Ryobi 31cc Target RPM


Expert Crash-Lander
Hey guys, My recent project has been converting a Ryobi 31cc wacker engine to put on a SPAD stick. The work on the engine seems to be done, and now I'm tuning all the kinks out of the system. Anyways, I was trying to check what RPM the engine was running at today, and I thought of a smart method. I took a video of it running wide open, then later went on my computer and found out what the frequency of the audio from the engine was. I found this to be about 158Hz. Assuming that the mic picked up a 'bang' for every cycle, or revolution, I then converted Hz to RPM (Hz x 60 = RPM) and got 9480! This seems rather high to me. The posts I've read say that guys are getting 7000-8000 with this engine on a 18x6 prop. So here are my questions:

Is this a reliable method to measure RPM?
Am I going to wear down my engine?
If needed, what can I do to slow it down? Richen?

This measurement seems very strange to me. When I got this engine, it didn't seem to have too much compression. Also, before I even started using it, there was light scoring of the head/sleeve. Thanks for any help.

Also, heres the video:
Click To View


Pro plane crasher
That is not how you measure rpm. to get it to turn slower you need more resistance (more prop), or you can adjust your throttle servo