Shawn,
another angle on this is vibration. I have used the technique successfully on FW light aircraft, although as yet I have not had call to do it on a piston engined helo. Essentially as you are aware, the reduced compression will generate an uneven force on the crankshaft, thus inducing a signal on the vib sensors at 2 x Crankshaft speed. This is backed up by a strong vibration at piston firing frequency. Of course the strength of the "Strong" signal correlates to the severity of the compression loss. I must concede that on engines where I have detected loss of compression using this method, then the loss has been significant enough for the pilot to detect vibration in the cockpit, although without exception, and despite the higher frequency of the vibes, the Pilots always attribute the vibration to propeller out of balance. As to whether you could use this as a definitive method of detecting loss of compression is open to several questions, primarily one of cost, you need at least two localised sensors on a 4 cyl engine, one sensor for every two cylinders is a good rule of thumb. Add to that the cost of a spectrum analyser and we are suddenly talking about a couple of grand at least.
This method though provides an indicator, ultimately, we still conduct a standard compression check to confirm the cause, and identify the specific cylinder(s) at fault.
GW
