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Differential compression at the bottom of the stroke

Sent: Tuesday, March 30, 2004 11:53 AM

Subject: compression test

Enjoyed the article about your facility in this months AOPA..
 
As I remember, there is valve overlap at the bottom of
the stroke.  How do you make your reading when the
piston is down?

G.R.
Instructional Assistant
Aeronautical Technology
T.C.C.


Thanks for the comment,

     Typically when we take the differential pressure at the bottom of the stroke, we leave the air pressure on cylinder just after the top dead position.  The piston will move from the pressure and we follow holding the prop on the combustion stroke.  From experience we know where to stop.  If the exhaust valve opens we rotate prop opposite of rotation until the exhaust valve closes.  The resulting reading is the bottom direct compression for the cylinder.  Tim

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