Hi all. Got into a discussion with a couple co-workers that ride as well and we couldn't come to a conclusion. Topic was on the M8 is it an either/or type thing for the breather bypass and the oil fill cap mod? Do they do the same thing as far as venting the oily mist to the atmosphere and not back into the air intake or do they do entirely different things?

 

Here's what I know , the and read about. the oil fill cap mod vents air out of the oil reservoir The M8 was pushing the crankcase oil vapor up to the heads and got pushed to the oil reservoir and some believed this was building enough pressure to cause the oil to be pushed into the bottom of the case and causing sumping in the crank case. So the oil fill vent was designed to allow that pressure to vent to the atmosphere. The external breather setup was removing the vapor to the atmosphere and going into the combustion chamber for reburn with fuel. The external breathers allow only air and fuel to go into the cylinders, no oil vapors. So IMHO, there are two different reasons and results for either mods. Just trying to undo what EPA what's done to inhibit the full potential of the V-twin engine.

I would like to know how other V-Twin makers are curing this problem.

 

I think about it this way....

Blowby causes a positive pressure in the engine (that includes the heads, engine case, and oil case as they are all connected). The positive pressure is a gas made up of combustion products and oil vapor. That pressurized gas is normally relieved through the head breathers and vented into the intake to be burned. The higher your rpms or the more worn your rings, the higher the pressure. If your scavenge oil pump doesn't keep the oil level low enough in the crankcase, it will be whipped greatly increasing the oil content of the pressurized gas (and robbing power). The hotter the oil, the more oil vapor in the gas.

Routing the head breathers away from the intake doesn't fix blowby or scavenge issues but it does keep you from burning a oil/gas mix. Venting the oil tank is just another way of relieving the pressure and not burning it as gas/oil. Doing both will help the system vent quicker but that's about it.

An old trick with car motors was to remove the oil cap and see how much blowby was coming out to gauge how worn the motor was.

 

Thanks guys