900 ACE heats up turbo wars

Ski-Doo’s new 900 ACE Turbo is the real deal!
2019 Ski-Doo 900 ACE Turbo Renegade X-RS
The 4-stroke is far from dead. While these motors did not overtake the 2-strokes for dominance like they did in the motocross bike market - and they are not going to - they are here to stay. There is also still room for growth of market share in the snowmobile 4-stroke segment. The folks at Ski-Doo are betting on it.

The snow-dust has settled in the 4-stroke versus 2-stroke war, as the 2-strokes are clearly preferred by most sport riders, but there is still a nice 25% (or more) slice of the marketplace that would like the reliability, wider power band and fuel economy of a 4-stroke. The biggest negative some sport riders feel is the extra weight of a 4-stroke. The motors are also taller so they naturally have a higher center of gravity, which means more body roll in the corners. The introduction of turbo charging has now changed the picture somewhat for a percentage of aggressive riders.

Turbo charging increases the power without as large of a corresponding increase in weight. An example could be that a 10-15 % increase in weight from adding the turbo equipment could increase power by 50-100%. Hence, a more positive power to weight ratio overall ... which equals more fun!

The new Ski-Doo 900 ACE Turbo engine increases power from 90 HP naturally aspirated to 160+, running on roughly 12 lbs. of turbo boost. That’s a 65% increase in power, at the stock 7750RPM. Since all the hardware is there on the turbo, 200 HP could be in sight by increasing the boost to 18. Ski-Doo feels there is a good slice of market share to be had in the mid to high power 4-stroke sled market. There is space in the market between where the 130 HP Yamaha 4-strokes reside and where the Yamaha Turbo Triple motor presently sits unchallenged.
2019 Ski-Doo 900 ACE Turbo
■ Rotax builds a bullet
The standard 900 ACE has been a great success in the mid-power market. Ski-Doo had basically no presence here in 2013 and went on to dominate the segment with a 95% market share in 2017, making the 900 ACE the most sold 4-stroke engine last year. The new 900 ACE Turbo engine is not some after-market turbo slapped on a stock engine. Ski-Doo spent a ton of time and resources developing this engine into a reliable but responsive power package. First of all, this is not an application of an existing turbo-package. Everything from the housing shafts, bearings and impellers Rotax designed along with Ski-Doo and are proprietary from the ground up, specifically for the ACE engine.

Turbochargers revving up to 200,000 RPM require an expertise that has a long history of evolution. Starting in the mid-1930s, these high-rev turbos were developed for fighter and bomber planes used in World War II. I was fortunate to meet a project engineer who was involved with developing airplane turbos during World War II and it was fascinating to learn about the technology required to make bearings live at 100,000 RPM. There were pressure balance techniques needed to build boost without flat spots in all load ranges. Needless to say, those problems and others have been subject of many books and would be too extensive to cover in this story. The point is, this is not just a hastily slapped on kit, but a thoroughly engineered application.

The triple-cylinder engine lends itself well to turbocharging with its even power pulses. In order to take full advantage of the exhaust energy pulses, Ski-Doo engineers wanted the manifold passage to be as short as possible, so the whole turbocharger is built right into the manifold itself, in a compact package right in front of the engine. The housing and bearings are water-cooled and use a special lubrication circuit to make sure the bearings survive at 200,000 RPM. The compressor wheel is precision milled for low inertia and the exhaust turbine is made of nickel alloy to withstand the heat-load. All rotating components are precisely balanced to ensure minimal vibration and maintain low noise level.
2019 Ski-Doo 900 ACE Turbo
Air from the compressor is routed through an air-to-air intercooler before it enters the air-box. Because of the increased horsepower level, larger fuel injectors are used to supply the increased level of fuel. Compression ratio was lowered from 11:1 on the standard 900, to 9.1:1 on the turbo engine. This is usual on turbo applications not only because of the extra turbo pressure, but also because the increased combustion chamber volume makes room for more charge mass. This required a redesigned piston with a dished top crown, which not only increases the volume but also provides a deeper and more efficient combustion chamber. This design is very close to those considered optimum in the World War II Rolls Royce supercharged Merlin and Griffin fighter plane engines.

The pistons are also strengthened and cooled on the underside by oil from strategically placed nozzles. This is basically technology now found in uber-expensive Formula One race-car engines. Not only is the piston stronger, but the connecting rods are also reinforced as well as the crankshaft with a wider rod-bearing journal. To back all the extra power up with reliability, the head is revised for extra cooling and a larger water pump provides more cooling fluid. The oil circuit is now fed by both a larger oil scavenging pump and a larger higher pressure oil supply pump.
2019 Ski-Doo 900 ACE Turbo
■ All-important weight game
Ski-Doo claims that the turbo sled only weighs 12 lbs. more than the standard 900 ACE, but adds 60 horsepower. In addition, the engine weighs the same as the old naturally aspirated 1200 triple 4-stroke. While the 1200 is maxed out at 130 HP, this easily has a 200 HP potential with higher boosts.

With all the strengthening of pistons, rods and cranks, increased cooling and oil flow, this turbo engine is most likely maneuvering to take on the 200 HP Yamaha. The engine is also used in the Can-Am Maverick side-by-side UTV, and the European models claim 172 HP. With a longer stroke than the 1200 triple, the 900 ACE is only slightly over-square. This is unusual, but may mean Ski-Doo could easily increase the bore by three millimeters and make it a 1000 ACE engine.

When the after-market hot-rodders gets their hands on this, they are going to go nuts! We would expect to see 300 HP asphalt sleds as early as this fall at the AmSnow Super Sled Shootout in Martin Michigan cranking turbo pressure up to mid-30 PSI. This is all exciting stuff right in the spirit of the innovative snowmobile technology Ski-Doo prides itself on as a market leader!
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