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S-1E Mechanical Tach Question

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Jun 15, 2013
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Shawnee, KS
The tach cable broke recently in my Pitts S-1E. To avoid future failures of that kind, I replaced the tach with a Flight Data Systems T-30, and I am pleased with it so far.

One of the T-30 configuration parameters is the reference RPM, at which one wall-clock hour of engine operation produces one hour of tach time on the T-30 recording tach. My mechanical tach was a GMC AC unit, but the decal on the side that should indicate the model number and reference RPM is mostly worn off. The face of the GMC AC unit includes a red arch from 2100 to 2350, within a green arch that goes from 500 to 2700, which suggests to me that the tach may have been supplied with the S-1E kit with those special bezel markings for use with a Sensenich 76EM8-0-56 metal prop, more-or-less aligning with the "Avoid continuous operation 2150-2350 RPM" warning in the Pitts S-1S POH. The Pitts S-1S POH equipment list shows GMC AC model RT-7. The S-1S Parts List says "RT-7 HIGH". I have not found the tach model number in any of my aircraft records. A photo I found on EBAY showed the decal on an AC RT-7 tach, which named 2310 RPM as the reference RPM. This ACS webpage shows 2300 RPM as the reference RPM for the AC RT-7 tach: Mitchell Clockwise Mechanical Tachometers | Aircraft Spruce
So, I'm supposing I should set 2300 RPM as the reference RPM for my T-30 tach, which only allows values of hundreds or fifties for that setting.

My question for the Pitts Brain Trust: What is likely to be the true reference RPM for an S-1E with a Lyc O-360A4A, using an AC tach? Whatever that is, I plan to set the reference RPM in my new T-30 tach accordingly.

- Dave

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