Crackcase beather flow

6 replies to this topic
  • MicahAdams

Posted September 15, 2007 - 04:51 PM


The large hose coming off the top of my valve cover pumps air like a leaf blower at idle. If you raise the rpm's if flows less. Is that normal?

If that was a car that would mean you are getting a ton of blow by

  • MicahAdams

Posted September 15, 2007 - 04:52 PM


BTW, its a 06 450

  • grayracer513

Posted September 15, 2007 - 07:42 PM


Yes, it's normal. The piston(s) in any engine displace air below them as well as above. In a single cylinder 450cc engine, that means that when the piston goes down, it pushes 450cc of air out of the way out through the breather, in this case. When it goes back up, 450cc is pulled in. At idle, this is very noticeable in a thumper. As the engine speed is increased, there is less time for the full displacement of the engine to be pushed out before the piston reverses the crankcase pressure again, so the pulse diminishes.

In a car, there is always one piston going up while one is going down, so the two actions offset each other, and there is little, if any, pulsing at the breather.

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  • Fastest1

Posted September 16, 2007 - 12:03 PM


There is a place on the cases where it looks like they could have tapped for an additional breather:thinking: . I have even read where people did it and claim faster revving. I have only seen that claim once or twice. I do have my motor out and dont have anything to do right now, so maybe:devil: Details soon!

  • MicahAdams

Posted September 16, 2007 - 07:31 PM


Ok that makes sence with the single piston vs multi cylinder motors. Thanks!

  • casperkc

Posted September 18, 2007 - 04:47 PM


Re-route that hose to your airbox especially if you ride off-road (mud). When you stall that hose sucks really hard. When you stall in a mud puddle that hose sucks really hard. Then it costs about 3500.00 dollars to fix the damage. I know this on a first hand experience.

  • swatdoc

Posted September 19, 2007 - 04:44 PM


bTW, what's a "crackcase" - is that something you store your crack in? lol :thumbsup:

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