Higher revs than what the engine was designed to work EFFICIENTLY at.

Don't get misled by the popular conception that more useful power can be gained by running even higher rpm. Every engine has a design RPM range at which it delivers its maximum torque, which, (in the case of a model airplane engine) when the right prop is fitted, will give best performance.

An 11 x 5 will make it rev its head off with little benefit for the model its powering.

You'll have to open your wallet and buy a more suitable sized prop.

For this sized engine try an 11 x 6 or 11 x 7 or a 12 x 6, but it all depends upon what model it will be powering.

My vintage ST .60 powers a Kyosho .40 sized Cub and spins a wide bladed 11 x 6 at about 11,000 rpm max. Fine for the Cub.