F650 REC and F650 COMP

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

Top speed, F650 REC on the stock pipe

45 to 46 mph at 6,500 to 6,600 RPM Group K's figure for its own package, on 91 octane at 40:1. Their stock baseline is 38 to 39 mph at 6,100 to 6,200 RPM on 87 octane at 50:1.

Top speed, F650 COMP on a Factory Pipe

50 to 51 mph at 7,000 to 7,100 RPM Group K's figure, on 91 octane at 32:1. They add that 89 octane is for short runs only and that sustained peak RPM wants a 20 to 30 per cent avgas or race blend.

Displacement

635cc stock, 657cc at 1mm over, 668cc at 2mm over Group K's figures, with a 76mm stroke. Quoted for comparison against the 750 they argue you should not swap in, which they give as 743cc from a 74mm stroke and an 80mm bore.

The most complete build recipe in this archive, and the only one that treats the hull and the engine as one job. Group K's argument is that a properly built 650 accelerates with a 750 while using less fuel and lasting longer, and that dropping a 750 into a 650 is neither mechanically possible on that bottom end nor worth the money. Whether or not you accept that, the restoration half of the page is the part with no commercial angle: seals, engine mounts, reed petals, deleting the oil injection, sealing the pump case and preparing the hull surface are work any thirty-year-old ski needs before power is worth adding. They give the pump case sealer an eighteen hour cure before running, and specify a 30 to 40 grit scratch pattern run front to back over the full length of the hull. Their handling argument is blunter than most catalogues will put in writing: the stock hull runs out of control long before the engine runs out of RPM. This site does not list the 650 SX as a machine, so nothing here is joined to a parts page.

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.