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How does a timed breather work? #783023 08/29/19 10:30 pm
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Danam Offline OP
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Can someone explain how a timed breather works? What the purpose of timing it, and not just having it open to the atmosphere like a B50?

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Re: How does a timed breather work? [Re: Danam] #783025 08/29/19 10:57 pm
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Think you want to vent blowby on downstroke but create partial vacuum on upstroke to aid power stroke.


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Re: How does a timed breather work? [Re: Hugh Jorgen] #783042 08/30/19 5:31 am
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Danam Offline OP
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Wouldn’t a vacuum in the crank case keep resistance on the piston traveling up?

Re: How does a timed breather work? [Re: Danam] #783051 08/30/19 7:46 am
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Yes but it also stops leaks and is so small anyway.

Re: How does a timed breather work? [Re: Danam] #783054 08/30/19 8:32 am
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It is obsolete technology that dates back to the pre-oil seal days when engines relied on slingers and scrolls to keep the oil on the inside.
Thus on the down stroke the fume vented through the breather and on the up strok e air got sucked into the engine via all of the shafts.
back then most engines were long stroke and reved quite slow by the standards of the 50's.
The shorter the stroke & faster the speed the less evective a breather is .
No one really woried much about it till Rex started getting things tested and found that the absolute best system was the treat the under side of the piston as a pump so suck in fresh , filtered air on the up stroke and expell it on the downstroke.
Yes. Pulling a partial vacuum does cost power, some of the Historic racers gaind around 1/2 horse by dumping the timed breathers in favour of a flow through.


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Re: How does a timed breather work? [Re: Danam] #783056 08/30/19 10:54 am
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Reducing the density of the crankcase atmosphere increases power, so they say.

But timed breathers aren’t very good at it.


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Re: How does a timed breather work? [Re: BSA_WM20] #783371 09/03/19 8:09 pm
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Moto gp racers use purpose pumps to create vacuum in the crankcase and the latest Ducati R bikes have such pumps. Why do they have these if it is obsolete technology and means loss of power?


Arnstein

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Re: How does a timed breather work? [Re: Danam] #783374 09/03/19 8:41 pm
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i use open tubes on my T120s. the 72 came that way, and i fit older crankcases with just a long tube.

maybe a timed breather or a reed valve would create some vacuum in there, but a simple tube long enough not to suck dust back into the motor is vastly more simple.



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Re: How does a timed breather work? [Re: Arnstein] #783377 09/03/19 9:41 pm
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Originally Posted by Arnstein

Moto gp racers use purpose pumps to create vacuum in the crankcase and the latest Ducati R bikes have such pumps. Why do they have these if it is obsolete technology and means loss of power?


It dont believe its obsolete technology in any way .... the crankcase has to vent blowby ...
Breathers are dealing with inadequate crankcase atmospheric volume
Where the crankcases are often sized just big enough to hold the flywheels , camshaft and a small oil sump .
The compact design of a motorcycle crankcase comes at a cost .
The pistons down stoke can significantly decrease crankcase volume .
Adding a timed angle of venting... through the camshaft ... on the pistons down stoke is one approach.
Adding the primary case volume to the crankcase volume
Was just another approach
Where increased volume already available was utilized in a different way .
The pistons downstroke has less pressurizing effect on a larger volume .


But gp racer are probably using vacuum , I'm guessing for a totally different reason .
Cyclic Crankcase pressure is a reaction to loss of volume as the pistons come down .
The stored pressure is returned on the pistons upstroke... there is no energy loss except for friction
But the pressure wave , i think , acts like a flywheel.
Decreasing crankcase pressure probably decreases any flywheel effect ... letting the engine rev more quickly ?

Re: How does a timed breather work? [Re: Danam] #783378 09/03/19 10:36 pm
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On my 72 and 74 Commando's I use a reed valve on the breather line. The 72 vents out the back of the crankcase and the 74 out the back of the timing cover with openings between the timing case and crankcase. Pre 72 Commandos use a timed breather similar to the BSA's. The idea of the reed valve is to reduce oil leaks by creating a partial vacuum on the piston upstroke. It seems to work on those two bikes. However, my BSA engine is also leak free with the timed breather.


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Re: How does a timed breather work? [Re: Arnstein] #783417 09/04/19 9:03 am
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Originally Posted by Arnstein

Moto gp racers use purpose pumps to create vacuum in the crankcase and the latest Ducati R bikes have such pumps. Why do they have these if it is obsolete technology and means loss of power?


They are not 360 degree twins.
Their crankcase volume does not fluctuate anywhere near as much as any BSA ever built .
They also rev a lot higher
Just because something is used on a track bike does not mean it is applicable to what we are riding
When Rex was doing the research on the Bunn Breather they actually found that ON SINGLE CYLINDER BSA and in particular the 441cc B44 on the test bed at Auckland Uni a 2 hp increase in power when using the strait through breathing.'
I think they also hooked up one of his spare B50 engines as well.
HD licensed the technology and they showed a Hp increase as well and Enfield did the same for some models but in their case I think it was to stop cranks leaking.
On all BSA's ( fit a gauge to your case if you don't believe me ) the timed breathers work really well at idle.
The crankcase pressure drops slightly up to a point some where between 2000 & 3000 rpm depending upon the actual model then increase from there right up to the redline


I have no idea about F1 race bikes
I do know, about BSA's from 1927 through to 1972
And once they put oil seals on the crankshaft the breather does very little and in many cases actually causes an increase in crankcase pressure at higher revs because the timed system can not compensate for the latency in the pressure pulses.

