FT: Frame (Twin) Elephant trunk breather

[youngjohn]

The interesting thing about a PCV, is that it is a 2 way valve. The design of the valve has a flat metal 'leaky' seal so that it lets air out of the engine and then restricts, but doesn't stop air coming back in. The important thing is the difference between the amount of air going out of the engine and the amount of air that the valve lets back in, and that is governed mainly by the weight of the 'plunger' and then the strength of the light spring. These variables are the difference between valves for different applications.

I'm not sure how well the flattened rubber tube 'duck bill' valves work as they are usually so far from the engine that the internal volume of the tube connecting them absorbs much of the pressure differences, reducing the valves ability to regulate airflow, particularly at high speed. This is why a PCV should be as close to the engine as possible.

 

[vibrac]

There is also the "condom" evidence to be considered (nothing ever went bang!) certainly in my experience the flow is hardly noticeable once a minute of running time has elapsed

 

[bmetcalf]

There is also the "condom" evidence to be considered (nothing ever went bang!) certainly in my experience the flow is hardly noticeable once a minute of running time has elapsed

Steve Hamel found that using a manometer connected to the timing chest.

 

[greg brillus]

Hold you hand over the ATD opening with the cover off on a running engine and feel the volume of air displaced by those two pistons belting up and down.......

 

[vibrac]

Hold you hand over the ATD opening with the cover off on a running engine and feel the volume of air displaced by those two pistons belting up and down.......

Yes so once the valve has stopped the air returning just think of the extra pumping effort passed back into propulsive power...

 

[stu spalding]

Hold you hand over the ATD opening with the cover off on a running engine and feel the volume of air displaced by those two pistons belting up and down.......

Doug Hart reckoned that the swept volume of the crankcases was just over 970cc.

 

[Dave M]

View attachment 17502

Thanks for the mention Bill! Hope you’re well.

From time to time I produce a batch of PCV breathers built into a rocker inspection cap which is designed to go onto the front cylinder inlet.
All PCV valves are not equal and it took me about 30 attempts and lots of patience to find a valve with the right characteristics which would work well on my Rapide and Comet.

It’s not a magic cure for a worn out engine, but it works really well on my bikes and I’ve had good feedback from many of people who have bought them.

Hi youngjohn I’ve just purchased a Rapide and I would like to fit a PCV breather, are you still making these?

 

[youngjohn]

Hi, I think I have a couple of kits in the garage, I’m not about until the weekend, if you pm me on Saturday I should have had chance to look for them.
Cheers

 

[oexing]

Heavy plunger type breathers are not a good idea in my eyes, there is too much inertia for that use. I made up several reed type ONE way valves on some engines successfully, placed at the end of some plastic hose , works without thinking about it . You just have to care for minimal contact faces when the reed blade (feeler gauge ) closes as some oil there makes things too sticky to open easily when required. I will definitely go for any other type of breather to avoid that horribly looking elephant contraption at this cosmetically sensitive mag inspection cover. Provided the valve does its job as designed there is very little air and practically no oil coming from the breather in use.

Vic

 

[Nigel Spaxman]

The problem with the timed breather is it seems like the factory, Phil Irving, Richardson and pretty well everyone got it wrong!! The breather doesn't need to open anywhere near TDC. BDC is the wrong place to close it. The idea seems to be that the crankcase is like a liquid pump and the outlet needs to be open as the volume is reducing. It is not a liquid pump. The crankcase has a compressible gas in it, but it is not filled with gas, unless the blow by is really severe. It needs to be open when the volume of the crankcase is smallest. If it opens anywhere during the top part of the stroke air is likely to go in through the breather, then it has to try to go out again near BDC, but then with the normal timing it closes.

The breather needs to open no sooner that 90 degrees after TDC and close about 90 degrees after BDC or it can be open a bit less. The main thing is it needs only to be open when the pressure is higher than atmospheric. You have to try to get it to do what a very quick acting check valve like a reed valve can do. It needs to be open only when the pressure is above atmospheric. It can do this really well if it is timed right, it doesn't even have to be that exact to work. The trouble is a lot of the information in the Vincent world encorages people to time them in a way that is not optimal.