Evacuating the bottom end of air pressure makes the rings work better, and frees up some horsepower that is lost when the piston goes down into a pressurized area.


There's a No duuh... and I never thought about it..
That might be worth messing with.

In the stock setup, the breather hose is connected to the air box and if it huffs/puffs in and out....so at least it is getting filtered air.  The hose is connected to the air filter box just before the carb inlet...so it is not subject to a lot of vacuum - but does get a bit and it does pull the vapors into the engine where they can be burned (oil and water droplets fall into the bottom of the air box and are drained out).  The vacuum is going to increase as the throttle is opened and the rpm increases - but it will only be as strong as the vacuum through the air filter.

I am not sure what kind of weird things would happen if you tried to connect the fitting to the engine side of the carb....downstream from the carb.  There are really large pulses going on....and being a single....the volume change inside the engine is equal to the volume change at the inlet port to the head - so given enough time to equalize the flow would be exactly the same.....the engine wouldn't try to pull anything through the carb! This would most likely have the biggest effect at low throttle settings when the air flow through the carb is low. The vent system is restrictive however, and the path from the bottom end, metal scrubby screen in the head, and flow through the vent nipple and hose does dampen out the pulses.  I would guess that as the engine is running, there is a positive pressure in the crankcase as the piston is near the bottom of the stroke, at a negative pressure as the piston is at the top of the stroke.....and the pressure varies with the rpm and throttle settings (as more throttle/rpm will create more blow by past the piston and rings).

I don't know if you will ever be able to create a vacuum in the engine - you might be able to increase the scavenging a bit.  When they have a PCV system on cars/trucks - does the PCV port somehow get incorporated into the position of the throttle plates in the carb....so that at idle the engine is not pulling too much air out of the engine and affecting the idle air/fuel flow?

Armen:

I have seen the drag race cars with the breather pipes running into the headers, with a valve to prevent the backflow of exhaust - I thought they were just getting rid of the crankcase vapors.....I didn't know that the were actually trying to create a negative pressure in the engine interior.

Dave,
Exactly.
The gag with not using intake vacuum is that the most vacuum in the intake occurs when you need it the least for crankcase pressure. Closed throttle-more vacuum/WOT and very little vacuum. The most ring blowby happens at full chat.
Rings don't seal because of spring tension, they seal because the pressure above the piston forces the ring out against the cylinder wall. Most of an engine's internal friction comes from ring drag. In the race motors that use crankcase evacuation systems, they also use low tension rings. By creating vacuum in the bottom end, there is a pressure differential above and below the rings, so even a low tension ring seals well. Less drag=more HP.

Serowbot-translation of your comment is "Why move forward?"
There have been threads about oil consumption when the bikes are ridden hard. A more sophisticated crankcase breather system might be a way to reduce that problem.

The advantage multi-cylinders have is only blow by is being huffed out the vents.  Whereas the thumper has to breathe and oil loss at high speed maybe from going from vac to pressure because of the 'small' diameter breather line.

Armen wrote on 08/09/16 at 15:20:22:

still the point is, Ø1/2" breather line vs Ø94mm piston = major restriction.
Driving a piston against vacuum is no better than pressure.

This the same as ram induction theory , might have a h/p boost on a thousand h/p nascar engine , but on a 33 h/p single cylinder it not going to do squat-crap-zero-nada but maybe blow a gasket & cause a leak!!!
There is plenty of H/P to be extracted from these engines without chasing your tails for 1/10 of one h/p ! The Topic Terminator has spoken !