And to really cloud the waters with somehting that is equally irrelevent to BSA's the breather on 4G63 Mitsubishi engines cross over at 3000 rpm & actually work backwards so when you hook up LPG ( also totally irrelevent ) you have to replumb the breather to the air filter before the gas delivery point or the breather fills the crankcase with LPG vapour .

Last edited by BSA_WM20; 09/04/19 9:09 am.

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Re: How does a timed breather work? [Re: Danam] #783441 09/04/19 3:07 pm
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To answer the original question, "How does a timed breather work?"

In principal it is a disc valve , similar , sort of, to a two stroke disc valve induction system. But unlike the two stroke which is timed to inhale on the upstroke of the piston and fitted directly to the crank, the A65 type is located at the end of the cam shaft, the disc has two small holes cut in it 180 degrees apart ( because the cam is half engine speed) and is driven by two small pegs which are married to two cut outs in the end of the cam, a light spring keeps the disc sealed against the case face where it lives. It is timed to open on the down stroke of the piston, expelling case gases, it closes on the upstroke to create a partial vac in theory keeping the cases below atmospheric pressure, however....
And its a big However,
to get from the crankcase to the end of the cam shaft takes time, gas is passed into the timing chest , makes a 90 past the idler pinion and cam pinion, makes another 90 into the end of the camshaft , squeezes through the 1/8" inch diameter hole then heads through the camshaft to the timed disc , through the tiny disc ports then out . All this wriggling around and squeezing takes time, compare the size of the holes going into the head for intake fuel and exhaust gas with the 1/8" diameter camshaft restriction and its a wonder it works at all. As Trevor mentioned above , as the revs rise it fails to cope, oil is squeezed out with the blow by and after a decent thrashing there will be a drip under the gearbox sprocket as a result, also the back wheel and swinging arm will never rust if the bike is ridden frequently.

You might well wonder why the camhole is only an eighth diameter? I do anyway, I reckon its because if it was any larger , even more oil would be expelled, I dunno, if anyone out there has opened it up and can comment first hand I would be happy to know?

When BSA first introduced timed breathers on the singles they were a big improvement over the earlier flapper breathers, but on the singles the gas path is a lot less convoluted, when the idea was carried over to the twins it didnt work nearly so well.


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Re: How does a timed breather work? [Re: Danam] #783488 09/05/19 2:44 am
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Gavin, how were the flapper breathers worse?

I noticed, on my '69 BMW R69US, there was a reed valve on the top of the crankcase. If I remember right, it was under the air cleaner. Now going on the popular myth that BMWs are the pinnacle of engineering, how would this reed valve rate as a breather? Seems simple enough...


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Re: How does a timed breather work? [Re: Danam] #783494 09/05/19 4:50 am
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The BMW reed valve breather works well. I’ve used it on a Triumph. Your R69S may have been upgraded from the previous spring-loaded disc design.

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Re: How does a timed breather work? [Re: Danam] #783498 09/05/19 7:14 am
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https://www.dropbox.com/s/78v5hgoatu9b2cv/P1000571.MOV?dl=0

This video was made in feb. -17 and posted in a similar thread then. I also took the motor up to 4-5000rpm without any difference in the vacuum. Might be that a different under or overpressure would have been showing if holding that rpm for longer period of time, I dont know. I would gladly test this again but my "new" motor is 90 degree so..
To pump air in and out of the crankcase in any engine does not seem to be the way to go of the ones lookin for as much hp as possible? Making underpressure with pumps seems to be.


Arnstein

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Re: How does a timed breather work? [Re: Danam] #783505 09/05/19 9:47 am
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"Gavin, how were the flapper breathers worse? "

it was part of the Goldstar development program, the fibre disc type flapper valves did not cope with high revs as well as the timed breather ( on the pre unit singles). Shame really, later reed types with low mass parts have proven themselves


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Re: How does a timed breather work? [Re: Danam] #783586 09/06/19 6:27 am
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I have both the timed disc and a reed valve on mine. The timed disc behaves as it was designed.
Mine breathes as a later Triumph, through the primary. The Mike's XS reed valve is mounted on the pipe from this breather.
About the only change I noticed after installing the reed was a smaller oil spot under the back of the primary when parked.


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Re: How does a timed breather work? [Re: Danam] #783587 09/06/19 6:46 am
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You will also accumulate some oil (from mist) in the cam, so as long as the end is open it will always drop a tiny bit out. More so if you haven’t got an extra breather. I found similar when I was using a 10mm pipe off my inner timing cover.

If your varying cam timing then you NEED an extra breather, otherwise the timed disc is out of phase, but otherwise these isn’t much on difference performance wise between with an extra breather (open pipe) and the timed breather.


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