Testing the Tachometer
Tachometer Testing Back to Projects
I was trying to evaluate the performance of my new propeller and needed a way to test the accuracy of my tachometer. I tested the tach by recording the running engine on my computer and analyzing the waveform to count cylinder firing. I set the engine RPM right at 2000 (on the tach) and made the recording. I used a simple microphone that came with the computer and stuck it in the engine compartment. The firing showed up as nice clear spikes on the recording. I have a sound processing program called Sound Forge that gives a nice display. I counted the firings in a given time period and calculated the RPM with the following formula:

RPM = count / time * 60 seconds * 2 revs per firing / 2 cylinders
For my measurement it was:
RPM = 86 / 2.62 * 60 * 2 / 2
RPM = 1969 = -2% from tach

-2% is as good as perfect since I couldn't very well expect to set the engine RPM more precisely than that anyway. One cylinder fires once every 2 revolutions (4 stroke engine). With 2 cylinders you get a firing every revolution. So the number of firings per minute is the RPM for a 2 cylinder engine. For a three cylinder it would be 2/3 (and so on). I also recorded at 1200 and 1500 RPM. It was harder to read the firing on the 1200 RPM recording because it was more masked by ambient noise. But it did work.