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FooFoo valves


The 2CV engine uses a non-return (foofoo) valve to control the pressure in the crankcase. It is fitted in the oil filler. I was in contact with Peter Johnson who is building a Moto Guzzi powered car and he mentioned that Silverfish has a breather valve system on his Guzzi powered 'hopper.Peter said he was going to fit one to his build. Both Peter and Silverfish are using big block Moto Guzzi engines. I'm using a middle weight (small block) Moto Guzzi engine which just pipes the two rocker cover breathers to an oil separator with the gases feeding the air filter box and no foofoo valve.

This got me thinking about the different pumping actions of a V twin and a boxer engine. I did a bit of maths (see below for details) and found that, to my surprise there isn't a great difference between the two types of engine. The graph shows on the horizontal axis the angle (in radians!) between the cylinders and on the vertical axis the swept volume in arbitrary units. The peak volume (4 units) is at 180 degrees (3.14159 radians) for the 'boxer' twin. The value for the Moto Guzzi V twin is found at 90 degrees (3.14159/2 radians) and a Harley Davidson V twin at about 1 radian.
I'm puzzled, why should the two Moto Guzzi engines differ in the breather system? Does it matter? The only reason I can see for the foofoo valve is the 2CV engine has a high oil pressure whereas the small block Moto Guzzi engine oil pressure is about 50psi (in old money). Anyone know the oil pressure for the bi block engine? The vacuum in the crankcase will stop oil being forced through the crankshaft oil seals.
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Click here for the maths - this requires Adobe Reader - if you need Adobe Reader then Click to download


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