I've lost track of who wrote what...
but the gage takes the ambient pressure into account ( it reads
zero before you start the test, right ? )
so divide the reading by 14.7 and thats it, right ?
John Kahoon
R->Suppose the compression ratio is 10:1. I assume that means
R->the compressed volume (TDC) is 0.1 of the uncompressed (BDC) volume.
R->If so, the ideal gas law says the pressure is 10 times the original
R->pressure. The original pressure (absolute) is 14.7 psi, so the
R->pressure at 10:1 is 147 psi (absolute) so the gage, which reads
R->"gage" pressure should say (147-14.7)=132psi. If all that logic
R->is correct then the compression gage reading (gage pressure)
R->will covert to compression ratio as follows:
R->CR=(Pgage+14.7)/14.7
R->This is different from what I intuitively though it should
R->be,( CR=Pgage/14.7psi ), so I'm puzzled.
R->Either way, the numbers Pat posted are more like 11:1 than 7:1.
R->I just ran a compression test on a Spitfire 1500 cc engine
R->that I'm negotiating for. Readings (cold) are (117,117,110,85).
R->(without propping open the throttle, so these are low).
R->(Obviously a problem in #4, I suspect rings, and retesting
R->with oil in the cylinder raised the pressure)
R->Back to the lecture; using the equation above, this means the
R->CR (in the good cylinders) is about (117+14.7)/14.7 = 8.9:1
R->I think this engine is supposed to be 8.9 CR.
R->How have I gotten so close to the "right" answer
R->without even propping open the throttle?
R->Is my theory all wrong? (been a long time since thermodynamics!)
R->Is my (new Sears Crafstman) gage inaccurate?
R->Is it not necessary to prop open the throttle?
R->I let the engine crank through 3-4 compression cycles for
R->each measurement, until the max. pressure stabilized. Is
R->this the wrong procedure?
R->Maybe this engine has old, flat-top pistons, and a higher CR...
R->Thanks,
R->Ray
R->-------------------------------------
R->Ray W. James, P.E., Ph.D.
R->Texas Transportation Institute
R->Civil Engineering Department
R->Texas A&M University
R->College Station, TX 77843
R->Phone (409)845-7436; Fax (409)845-3410
R->E-mail: r-james@tamu.edu
R->Date: 9/12/97 Time: 8:27:43 AM
R->-------------------------------------
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þ SLMR 2.1a þ john.kahoon@pcohio.com
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