compression ratio vs. pressure tester

From: Kipp 98Leland (Kipp_98Leland@usmma.edu)
Date: Thu Feb 20 1997 - 11:54:31 EST


     Hello everyone, this is my first post on this mailing list (I am also
     a member of the Mopar Mailing List) and even though I don't have a
     dakota I love to hear theoretical discussions about engines and cars
     and trucks. This higher pressure measured by the compression tester
     than the computed pressure based on the compression ratio got me
     thinking.
     
     Perhaps the pressure tester has higher psi than the compression ratio
     would predict because the fitting that screws into the spark plug hole
     takes up more volume inside the combustion chamber than the spark
     plug, yielding a higher compression ratio while it is in there (the
     spark plug displaces less volume while screwed in then the fitting for
     the compression tester while screwed in).
     
     I guess that would be a quick and cheap way to bump up the compression
     ratio (slightly, of course), a plug company could make a spark plug
     that takes up the most volume without causing piston interference and
     the compression ratio would go up .3 or something.
     
     The only other explanation I can think of is that the piston moves so
     fast it produces a pressure "wave" that slaps into the gauge and gives
     a momentary higher pressure (due to its velocity) than the average
     pressure in the cylinder.
     
     Anyone think this is likely?
     
     -kipp leland (in digest mode)

 



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