The 89 is a different bore and stroke, off the top of my head I believe a 67.4mm bore and a 70mm stroke. The newer style 250cc engines are 66.4mm bore x 72mm stroke so a newer style cylinder would not cross over to the older engine. I would say if you running a UAS flat kart I would go with the 86 as it might be more user friendly in that situation. But with a champ with a big wing you should be able to use more power that the newer engines produce. Either will get you going if you are just starting out and will definently be fun, but don't expect to run up front on a big track.jmo