|
Tech Feature: Dyno Tuning the Ski-Doo MX Z 600
-Snow Tech Winter 1998-99, Page 98
The new cylinder reed intake 600cc (Rotax type 593) twin from Ski-Doo marks a significant switch away from the rotary intake Rotax twins we have enjoyed for so many years.
Why cylinder reed intake? Ski-Doo claims this design has all the power of the case reed designs, with the advantage of being able to mount the carbs up on the cylinders instead of down on the crankcase. This allows for zero engine tilt, and an engine mounting position that is lower and closer to the drive axle. Lowering and centralizing the mass was the objective to reduce the moment of inertia and improve the handling. This design change is critical to maintain vehicle cornering with today's long travel suspensions and the higher chassis center of gravity they create.
This engine is completely all new. The cylinders have triple exhaust ports, cast iron sleeves, and Rotax RAVE exhaust port modifiers. The cylinder mounted reed cage has a very large intake area and is really a hybrid between cylinder reed intake and case reed intake, with 70% of the intake charge going directly to the crankcase and only 30% of the charge going under the piston skirt (all of the high performance motocross bikes are using this design). The transfer ports are large volume flow loop charged for excellent flow efficiency. A decompressor notch on the auxiliary exhaust port allows for easier pulling over and starting of the engine. A high output 290-watt magneto provides enough juice for the dual headlights, and there is enough room to house two trigger coils for future development.
The new water pump design is claimed to force 50% more coolant into the cylinder, keeping the coolant moving at a constant flow and eliminating any hot spots, allowing for the use of a cast iron sleeve. This reduces the need to use fuel as a coolant, resulting in improved fuel economy. Ski-Doo claims that the type 583 required 400 grams of fuel per kilowatt hour (think of this as "metric" fuel consumption; instead of pounds of fuel per horsepower hour, the metric version is grams of fuel per kilowatt hour), and the new type 593 only requires 330 grams of fuel per kilowatt hour, a 17.5% reduction in fuel requirement.
The Elko pistons from Austria are a new lightweight cast alloy with a single ring for less drag and efficient sealing, which should also reduce hot spots and detonation problems. This piston is similar to what the 670 High Output engine uses with its flat top, which improves performance due to faster flame propagation in the combustion chamber. The two-piece cylinder head features hemispherical combustion chambers with wide squish bands, which were designed in conjunction with the new flat top pistons.
The crankshaft is heavy-duty unit with 809 type big end rod bearings, along with new nitrated steel cage ball bearings, including a nitrated solid retaining pin. The mag end features a new style seal for greater durability, while the PTO end has a 7.5:1 taper and a new fully oiled outer bearing. This oil line also is routed to the center water pump gear for excellent lubrication and durability.
Rich Daly at Dyno Port (315-258-5618) was able to obtain one of the first MX Z 600s out of the factory and immediately placed it on his Huff-Depac dyno. Ski-Doo personnel at the Snow Shoot in Quebec in March were estimating 105-110 HP from this new engine, so our expectations were high. Much to Rich's surprise, the new engine only produced slightly more than 101 HP. Ski-Doo informed us that the 600 has an ignition feature that retards the ignition timing by 3 degrees for the first four hours of operation beginning at 4,000 rpm, after which the engine would make full power. After the four hour break-in period, the following dyno run was recorded pulling 104.0 HP at 7800 rpm. The stock jetting was pretty lean compared to typical fuel flow numbers, but Ski-Doo insists that this is normal for this engine. Ski-Doo also told us that the pipe needed to be good and hot before it would make full power, but Rich found over the course of four dyno runs that the power did not increase appreciably with a hotter pipe.
 
Rich first built and tested a canister silencer, retaining a larger volume for low frequency sound reduction as well as a dissipative glass packing for high frequency attenuation. This silencer was 6 pounds lighter and measured only 2dB louder than the very quiet stock muffler, and the following dyno run was pulled, still using stock jetting. Note the similar fuel consumption numbers, now producing 105.7 HP at 7800 rpm, with a slight increase in torque. Rich also noted that the airflow had increased slightly by 4 pounds per hour. Rich made an attempt at making more power by reducing the stinger outlet diameter of the stock pipe, but this resulted in a loss of power!
Looking at the stock exhaust, Rich was concerned with the sharp "U-shaped" return path of the exhaust coming out of the pipe into the silencer, and built a single pipe that eliminated what he felt could be a restriction. The single pipes from Ski-Doo on the 500, 583, 670, & 700 engines have been very excellent power makers and have been very difficult to improve upon, but the 600 appears to be different. He was surprised to see a significant power increase, and strongly suspects this is due to the sharp curve at the tail section of the stock pipe.
The Dyno Port single pipe peaked at 111.7 HP at 8300 RPM, and produced a wide band of over 110 HP all the way from 8100 to 8400 rpm, with a torque peak of 71.8 foot pounds at 8100 rpm. With this pipe, the jetting was increased one size and the fuel consumption numbers are slightly leaner. Rich did jet this set-up down one size and lost power, so ideally it should be jetted up 1 more size to maintain the stock-like fuel consumption. This will be two sizes richer than stock, producing a safe 110+ HP while reducing weight by another 5 pounds, for an 11 pound total weight reduction.
 
Next, a thicker base gasket was installed to raise the port timing and the head was machined to get back to stock compression. This produced 112.5 HP with similar fuel consumption. The next step was to perform some slight trail porting to the cylinders to provide a wider power band and bring the rpm back down. Being able to make more HP at a lower rpm with greater torque increases acceleration. This provided for a significant increase in power, resulting in nearly 116 HP at 8100 RPM, and a power increase at every rpm above 7000. Notice the fuel consumption, it was a bit richer than the stocker, and Rich felt that if the jetting had been reduced one more size that this package would have pulled 118 HP with fuel consumption numbers in the 5's like the stocker.
 The new type 593 Rotax engine is clearly stronger than the rotary valve 583 across the entire power band, but surprisingly Rotax has left some power on the table for the aftermarket to deliver. This may be caused by the choice to abandon the center outlet pipe design, or possibly due to the fact that tighter noise level targets were desired with this next generation engine. The fuel consumption numbers indicate that this engine will operate safely with less fuel than most every engine previously tested, but we want to verify this on the snow during the course of the Winter. A new engine out of the box will be down slightly on power due to the electronically controlled ignition that switches to full advance after a four-hour break-in period. This feature should result in improved reliability and reduced warranty claims for Ski-Doo. While some buyers may be slightly disappointed that the engine does not produce closer to 110 horsepower, it will be very competitive and time will tell if it is the fastest single pipe 600 on the snow. For those who want even more, the engine responds very well to trail porting and can deliver close to 118 horsepower with a quiet single pipe. What about twin pipes? As with any brand new engine design, give the aftermarket some time to digest and understand this new engine, and they'll likely find out what options work best as the season progresses.
|
