Oil Breather / Recirculation and Catch Cans

29 replies to this topic
  • little_twin

Posted February 21, 2008 - 07:52 AM


That may be, but if it is sucking air it could be causing excessive crank case pressure. Not only did mine suck in a little garbage, but it began leaking small amounts of oil from the counter shaft seal, kick stater shaft seal, and from between the case halves. Needless to say it was not an easy fix.

  • swatdoc

Posted February 21, 2008 - 02:09 PM


OK thanks - I'll keep an eye out

  • grayracer513

Posted February 21, 2008 - 03:16 PM


There is no reason (or even, I venture, a possibility) of creating a one way pressure condition, either positive or negative, with the configuration swatdoc has pictured as long as there is a path from the cam box through the separator and out to the air boot. The crankcase and the top end are atmospherically linked to each other via the cam tower, so any pressure delta between them is transient at best. Pressure resulting from blow by is vented off through the breather as normal. At first, it seems reasonable to assume that the one way passage draining the separator to the crankcase could allow for some positive pressure from within the breather hose to travel into the crankcase rather than out, but that ignores the fact that the positive pressure in the breather is created by the downward motion of the piston into the air volume of the crankcase. Thus, when the pressure is positive, the one way valve leading to the crankcase closes, and the outflow of air is the same as the OEM setup.

On the upstroke, pressure reverses and becomes negative. The difference in the separator setup is that the inflow of air from the terminal hose (in this case, the one from the air box) is no different than it ever was. At the separator, the air flow back to the cam box is reduced by whatever amount of flow there is in the hose leading from the separator to the crankcase. Otherwise, there is no net change in the crankcase pressure as a result, and no net increase in inflow, nor reduction in outflow through the breather system.

  • William1

Posted February 21, 2008 - 03:24 PM


The only value I see from the valve would be oil spun off from the flywheel up into the tube would be stopped. The bottom line would only be able to drain.

  • grayracer513

Posted February 21, 2008 - 03:30 PM


The only value I see from the valve would be oil spun off from the flywheel up into the tube would be stopped. The bottom line would only be able to drain.

With the fitting tapped into the ignition chamber, which is outboard of the actual crankcase proper, there is no risk of oil slung from the crank ever getting to the end of the hose.

The advantage of the one way valve, especially when a small drain back hose is used, is that it prevents oil draining down in the hose from being pushed partway back up by the next positive pressure pulse. Oil and air will only go down, never up.

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  • swatdoc

Posted February 21, 2008 - 04:28 PM


Gray - thanks for the good explanation. I was starting to have some doubts. Yes the one way valve was used on the return line just to insure that the oil goes back into the case. I wasn't real sure if there were any pressure pulses in that outer ignition chamber or not, and if there were how strong they would be. So I just used the valve to make sure the oil wold flow down and not just be suspended in the can or the line.

  • grayracer513

Posted February 21, 2008 - 05:17 PM


The pressures in the ignition area are identical to the crankcase except for the time it takes the air to move from the crankcase out into that area. The CC also vents to the clutch side, and then to the trans cavity (which also connects to the actual sump, as does the ignition side. Remember that the only route crankcase pressure has the cam box is through the chain tower, which starts at the ignition case. The entire oil exposed air mass within the engine is connected, and the only difference in pressure anywhere at any given moment is due to the time it takes the pressure waves to move from the piston to the far reaches of the cavity and return.

  • ca412686

Posted March 21, 2008 - 03:12 PM


Have you had the bike running? Does your set-up seem to work ok? I need the smae kind of thing on my bike. The track I race on has long straights, so im spitting oil out all the time into my catch can.

  • todds924

Posted March 24, 2008 - 02:43 PM


Very clean and nice work!

  • swatdoc

Posted March 24, 2008 - 07:12 PM


thanks guys - bike ran about 30 - 45 minutes so far and all seems to be just fine.
Honestly, if this setup works for the number one supermoto team in the country, (if not the world) I really think it'll be fine for us.

Not only that, but this setup is almost EXACTLY like the oil catch and recirculation setup on the Suzuki DRZ400 SM as it comes stock from the factory. Oil breather from valve cover goes to can, air breather from can to airbox, and oil return line back into cylinder. Only difference really is Suzuki doesn't use a one way valve.

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