helps with lower ring sealing to some extent depending on level.

Total flow clearly is ~linear with rpm and (blowby is fraction of this) so no major blowby flow volume issues at low rpm. High rpm overrun can be particularly bad - except that on a 928 overrun fuel cut off helps for closed throttle cases (although even then doesn't eliminate some pure compression blowby).

So I think worse case is slow decelleration from high rpm - with throttle still open.

BTW the vacuum pump doesn't necessarily move much air... it can only move whats available to it. That is crank (etc) leakage, deliberately introduced fresh air flush flow, recirculation flow that you allow in the system design, actual blowby and a tiny amount of evacuated air volume to achieve vacuum.

Unless you are deliberately flushing with air (or have lousy sealing) the net evacuation flow out the AOS vent will always be ~the actual blow-by flow. Due to improved ring sealing its also likely to be a lower flow under consistent vacuum than it would have been without vacuum.

Alan
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Old 12-16-2013, 01:13 PM
  #60  
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Originally Posted by ptuomov View Post
...The pump volume flow scales about linearly in rpm as you say...
I didn't actually say that... and I don't see it's close to true for the system - though many assert that it is.

The pump flow is limited by its sources in the specific configuration... without massive flush capability there simply isn't a source of infinite air to pump. So pumped flow is related to RPM via the recirculation % and is additive to the actual blowby flow... and the blowby flow can be large & very variable relative to the recirculation flow. The comparison here should net flow out of the system @ ambient pressure.

I'm not sure we are disagreeing much, fully closed thottle is a lower blowby case here - agreed - but partial throttle is not - so you need to be careful. Allowing flushing at high rpm may flow a lot of air out the AOS vent - which up to a point is good - but not if it overwhelms a filter element type AOS.

Except in a race car high RPM is rather infrequent and rarely sustained so flushing at lower rpm's is more consistent in effect - so for me preferred. At lower pump RPMs the AOS flow is much less concern.

Alan

Last edited by Alan; 12-17-2013 at 10:32 AM.
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