Author Topic: 1961 on later breather  (Read 185 times)

Offline edboy

  • Valued Contributor
  • ****
  • Join Date: Oct 2012
  • Posts: 417
  • Karma: 3
1961 on later breather
« on: 03.08. 2019 22:11 »
hello,
can anyone explain the change to the 1961 breather. i guess the 2 holes in the driven pinion were relocated?
does the change make a great difference?

Online BSA_54A10

  • Wise & Enlightened
  • *
  • Join Date: May 2008
  • Posts: 1843
  • Karma: 29
    • BSA National
Re: 1961 on later breather
« Reply #1 on: 04.08. 2019 08:40 »
The breathers were originally biased towards being timed correctly for idle.
This ignored the latency of the air flow within the crank case.
Thus by the time you are at 4000 rpm it is working backwards to the pressure pulses.
So the revision supposedly pushed this higher up the rev range.
The timed breather was a good idea in theory but does not work particularly well in practice.

Think about the massive difference in performance that 2 stroke engines get with variable port timing.

For a timed breather to work at all engine speeds you need to do the same thing
Bike Beesa
Trevor

Offline duTch

  • Ricketty Rocketty Golden Flashback
  • Wise & Enlightened
  • *
  • Join Date: Oct 2011
  • Posts: 4445
  • Karma: 40
Re: 1961 on later breather
« Reply #2 on: 04.08. 2019 09:23 »

 
Quote
Think about the massive difference in performance that 2 stroke engines get with variable port timing.

For a timed breather to work at all engine speeds you need to do the same thing

 So if some enterprising young lad were to adapt a ATD to the BTH (Breather TopHat), he could be rapidly on his way to his second million $£$£-  *eek* in debt...?
Started building in about 1977/8 a on average '52 A10 -built from bits 'n pieces never resto intended -maybe 'personalised'
Have a '74 850T Moto Guzzi since '92-best thing I ever bought doesn't need a kickstart 'cos it bump starts sooooooooo(mostly) easy
Australia

Online muskrat

  • Global Moderator
  • Wise & Enlightened
  • **
  • Join Date: Jul 2009
  • Posts: 7937
  • Karma: 106
  • Lake Conjola NSW Oz
    • Shoalhaven Classic Motorcycle Club Inc
Re: 1961 on later breather
« Reply #3 on: 04.08. 2019 10:09 »
All hail Rex Bunn *grins*
'51 A7 plunger, '57 A7SS now A10CR, '76 XT500, '77 AG175 '83 CB1100F, '81 CB900F project.
Australia
Muskys Plunger A7

Online Triton Thrasher

  • Scotland
  • Wise & Enlightened
  • *
  • Join Date: Feb 2009
  • Posts: 1434
  • Karma: 20
Re: 1961 on later breather
« Reply #4 on: 04.08. 2019 12:33 »
Quote
Think about the massive difference in performance that 2 stroke engines get with variable port timing.

For a timed breather to work at all engine speeds you need to do the same thing

The other thing that two-strokes got was the reed valve.

Online BSA_54A10

  • Wise & Enlightened
  • *
  • Join Date: May 2008
  • Posts: 1843
  • Karma: 29
    • BSA National
Re: 1961 on later breather
« Reply #5 on: 05.08. 2019 10:28 »
Quote
Think about the massive difference in performance that 2 stroke engines get with variable port timing.

For a timed breather to work at all engine speeds you need to do the same thing

The other thing that two-strokes got was the reed valve.

Reed valves also suffer from resonance and latientcy.
At lower speeds the air moves almost instantly which is why the flapper valves on M series worked so well
As the speed increases the air in the crankcase becomes more resistant to moving so rather than flow it compresses, then flows, then decompresses till it is actually lower than atmospheric pressure.
Not sure if Rex put them in the books but the graphs of pressure ,piston speed and flow plotted against a time scale were facinating .
And the amount each of these were out of phase with each other increases with engine speed
In fairness to BSA the sort of gear we have available now days that can record data a million times a second was not available back then.
And we can punch a few buttons to get a dozen graphs printed out at a massively amplified time scale so we can see what is happening micro second by microsecond where as even if BSA could record the raw data it would have been a years work for a team of draftspersons just to plot the data to make the curves.

We had a member who used to race A7 powered go carts and he modified the timed breather by changing the shape of the hole making sort of comet shaped.
Right now I can not remember if it was sharp at the front or at the back but he reckoned it made a big difference to the engine, one of his little tuning secrets that we formed the BSA club to archive but failed miserably to do.
Bike Beesa
Trevor