Crankcase breather??

[cowb0y]

Can any explain the true function of a crankcase breather? I am wondering if it will increase the performance of my '80 GS1100?

[mike-s]

The crankcase breather is used to capture any blowby or oil vapour from the crankcase and put it into the engine in an effort to reduce the quantity of unburnt fuel/oil pollution sent into the air.

If you were to use a cannister (i.e. a film cannister packed with air filter foam, which would have to be periodically emptied as opposed to being allowed to vent directly to atmosphere) to capture the majority of the excess oil and fuel vapour the biggest benefit i can see would be a reduction in the temperature of the air going into the engine (thusly denser air going in) as well as a possible reduction in the level of CO2 going into the engine.

You might gain one, possibly three HP by doing so, maximum. A downside is if this happens and you live in a colder climate and still ride, you would loose one method of the engines keeping the carbs warm and the carbs may be slightly more prone to icing up.

There may be more pro's and cons, but those are the issues & benefits as i see them.

[fast pom]

Most engines have a breather somewhere prob no top of the cam cover the point of adding extra breather to the crank case prob using the oil filler cap is to vent excess pressure from the cases on tuned engines , not nes on standard. The cause of the pressure is blow back past the piston rings when using hi comp pistons.

[mike-s]

I took the original question to mean "why do i have this bit of tubing on my engine and what do i have to gain/loose by having it on/off my bike?", hence a general explenation of what its for. It is true that high comp pistons would increase crankcase pressurisation and so on, but i took that to be outside the scope of the question here.

Further to all that, on stock tuned engines the purpose of the breather being recycled back into the airbox is to reduce emissions. in California (With their zany strict emission laws) they take it a step further and you have to have a charcoal filter and other associated paraphenalia in there to capture the emissions and possibly feed the captured vapour (back in liquid form) back into the crankcase via one way valves & send the air back to the carbs to have the residue burnt off.

Thankfully these old birds are (for the most part) excempt from the crazy emission laws that have come into play (euro3 & euro 5 level emission regulation, etc).

[silverhorse47]

I thought the crankcase breather was to relieve pressure inside the crankcase caused when the sealed piston rings travelling down the bores? The valves relieve pressure...or hold it...on the upstroke, and the crankcase breather bleeds it to the atmosphere on the downstroke.

The routing of it back into the precombustion side of the engine for pollution control is a fairly recent thing.

S.

[paco13]

+2

What silverhorse said.
Without a place for the pressure to go it will make its own vent, through your seals.

[Nessism]

The routing of it back into the precombustion side of the engine for pollution control is a fairly recent thing.

S.

Recent depends on your perspective; bikes have been using PCV systems for about 35 years, and cars are pushing 40 or longer.

[Steve]

I thought the crankcase breather was to relieve pressure inside the crankcase caused when the sealed piston rings travelling down the bores? The valves relieve pressure...or hold it...on the upstroke, and the crankcase breather bleeds it to the atmosphere on the downstroke.

Since there are an even number of pistons, and they are moving in opposite directions, the actual volume inside the crankcase remains rather constant. On a BMW boxer twin, however, there are rather dramatic changes in volume as both pistons go in and out at the same time. Also true on single-cylinder bikes, but on most multi-cylinder bikes, the pressure-relieving function is not as critical.

The routing of it back into the precombustion side of the engine for pollution control is a fairly recent thing.

Define "recent".
Not sure when the whole concept of venting the crankcase happened, but back in the '40s and '50s, cars had vent tubes from the valve cover that hung down below the engine so that the vacuum created by the moving air would draw vapors out of the crankcase. In the early '70s, that tube was re-routed back to the intake manifold through a PCV valve (Positive Crankcase Ventilation) that would purge the vapors under certain conditions, like high vacuum due to decelleration. I had Kawasakis in the late '70s that had vent tubes that routed fumes to the airbox. The idea of routing to the airbox instead of the intake manifold was introduced because most bikes had individual carbs and this would even out the diluted air to all cylinders without the complexity of a PCV valve. Recent? It's been happening for over 30 years, in cars and bikes.

Back to the original poster:
Your question is a little ambiguous. Will the breather increase performance? The breather by itself won't do much to performance. As mike-s explained, it can slightly dilute the oxygen content of the incoming air if you have a lot of blow-by due to worn cylinders or tired rings. If you are building a drag bike and are looking for that last tenth of a horsepower, keep the vent out of the air intake path. If you have a street bike, it's best (for the environment) to route it to the airbox. If you are running individual pod filters on your carbs, find a way to fashion a catch can to catch anything that drips out of there and be sure to check that can on a regular basis. Normally, there should not be much in there, so an increase in collection can indicate a deteriorating engine.

.

[silverhorse47]

Recent? It's been happening for over 30 years, in cars and bikes.
.

So that would make 'recent' less than 30 years.

S.

[JMHJ]

why does '66 or '67 stick in my mind as the last year for non-emission controlled engines in GM cars? that may not be right. at any rate, my '69 350/4 impala had the pcv system. and it was a runnin' machine.

[fast pom]

Jeeez you guys do go off at a tangent some times

[Steve]

Jeeez you guys do go off at a tangent some times

Yeah, but even tangents touch the target.

.