As a result, a production run of a hundred or so units was made from stainless. Well, that's not necessarily the case. With the stinger, the 12 horsepower gain is at a lower rpm than that of the peak. If you understand that then skip the next paragraph. If not, read on.
Horsepower is derived from a formula that calculates torque and rpm.
We'll spare you from the boring math, however, here is an explanation in layman's terms. Since the engine is working harder, it can do more, and in the case of PWC, this increased work is put to use by spinning a steeper-pitched impeller. By increasing the engine's torque at a lower rpm, its horsepower is also increased at that rpm, but that number may not be the peak power rating of the engine. This explains why the horsepower rating of the XL1200 Limited with the Riva stinger installed may not be 167. The stinger increases the engine's torque at a slightly lower rpm than where peak horsepower is developed.