[DeTomaso] Compression ratio question

Göran Malmberg hemipanter at hemipanter.se
Sat Sep 22 15:56:13 EDT 2007


A great deal, as a big cam has a lot overlap the cylinder will not build 
very much presure at
lower rpm:s, and defineatly not at cranking speeds. Therfore you must 
measure cylinder
pressure at an rpm at which the cam is breathing at its best, provided you 
want to know
the cylinder filling ability of the engine. To compare cylinder condition to 
each other, it
dosent matter.
You ask how much it matters. The same amount as the difference in Tq output 
depending on
different engine combinations.
Goran



and it right,the compression pressure in a fresh engines
cylinder is the result of compression ratio + eventually charging from the
intake air , minus pressure escaped from bypassing pistonrings,minus
pressure lost from the cams exhaust overlap to help get rid of the exhaust.I
understand that the rpm. and thereby the airspeed will affect this,but how
much?
One thing i know for sure ,is that its very important to have the throttle
wide open when measuring the compression pressure,as you otherwise build up
vacum on the intake side and get wrong result :)

Øyvind Bakken
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