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Ryobi 31cc Target RPM

jsut210

Expert Crash-Lander
#1
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
 

c172ae

Pro plane crasher
#2
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