The published HP figures are not far off with an open exhaust. In actual use think of a 90 as around 20% more power than a 60. Supertigre 75-90s are very good "60" sized engines with carbs that many people struggle with. OS have great carbs but the engines are expensive with occasional issues that the mfg wont resolve. Other cheap engines are OS clones that can run every bit as well, or not. I now pull carbs off old OS 61s and put them on new Supertigre 75-90s. This makes for a cheap engine with consistent quality that runs good and lasts forever with good parts support if needed.