I had an OS 91 Surpass II in my RV-4 and it flew it scale but it did get boring. The plane at 10 lbs. is too heavy for this engine. I know it's a scale plane but I've discovered I want to have a little more fun with my RV. I've recently replaced the 91 with a brand new OS FS-120S-E Surpass. The engine fits beautifully inside the cowl with no cutting required for the cylinder. With the fit so nice, I don't know why GP doesn't suggest this engine as the maximum for the RV-4. Because of weather, I haven't been able to fly this combination yet but I hope to within the next couple of weeks.