Idea for the breather tube that comes out of head...

18 replies to this topic
  • DoctorRzed

Posted July 09, 2004 - 12:07 AM


I noticed that the tube that comes out of the head blows out alot of air. What if you route it so it puts the air into the airbox toward the carb? Will this give better airflow? Also I assume oil can come out of it so I would have to put something on the end?

  • Hamish

Posted July 09, 2004 - 02:01 AM


it blows and sucks as the piston goes up and down. Anyway, it's hot air which doesnt do your performance any good if inducted into the engine. Most cars have a PCV valve (posative crancase ventilation) which links the crankase breather to the inlet manifold. Bad for power, but good for emmision control.

  • grayracer513

Posted July 09, 2004 - 03:40 PM


The purpose of the tube is to relieve crankcase pressure. Remember that a piston displaces the same amount of air on both sides of the crown. In a 2-stroke, this is put to work moving air and fuel from the carb to the combustion chamber. In a four-stroke all that pumping is a parasitic loss.

The simplest way to get this out of the way is to give the air a way to come and go from under the piston with as little resistance as possible. This is done with large passages leading from the crankcase to the top end, and big hoses leading out of the engine. For the most part, it's a push one out/pull one back kind of thing where the air mostly moves back and forth. However, piston rings don't seal perfectly, so at heavy throttle some combustion gases "blow by" the rings and end up leaving through the breather tube, taking a very minor amount of oil with it.

As was pointed out, such hot and contaminated air would not be conducive to adding power if dumped in the intake. The best thing would actually be if the crankcase could be evacuated to about half to one quarter atomosphere, since the ramaining air would be less dense and easier to pump. Drag racers have fooled around with running the crankcase breathers into fittings at the end of the exhaust designed to use the exhaust stream to drag air out of the crankcase, but I don't know if anyone still does it. In multicylinder engines where one piston is going up for every one going down, this works well. In a single however, it gets to be a sort of trade off because the vacuum actually resists the piston's upstroke as much as the crankcase pressure resists the down stroke.

Any gain you would get this way would be very minor in a small motorcycle engine.

As far as automotive PCV valves are concerned, you will find that they require the manifold vacuum to be at mid-throttle levels or higher in order to pass crankcase vapors to the intake. At higher throttle openings, the vacuum is too low, and they close off and thus have no effect on engine power.

  • grayracer513

Posted July 09, 2004 - 06:25 PM


Confusion and education can cause similar symptoms, no?

  • stroker101

Posted July 10, 2004 - 04:37 AM


factory honda(?) routes the tubing in a canister then out and ties into the exhuast. check out the photo in the newest addition of dirt rider mag.

  • TonyDeCoster

Posted July 10, 2004 - 05:11 AM


Factory Yamahas have a reed valve setup on the crank vent that acts as a one-way valve to lower crankcase pressure. The new works bikes seem to tie this into the exhaust pipe to increase the effect.

  • DoctorRzed

Posted July 10, 2004 - 08:49 AM


WHAT ABOUT IF i PUT A BATTERY FAN IN THE AIRBOX :awww: :devil: :thumbsup:

  • Hamish

Posted July 10, 2004 - 12:36 PM


the idea of linking the crankcase to the exhaust has been around for ages. We used to do it on cars, weld a 1 way valve to the header pipe and connect a hose to the tappet cover. Pro stock bikes (and cars I guess) go 1 step further and fit an electric vacum pump to remove crankcase pressure.

  • SFO

Posted July 10, 2004 - 02:11 PM


Using the vacuum of the exhaust pulse will change the tuning of the pipe.
Just a point to consider.
The system used on the yzm of Doug Henry was for oil control and it would also return the oil to the crankcase.
The billet yzm motor had a minimal sump resevoir so oil control became critical.
Changing to a pcv valve for the crank breather has netted some interesting bonuses on some of the singles I have dyno tested not to mention twins, but you must remember by generating a vacuum with a pcv valve you are also asking the oil control ring to work differently and might have to experiment with cylinder finishes to get the results you are looking for.
Without being willing to adress the system you are not going to be able to troubleshoot the problems you create.

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  • aussiequadrider

Posted July 11, 2004 - 01:09 AM


So don't run either the head or the crankcase tubes into the airbox.
Is that right?


  • Hamish

Posted July 11, 2004 - 01:39 AM


not sure about your quad, but on the bikes, the crankcase tube comes out of the tappet cover. It's linked to the crankcase via the cam chain tunnel. If I were you I'd just leave it as is, unless you do deep water crossings then attention is required.

  • crf450f

Posted July 23, 2004 - 07:12 AM


I would suggest leaving it the way it is, air flow at the tube is usually blows out rather than in, so dirt does not enter. Improving the air flow to the tube would do nothing.

  • Ga426owner

Posted July 23, 2004 - 10:38 AM


I have a good idea - add some more tubing to where the breather tube will sit just on top of the countersprocket. Overfill you oil capacity & ride like a banshee - you will have the only self contained oiled chain all day long!
man you guys must be getting bored with the daily topics! :thumbsup:

  • Guest_monsieurfurious_*

Posted July 23, 2004 - 07:01 PM


Or you could just have a leaky countershaft seal and have a continuously oiled chain also. Been there still doing that.

  • bushy

Posted July 24, 2004 - 07:20 AM


How about you overfill and route it so it points out under the back fender.. You could spray everyone behind you. :thumbsup:

  • crf450f

Posted July 26, 2004 - 04:43 AM


ha ha, like this answer! Never thought there would be so many ingenius ways to route breather tube. That's putting it to good use. LOL :thumbsup:

  • crf450f

Posted July 26, 2004 - 04:46 AM


he he, good one!!

  • Ga426owner

Posted July 26, 2004 - 04:53 AM


How about you overfill and route it so it points out under the back fender.. You could spray everyone behind you. :thumbsup:

that is exactly what TMR did to Josh Grants CRF250 - oil breather attached to rear fender

  • DoctorRzed

Posted July 26, 2004 - 11:01 AM


I dont understand what the point of that was???

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