Ive seen a couple of high spec race V4's with an additional breather coming from the oil filler. Does anyone know why this is done and what benefit it gives?

I know the other consensus is to run a hose from the top breather to the filler hole, so there is a loop

You would generally run two breathers to help the motor cope with pumping in the crank case.
You could run a breather from the head and another from the lower case.
I would guess more importantly on the V4 you would run a breather from both heads.
Remember when the piston moves down you have 100cc of air below the piston that has to go somewhere.

porndoguk wrote:I know the other consensus is to run a hose from the top breather to the filler hole, so there is a loop

Yes, often used on enduro racers so that any oil that would have been blown out of the engine is returned to the sump.

So the more breathers the better then?

I wonder if its possible or even worth fitting a rear cam cover to the front head so the front head has a breather also?

Will that even work?

vfrman wrote:Will that even work?

No idea lol Might try it later

I read somewere that motogp and F1 use a vac pump to suck out the gas in the crankcase, dunno if its true tho, but would help to get rid of alot of the pumping losses. There was also something i read about nascars using a one way valve and venting breather tank into the exhaust so that the negative pressures ing the exhaust would suck out the crankcase gases.
Cheers
Ian

But what happens when the piston is in the compression stroke? Wouldn't you need to fill the volume created by the piston moving up the bore?

The one moving down the bore