Forum

[Graham43]

In the past we fitted breathers to our bikes with an open end so why has the Electra X (and others?) have a duck's bill fitted? I assume the duck's bill allows air to escape but not enter but surely all this does is build up a small amount of back pressure (resistance of forcing the rubber lips apart) for what reason? Anyone got any thoughts on this?

[Craig]

In My Days of BSA's & Triumphs and a miss spent use. most people (Round Me in Nottingham) had Quite Large Diameter Pipers to there Rear Number Plate..Or Very Short in to a Coke or Oil Tin On the Engine..Perhaps (With rose Tinted Hind sight) a Bit of Oil in the Long Pipe Provided enough back pressure till a Cough fired it out..later to be replaced by more..While the Can Brigade where for ever re tipping it back in to there (With hind sight overfilled Oil Tank's) so perhaps the weight of Oil itself provided back pressure...Back in the 70's what Ever Bike I had seemed to use Oil by the quart ,so in comparison all my oilfields "Sorry" Enfields seem a lot better...IMHO ;0)

[sofiaspin]

For some inexplicable reason that has no bearing on fact, the former Chairman of a prestigious British bike brand club keeps calling my utterly reliable oil tight classic Enfield as 'Oilfield'. I have no idea where this prejudice comes from but admittedly he is a good 15 years older than me and obviously has a chip on his shoulders. I suspect it has more to do with the British class hierarchy. The irony is though, his preferred brand and every bike from that brand that I have come across, leak like a sieve. I am on my 4th Enfield and just two of them have left a few drops here and there.

[PeteF]

The reason for the duckbill as I see it.

Piston goes down, pressurizes crankcase, pressure escapes via duckbill.
Piston goes up, causes partial vacuum in crankcase as air cannot re-entry via duckbill.
Theoretical this should result in a partial vacuum in the crankcase but due to other factors like blow-by it probably doesn't. It should, however, result in less pressure in the crankcase than with no duckbill. This should help to prevent oil leakages.
Other designs use timed breathers to achieve the save end but any breather system should have some sort of nrv in the system. The reason we all fitted plain pipes on our breathers in the past was probably that we didn't realise what a nrv would achieve.

[Adrian]

This sort of thing is also used by some owners who want a more efficient non-return valve in the breather hose.



http://www.ebay.co.uk/itm/311446346729? ... EBIDX%3AIT



A.

[Tim NZ]

Large bore open breathers were all the rage on High Speed Daytona race bikes in the mid 60's...


For average engine speeds of sub 4500rpm some sort of low pressure differential valve is needed to maintain a net average Negative crankcase pressure. By maintaining a partial crankcase vacuum it improves ring seal efficiency.


The Duck-bill is actually an Open vent, rather small but open all the same. It relies on external surface air pressure over the flap area to close off, and as such it is very effective. (No actual moving parts) Cutting the flap back to stop it 'quacking' at low speed rather much destroys its effectiveness.


Do not think that the engine breather is pumping/sucking a volume equal to the engines displacement. It may appear to do so at slow cranking speeds, but the total Cubic Feet Per Minute of fume actually displaced is determined by the area of the smallest diameter in the breather, which is typically quite a bit less than 10 cfm
before 'back - pressure' (Net differential pressure) starts to occur at engine speeds over 1500rpm.


Over 4500rpm most singles (This includes 360 Vertical Twins)breathing capacity becomes saturated and a Large Open pipe is more suited.

[Craig]

Yes ! As Tim say's (For a good Tech answer.) :0)