The only real limiter is the rear bearing with standard size props. The engine is good to 19000 plus in all other respects and you'll never get it that fast even in a dive with a 12.75" prop. Get a good plastic retainer rear bearing for it at www.pspec.com It uses a 15mm bearing like so many other engines in this size category.. Just remember that crimped or riveted cage bearings aren't really much good above 15 or 16000 rpm. They tend to come apart with disastrous consequences for the upper end.