Actually if it took the abient pressure into account it ought to read
14.7., give or take, I hate to ad another wrinkle here, but... 14.7
lbs/sq ft is average ambient pressure at sealevel. Most of us not rich
enough to live directly on the beach are a bit higher than that. Then of
course there's the problem of variations of the actual pressure at any
given moment from the average. That's what the television weatherman is
talking about when the talks about a low or high pressure system, and hence
why a barameter is a reasonalbly good predictor of weather.
At 09:12 AM 9/13/97 -0500, John Kahoon wrote:
>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 ?
>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)=3D132psi. If all that logic
>R->is correct then the compression gage reading (gage pressure)
>R->will covert to compression ratio as follows:
>R->This is different from what I intuitively though it should
>R->be,( CR=3DPgage/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 =3D 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->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->Date: 9/12/97 Time: 8:27:43 AM
> =FE SLMR 2.1a =FE email@example.com
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