I ran my stock single ring Ryobi 31cc today, and that thing seems to pull pretty hard with an APC 18x6. I didn't get my tach out and see what it was doing, but perhaps tommorrow I will get some readings. I did take a short video of it running and posted it on you tube. Not a real good video because I just thought to take the video as my tank was nearing empty, but thought I would post it anyhow. Neighbor came over and watched it run for a bit, and then came back and gave me another Ryobi weedeater he had sitting in his garage. He said it pulled to hard for him and he bought a newer easier pulling weedeater. The one he gave me is the older 28cc engine so I am hoping that maybe it has lots of compression due to an extra ring, but I will have to dig into it to see.
http://www.youtube.com/watch?v=XxFcxyWr7Dc