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Old November 11th 04, 12:39 AM
Raymond Lowe
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Shaggie wrote:
> On Wed, 10 Nov 2004 21:59:07 +0200, Jan Andersson
> > wrote:
>
>> John Connolly wrote:
>>>
>>> it flows both ways in that tube Jan. It flows from crankcase to air
>>> filter
>>> when the crankcase is pressurized (when the engine is making
>>> power), but it flows the OTHER WAY during high vacuum decelleration
>>> (downshifting).

>>
>>
>> Sounds like your rings are shot John Either that or you
>> converted to 2-stroke
>>
>> Please explain to me where the vacuum inside the engine case comes
>> from?

>
> I've been wondering where the vacuum during deceleration in the
> crankcase comes from. I didn't know that happened. I can understand
> the vacuum in the intake manifold during deceleration, but not in the
> crankcase. I mean the throttle closes so air/fuel isn't dumping into
> the intake, but the pistons are still going and sucking in what they
> can through the intake valve on the intake stroke so that would cause
> vacuum in the intake area. I don't get it for the case, though.


On the power stroke, the hot gasses are forced behind the rings and the
pressure expands them outward to the cylinder surface. This is how the
rings seal. On the intake stroke, the rings have only internal tension
holding them outward which means they seal far less.

When the vacuum spikes on deceleration, this is felt in the combustion
chamber during the intake stroke. Especially since you no longer have
the charge velocity to compensate. And, with very little happening on
the power stroke to offset this, the net result is vacuum (or suction)
drawing crankcase air around the rings and through the ring gaps.
The resulting vacuum in the case draws air in from your vent.


RT
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