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Another diff breather question
https://auszookers.com/forum/viewtopic.php?f=5&t=38227		 Page 1 of 1
Author:  whincup [ Wed Jan 09, 2013 9:22 pm ]
Post subject:  Another diff breather question
My long range fuel tank doesn't use the line from the charcoal canister, is the diameter of that line big enough to use as the diff breather line? is it thick enough to allow it to breath as much as it needs to?

i plan to remove the charcoal canister and use those little filters on the end of the line in the engine bay
Author:  jdk81 [ Wed Jan 09, 2013 10:57 pm ]
Post subject:  Re: Another diff breather question
Plenty big enough... i did calcs on that a while back.

Just curious, how does your long range tank vent?
Author:  fordem [ Thu Jan 10, 2013 12:50 am ]
Post subject:  And another one ...
How large does a diff breather need to be?

OEM size on a SQ series Grand Vitara is 5/16" ID but on my Mitsubishi Pajero iO it's an unbelievably small 1/8" ID, same as the vacuum lines used in the front freewheel disconnect.
Author:  whincup [ Thu Jan 10, 2013 3:34 pm ]
Post subject:  Re: Another diff breather question
yeah i had no idea how big or small they had to be. the tank has a standard vent that attaches to the filler neck up the top.
Author:  whincup [ Thu Jan 10, 2013 3:34 pm ]
Post subject:  Re: Another diff breather question
yeah i had no idea how big or small they had to be. the tank has a standard vent that attaches to the filler neck up the top.
Author:  droverdave [ Thu Jan 10, 2013 3:48 pm ]
Post subject:  Re: Another diff breather question
The idea of a diff breather is just to allow the air inside the diff to expand and contract with temperature. if it was sealed up, when the air inside the diff got hot, it would tend to pressurise the differential casing, and potentially force diff oil out through the axel seals.

The amount of air movement we are talking about is incredibly miniscule. So pretty much any hose will do the job, it just needs to be not sealed up. A pin hole would be big enough.
Author:  Gwagensteve [ Thu Jan 10, 2013 3:57 pm ]
Post subject:  Re: Another diff breather question
- a pin hole? Sortofmaybe but not quite Droverdave.

I agree that the volume of air is very small, however, I think you've only taken into account 1/2 the problem.

It's completely true that if there was no breather on the diff, the pressure increase as the diff got hot would push oil past the seals.

In my opinion, that's not where the real problems lie.

the bigger issue is snap-cooling the diff. Drive along in high range for a couple of hours on a hot day, then dunk the diff into a water crossing or bog hole - Whilst the volume of air moved is small, the temperature change is quick. If the caps on the diff breathers do their job and are pulled shut in this scenario, then water is pulled into the diff past the axle/pinion seals - so a "pin hole" might work for the heating cycle, which is slow, it's the cooling cycle where the damage might be done if the breather is too restrictive.

I don't think it really matters what diameter hose is used - it's more about a size that is easy to work with. Air locker line is fine, so is 6mm fuel hose - it's more about the fittings than the hose (when it comes to size, not construction.

Steve.
Author:  jdk81 [ Thu Jan 10, 2013 4:11 pm ]
Post subject:  Re: Another diff breather question
I did some rough calcs on this
1/4" hose, 2m long on sierra front diff, snap cooled from 60-10 degrees.
Pressure due to hose diam and length effects was 0.075psi (515.6 Pa)
Fairly extreme conditions, as even under snap cooling, all the air losing 50 degrees instantly, just wouldn't happen. It would still take a while, meaning this figures are still hugely cautious.

http://auszookers.com/forum/viewtopic.php?f=14&t=25760&start=30

But your seals can still fail. You are just reducing the likelihood that they will fail.
Author:  droverdave [ Thu Jan 10, 2013 6:05 pm ]
Post subject:  Re: Another diff breather question
Gwagensteve wrote:
- a pin hole? Sortofmaybe but not quite Droverdave.

I agree that the volume of air is very small, however, I think you've only taken into account 1/2 the problem.

It's completely true that if there was no breather on the diff, the pressure increase as the diff got hot would push oil past the seals.

In my opinion, that's not where the real problems lie.

the bigger issue is snap-cooling the diff. Drive along in high range for a couple of hours on a hot day, then dunk the diff into a water crossing or bog hole - Whilst the volume of air moved is small, the temperature change is quick. If the caps on the diff breathers do their job and are pulled shut in this scenario, then water is pulled into the diff past the axle/pinion seals - so a "pin hole" might work for the heating cycle, which is slow, it's the cooling cycle where the damage might be done if the breather is too restrictive.

I don't think it really matters what diameter hose is used - it's more about a size that is easy to work with. Air locker line is fine, so is 6mm fuel hose - it's more about the fittings than the hose (when it comes to size, not construction.

Steve.


yer point taken.

the pin hole comment was a bit of an exaduration...but you get the idea.

I'm not sure how "snap" it would be for the air though, but fare enough, don't make it a pin hole :P
Author:  fordem [ Fri Jan 11, 2013 12:13 am ]
Post subject:  Re: Another diff breather question
Thanks fellas - I actually have the cap from the Pajero breather on my desk - I'm in the middle of extending the line from the original position (just above the right frame rail, say 20~24" over the surfcae on which the wheel rests) up to top of the firewall, the opening in the cap is maybe 3/32" diameter, I think I'll put a second one for good measure
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