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RE: PSI Changes: Plugs vs. Gauge

David Hoerl, you asked about cylinder pressures:
 
 > That is, for my 84 3.2L engine, Bentley's says I should get 160 
 > PSI on each cylinder. I measure 140 on lots of cylinders, nothing 
 > higher. Does this 20PSI differential sound plausable, or should I 
 > lay out another $50 or so for a newer gauge with a longer spark 
 > plug adapter
 
1. Your gauge could be miscalibrated; 140 is only 12.5% less than 160.  If all 
the measurements are close to one another, suspect guage error.  If they're 
not close to one another, the gauge could _still_ be wrong.
 
2. If your gauge is accurate, 140 psi vs. 160 implies the volume above the 
piston crown (when they are at TDC) is about 13% greater than the volume would 
be if you had measured 160 psi.  Let's call the volume above the piston crown 
the compressed volume, Vc.  The next step is to figure out exactly what the 
compressed volume is when a plug is installed, and to see if 13% of this 
volume is close to the volume you gain by using the gauge fitting instead of 
the sparkplug.  You'll need to know the volume swept as each piston moves from 
BDC to TDC, and the compression ratio (CR).  I presume you have the CR and the 
bore and stroke available from some sort of manual.
 
3. Calculate the swept volume: Vs = (3.14)*(bore ID)*(bore ID)*(stroke)/4
If you plug in the bore and stroke as inches, your Vs will be in cubic inches.  
If you have these dimensions in mm, divide them by 25.4 to convert to inches, 
_then_ make the calculation.
 
4. Calculate the compressed volume: Vc = Vs/(CR - 1)
 
5. Calculate 13% of Vc.
 
6. Check to see how it compares with the volume gain.  If you had a 
measurement of the _actual_ volume difference between the gauge fitting and 
the plug, you could of course do the whole process in reverse.
 
For all the _rest_ of you nerds out there, I have made the famous 
"engineering estimate" that it is not necessary to account for heat loss, and
that there is no appreciable change in the Z factor from 140 to 160 psig.
 
 Regards,
 Jerry
 Arlington, TX (25-Jul-96, 18:58)
 '95 540i-6 spd