I never pay attention to the max rpm they list. I ran my 120 AX engine over 11,500 on the ground. The irvine I had only had a couple of tanks thru it. Yes keep it and they are great looking engines. i both a OS 160 and this one and maybe I can run them this weekend. I usually run most engines open exhaust as that is a great indicator of its performance and a good standard. mufflers can vary greatly so open exhaust is best. The irvine 150 has a small carb so it will draw very well and make great torque at low rpm. I would prop it so its in the 8,000-8,500 range for most sport planes. I plan to make an adapter so I can run a Jett 120 muffler on it and see how it performs.