28cc engine, 27 is the model of the trimmer portion, sometimes it stands for the cutting diameter.

they're really easy to convert, cut up some of the magneto mount, cut off the snout of the crankshaft, and cut up the backplate some and get a prop hub (standard 5/16-24 threads) and you're done, the 32cc version is preferable usually, it's been a couple years since I've ran mine, but it's being put into use again soon.