Resto-mod practice ski, Sea-Doo HX

Quoted from Group K's page. Not measured here, and not reproducible without knowing the conditions, which are mostly not published.

Top speed

54 to 55 mph at 7,050 to 7,100 RPM Group K's figure for its own package, against a stock baseline they give as 50 to 51 mph at 6,800 RPM. Not measured here, and the page states no water condition or rider weight.

Displacement

718cc, 68mm stroke Group K's figure. This site's own model reference gives the 1995 HX as 717cc, so the two sources disagree by one cubic centimetre and neither has been checked against a factory document.

Range

40 or more miles per tank under hard riding Group K's claim, and it is the stated goal of the build rather than a side effect. No conditions are given.

Safe RPM ceiling

About 7,100 RPM Group K's limit, not a factory one. Their reasoning is that the long 718 crankshaft and its bearings fail quickly above it, and faster in rough water. The whole package is chosen to stay under it.

This is the only documented build we hold for the HX, which until now was the emptiest machine page on the site. It is also deliberately not a race ski. Group K's stated goal is mid-range torque, long range and durability, and the package stops short of racing RPM on purpose because they treat the 718 crankshaft as the limiting component. That makes it more useful as a worked example than a peak-power recipe: almost every choice is explained by the ceiling rather than by a power target. Note what they refuse. They advise against 44 to 48mm race carburettors because the fuel burn destroys the range goal, against boring past roughly 2 to 3mm because the O-ring groove on the cylinder flange loses sealing surface, and against pod arrestors as a straight upgrade because they flow better and protect worse against water ingestion. They also warn that carburettors left unbraced with arrestors fitted will crack the inlet manifold inside about thirty hours.

Whether any of it is legal in your class. A builder describing its own product, including by a class name, is not a check against a rulebook. Class legality holds the checks that name a section and a person.

Whether it would do the same on your machine. A quoted top speed is one builder's result on one craft, usually without the conditions, the rider weight or the fuel.