The design of any marine powertrain begins with a propeller that produces a desired amount of thrust. To elevate the performance of the new Sea-Doo RXT-X and RXP-X watercraft models, engineers at BRP Rotax in Austria worked with BRP propulsion engineers in Canada to devise an impeller with a thrust curve capable of propelling these craft from zero to 60 mph in a mind-bending 3.4 seconds. They also designed an engine with a torque curve able to turn that impeller at the desired rpm. BRP will brag on the class-leading 325 hp of its new Rotax 1630 ACE engine, but that rating is almost irrelevant. The top speed of all PWC is governed not to exceed 69 mph, so this new engine rarely comes close to its 8,250 rpm limit, which is where it makes peak power. Acceleration is the goal, and on that count this new powertrain certainly does deliver.
While the 325 hp engine shares its architecture with 1630 ACE engines rated at 230 and 300 hp—with a 1.6-liter three-cylinder, single overhead camshaft, supercharger and closed cooling—Rotax recognized that many components of this platform reached their design limit at 300 hp and would need upgraded to support the pressure required to achieve the desired new level of performance.
More power requires more air and fuel, and Rotax and BRP designed an entire new intake tract with a larger-volume air box, low-restriction intake tubing and a stronger plenum on the pressure side of the supercharger. The fuel system features a high-flow fuel pump and high-flow injectors.
An all-new centrifugal supercharger is optimized for efficiency. As in the past, the supercharger is driven directly from the crankshaft, but a new planetary gear set within the supercharger housing permits an astounding 100,000 rpm from the impeller. The new 75 mm impeller is 44 percent smaller than the previous 135 mm impeller, resulting in a more compact housing. The new design eliminates the friction clutch that has been a service issue on the previous supercharger. Despite boost pressure raising from 12.8 psi to 15.2 psi, Rotax says air temperature at the supercharger outlet is reduced by 50 degrees F, and cooler air makes more power.
The entire valve train is upgraded with new forged aluminum rocker arms, stronger springs, and a polished rocker-arm shaft to reduce friction. New pistons are fitted with low-friction rings, and new oil jets help cool the exhaust side of each piston. The engine block casting was strengthened and is now die-cast rather than lost-foam-cast. Even the coupler clamps on the output shaft are beefed up.
Our highly calibrated seat-of-the-pants test confirmed the result: This powertrain produces unprecedented acceleration. Pull the throttle and 60 mph pops up on the GPS speedometer before you have time to focus your eyes, with rpm rising to about 8,000 before dropping down to about 7,600 at the 69 mph limit. We rode the Sea-Doo RXT-X and RXP-X models back to back with the 300 hp GTR-X model, and there is simply no comparison. If you crave unparalleled power and the thrill of thrust, there’s simply not a more exciting option.
