What is it (breather on tappet cover?)

[Swarfcut]

 Musky, as this thread takes on a life of its own, that's all fine an' dandy, but can anyone explain how a small bore pipe open to atmosphere for a nanosecond can achieve any form of crankcase ventilation, with good bores and no piston blow by, let alone on a well worn engine? It would make more sense if the rotating breather bush was slotted to present a larger time period open, rather than just a hole that matches the bore of the breather tract.

  All I know is that by some miracle the system seems to work.  The larger the volume into which the swept engine capacity can be pushed, the lower the crankcase pressure above atmosphere will be, hence linking the crankcase to the primary chaincase on some Triumphs, in an attempt to reduce pumping power loss (or avoid someone else's patent?).

 Swarfy.

[muskrat]

G'day Swarfy.
I'm no engineer (just a plain old fitter & turner) or physics nut just a feel of the pants rider. All I know is it works. Lap times improved by 5 seconds a lap on a 2 minute lap (that's a lot to a racer).
I can understand your argument (nice not naughty) and yes it baffles me just as how can the suck, compress, bang and blow happen so quickly.  (don't have one for WTF) lol.
Cheers

[Butch (cb)]

So how much of this becomes academic if you switch your parallel twin from a 360 to a 180 crank?

[muskrat]

G'day cb.
It would still need to breath, but wouldn't need to be timed. There will always be some amount of blow by the rings producing positive pressure. Most multi cylinder car motors have PCV's. My rHonda 1100 has two 1/2" breather tubes exiting the crankcase.
Cheers

[kiwipom]

yeah Musky that is obvious so what is the point of the argument ? the engine is going or not going so dont start the bike and save energy,

[duTch]

 

So how much of this becomes academic if you switch your parallel twin from a 360 to a 180 crank?   

  I guess Beemers be also similar.... maybe
  For what is worth, Gutzzis are similar to that but slightly different with shared crankpin and 90°/270°(ish) firing, and I went through this a bit when I thought I was having breather issues with my Gutzzi which has a LM II type remote breather box of a design much like Sluggo's diagram.,  and concluded with Tom my local Gutzzi guy that the breather box needs to have a volume of half the engine capacity (rule of thumb,  and box is about 100mm x 90mm x 50mm for 846/946cc engine capacity)- I vent it into a juice bottle between the filter pods and that vents out a ' T ' piece towards the pods

[Daithi]

Here is a set-up I ran on my T120. I don't have any data to say if it made any improvement to power or not. The motor did "feel" more powerful and revved out quicker. Wish I access to a dyno back then just to see but there was a definitely a noticeable difference. I miss that bike. Dave

[Rocket Racer]

I'm sure the standard breather arrangement is a fine compromise for average road rally/club run use.
However on a race engine there are so many references to Nortons, BSA's and Triumphs of the period having breathing issues when working hard. Just look at the preunit singles, the XB motors had a breather, which was changed for the ZB motors which was further amended for the goldstars. On top of that articles typically talk about extending the timed breather duration due to the higher road speeds of today.
Add to that 6000-7200 (thats all I'm admitting to) on my road rocket lap after lap is not what the factory had in mind for their over the counter bikes breathing system.
Like Muskrat I run a bunn breather off mine. With the 6 speed box my engine sits continuously in the 5500-6500 range
The breathers allow my engine to not leak but they do deposit a modest amount into the breather catch bottle after a full race weekend.

[berger]

I have just come across a geezer on that face book thing that has been doing some incredible speeds 123mph + on one and 160 on the other[ photos shown of bikes] running A65s on nitrous and turbo's . one of the photos showed a crankcase breather that could have been 1 inch bore. I can't remember his exact words but he said he ended up blocking off the big breather and said BSA must have known that their breather was good enough as the extra one he fitted wasn't needed after all---------- something to ponder on over your mince pies 

[muskrat]

G'day berger.
As RR  suggests on a full bore ride it like you stole it the std breather can't do it. My 57A7SS saw 8 grand (in 1st,2nd & 3rd) and 125mph at Eastern Creek with a Bunn (only a std 4 speed). At the end of a race it might have 2oz in the catch bottle.
Even on a road motor it allows the motor to rev more freely not having to suck and blow.
Cheers

[berger]

8 grand musky I have taken mine to 7 in first and second and that sounded scary, I do agree with what you say because mine leaves its mark from the breather when I ride it like I stole it. it was just something that was put on face book and a question was asked about his big breather and he said he blocked it off. no pub for me MAN FLU

[Greybeard]

no pub for me MAN FLU

Oh, that's serious. Have you been to Casualty? I hope you can enjoy tomorrow.

[BSA_54A10]

G'day berger.
As RR  suggests on a full bore ride it like you stole it the std breather can't do it. My 57A7SS saw 8 grand (in 1st,2nd & 3rd) and 125mph at Eastern Creek with a Bunn (only a std 4 speed). At the end of a race it might have 2oz in the catch bottle.
Even on a road motor it allows the motor to rev more freely not having to suck and blow.
Cheers

It is all in Rex's blog if you can wade through it or the 2 books he has published on motorcycle breathing.
Auckland university is still doing research and no doubt there are a few papers in their archive on breathing should you wish to subscribe .
There is nothing magic about it.
It takes energy to compress a gas and also energy to pull a vacuum.
It takes a lot less energy to pump the air through

Rings do not make a perfect seal so if the rising piston on compression has a vaccuum below it some of the compressed charge will bypass the rings due to the pressure difference.
If the air under the piston is at full atmospheric pressure then there is less pressure difference so less pressure on the charge to bypass the rings.

The same applies on the exhaust stroke.

All the rest is about resonance, everything that applies to exhaust pipes applies to breather pipes.
This is where it gets very complicated because you have to take the stiffness of the tubing & it's diameter into account as well as the length.

The final part is the actual valves used, how much pressure difference is needed to open & close them, at what frequency they resonate , how fast they open & close and how good the seal is.

The last bit is the fluid dynamics of the crankcase gas itself.
Air has a weight thus it has a lag time from the time you start the piston moving down to the time that the air starts to come out of the outlet.
And similarly some air will still be coming out the end of the tube after the piston starts to rise.

The same applies to the air coming into the engine.

For the average rider the performance difference would be hard to notice apart from the fact that the engine no longer leaks.
To the Muskies of the world, it is a very cheap performance enhancer where every little bit counts.

[bsa-bill]

So how much of this becomes academic if you switch your parallel twin from a 360 to a 180 crank?

If your upto doing that I doubt you'd be stuck with getting it ti breathe properly 

[Black Sheep]

I know I'm being a bit simplistic, but if you use energy to compress a gas, you regain energy as it expands. So all you need is a timed breather to cope with the excess gas from piston ring blowby and the majority of air in the crankcases is just squeezing and expanding and staying almost but not quite energy neutral.
If you are pumping it continually, might you not be expending more energy?