At 07:50 PM 9/18/2010, Don Hanson wrote... >how does a fellow or a gal determine the compression ratio of a motor? > Measurement and mathematics. You can usually find the bore x stroke pretty easily. Then you only need to measure the head volume at TDC. That's often done by pouring a light oil in through the spark plug hole (but that depends on the layout, won't work for a plug on the side or bottom). If you start with a known qty then fill the cylinder, how much you have left will let know how much you put in. From there, it's just calculating. > I've been thinking to take a reading with my compression > tester, then divide that by the barometric pressure. Is that > correct? That won't even get you close. First, the compression is mostly adiabatic, the pressure rise increases temperature, which in turn makes the pressure rise further (this is how a diesel works). The pressure is effected by valve timing and cylinder/piston/ring wear. It's effected by cranking speed. Basically, there are too many variables other than compression ratio. |
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