A Tool question

[PA Dodger]

OK, the suspense is killing me.  What does a "dual gauge compression tester" do???  Yes, apparently it tests compression, but how is it better than the single gauge tester I already have?  I like to have the best equipment to tell me what that funny noise is coming out from under the hood of my Challenger.  It amazes me what all a header leak can sound like.
Dan

[duodec]

Sounds like a leakdown tester.  Compression gauges measure peak pressure the cylinder can build up when cranked.  The leakdown tester is used to meaure pressure loss from peak when the piston is stopped at TDC (how long till it loses X% of peak pressure, which is an indicator of wear or problems).

Picture of one is here (first one I found; I'm not familiar with the unit and this is note a recommendation for it...) http://www.racerpartswholesale.com/long29.htm

[mmccarty]

Yes, a leakdown tester.  You set the piston at TDC and attach a compressor to one side of the set.  You set the regulator on the first gauge to 80 lbs (aircraft industry standard, you can use higher pressures as long as you use it consistently in your tests).  There is a precision orifice between the first and second gauge and any leakage in the cylinder will register a pressure drop on the second gauge.  Measurements are recorded on each cylinder with 80/80 being a perfect seal with no leakage.  These are used for preventative maintenance checks on aircraft engines.  A running record would be kept of each cylinder everytime an inspection was performed.  Typical measurements would be better than 70/80 for new rings, climbing to 78/80 or so for broken in rings and then dropping over time with wear.  Below 70/80 a rebuild is in the near future.  Of course any deviation of a cylinder compared to the rest during an inspection would show a prolem with that cylinder that needs to be checked into further.

[mmccarty]

Read this, FAA Advisory Circular 43.13-1B  Chapter 8
See the part on differential compression testing

http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/99c827db9baac81b86256b4500596c4e/$FILE/Chapter%2008.pdf?bcsi_scan_4FFFF72CDE3A7F15=0&bcsi_scan_filename=Chapter%2008.pdf