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crankcase breather

7K views 9 replies 5 participants last post by  bpounds 
#1 ·
do all bikes have a crankcase breather? if not, why wouldn't a bike have a crankcase breather? why do some have an air filter attached to theirs but others don't? why does the vent hose direct back to the airbox so that it can be recycled back into the engine? why would you want that?
 
#2 ·
Far as I know, every ICE has to have a breather. Seals and rings are not perfect. There will always be some pressurized gasses entering the crank area. Positive ventilation is also a good thing, and reduces carbon and sludge buildup. Old engines just had a drip hose, which let pressue escape, but made a mess dripping oil, and did nothing to help clean the engine internally.
 
#3 ·
Crankcase breathers are to release the blow by from rings and eliminate a pressurized engine block resulting in blown seals with oil leaks. For years, up to mid to late 60s the crankcase breather was an open tube to atmosphere. This was an emissions concern with oil vapors entering our atmosphere. The manufacturers found that by simply putting the vapors into the intake they would burn without too many issues. The biggest issue is when the blow by get worst in higher mileage engines the added oil mist would foul plugs. Now days the same system is still used with a vapor canister to help eliminate this issue. Motorcycles have always had a smaller emissions footprint so crankcase vapor being rerouted into intake hasn't been around all that long. Speed oriented people learned quickly that the oil vapors were a deterrent to making the most of of fuel charge entering the combustion chamber, so they vented to atmosphere using a small filter. This has been the norm for performance alterations to the intake system. So, long story short, it's good for the environment and not to good for performance.
 
#4 ·
on a balanced 3 or 4 cylinder engine the volume of air in the crankcase can be constant as the pistons move up and down

but with a single cylinder and a Vtwin the pistons would be compressing the air in the crankcase on every stroke, if it had no way to breath

on our v twins the air pumps out and is sucked back in thru the breather tube on every stroke of the pistons - thats why it must have someplace clean to go (the air box) - because its like breath - it goes out and comes back in.
 
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#5 ·
Positive crankcase ventilation - pulling those vapors out before they condense as oil contamination.

In the early days of controlled ventilation, they used engine vacuum to create a negative pressure in the crankcase. This was done with a PCV valve. The PCV is commonly referred to, incorrectly, as a pollution control valve. In fact, it means Positive Crankcase Ventilation. Few cars come with a PCV these days, because most use CCV instead, which stands for Continuous Crankcase Ventilation. Usually this involves a duct into the intake plenum, which uses the venturi effect to create the vacuum needed to ventilate the crankcase. In both systems the oil laden vapor is burned during the combustion cycle.

Vacuum production is in short supply on most motorcycle engines, so it seems natural at least to me that they use the CCV system. On my 1100 it is nothing but a short hose from the top of the front cylinder to the rear side of the air filter housing. It isn't even a venturi systems as far as I can see, it just uses the negative pressure created by drawing the air through the filter element. To be honest, I just had it apart, and I did not study the shape of the housing closely, but it appeared to be nothing but an air port. So I am assuming a bit.

Did any bike ever use a PCV? As Les said, only recently has CCV become a thing on small engines such as motorcycles, so I suspect PCV had fallen out of common use before bike applications appeared.

Racing applications aside, my opinion is the PCV/CCV is something you really want on your car and truck engine. Keeps stuff clean, both inside and outside. The old days of opening up an old engine and finding it filled with sludge are largely over. Some of that is due to detergents in our oils, but much of it is due to crankcase ventilation.
 
#6 ·
Modern CCV uses manifold vacuum, the main reason for the changeover from PCV was by putting the case under vacuum it reduced the ammount of oil that made it's way into the combustion chamber,
Cleaner emissions that way.
The side effect is that a leaking seal or gasket now constitutes an intake air leak leading to lean codes.
Even something as petty as a chipped oring on a dipstick can cause problems.

On the race bike we run an electric dynamic vacuum pump attached to the cases to keep oil out of the chambers.
 
#7 ·
I haven't seen manifold vacuum plumbed directly to the engine. How do they set that up Jaguartech? Is it regulated down somehow? Is it a full 20"hg or whatever vacuum?

My old Jeep has a PCV, even though I have installed a fuel injection system on it. That uses ported vacuum from the throttle body. The PCV valve keeps the flow at a low level. Just a few inches I would guess, never measured it.

Some of you guys are old enough to remember the little round gauge you could buy to check your PCV. You would open your oil fill port, usually in a valve cover, and set this little disk shaped gauge over the port. If it was pulling in air as it should, your PCV was working.
 
#9 · (Edited)
Jag V8's have a dual system, part load and full load breathers. Both come off the valve covers, the full load system is diaphragm controlled so it closes at low load conditions. Both go to the throttle elbow after the throttle plate.
BMW and Benz use single systems that are capable of controlling CC pressure at any load.
VW and Audi systems are similar but they are know for problems. All have separators that prevent excessive oil vapor from making it into intake systems.
The negative pressure the systems operate at are simply higher than the older PCV system and provide better oil control.
the pain in the butt part is when something fails and lets unmetered air in the system it disturbs mixture. It can be very difficult to locate the leak, that's where a smoke machine comes in handy. A leak over .020" will cause a ck engine light.
 
#8 · (Edited)
All high end, high horsepower motors have some type of crankcase evacuation system for performance gains by creating a vacuum in the crankcase. First picture is typical race set up. Second picture is electric system for street/strip use.

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