Actually, you can run an engine wherever it gives you good throttle response. The RPM I mentioned above is where the SuperTigre 90's tend to give the best all-around performance, with good idle and acceleratioin, as well as developing good top-end power. It's just how they're ported.

Peak horsepower is developed at about 15K for most of the SuperTigre 90's. You'll lose low-end performance if you try to run it that high.