BOMBARDIER 1996 Ski-doo MX Z440 Handbook page 244

The following chart illustrates the effects of increased R.P.M. on delivered horsepower. As motor
R.P. M. is raised to attain higher maximum horsepower, efficiency of both the drive and driven
clutch drop considerably. This loss will often exceed the horsepower gained from the installation
of aftermarket exhausts or engine modifications. The only way extra horsepower can increase
your snowmobile performance is if it reaches the track.
CRANKSHAFT H.P.
115
115
115
115
115
115
115
115
115
115
115
115
Because newer clutch designs shift beyond a 1:1 ratio, belt speed increases dramatically and the
diameter that the belt follows around the driven pulley decreases considerably. This wastes
energy and efficiency as the belt is being bent around a smaller diameter and centrifugal force is
trying to pull the belt into a circular path instead of following the pulleys.
This is why for years manufacturers kept their clutch ratios around 1:1 to keep belt speeds down.
Now with the advent of larger displacement, high torque, lower RPM engines, we can use
"overdrive" transmissions and still keep our belt speeds within reason.
As we mentioned, as belt speeds go up, efficiency drops. This is one reason many drag racers and
radar runners gear extremely high sometimes even approaching 1:1 in the chaincase. They have
found through diligent testing that they can achieve a higher top speed without shifting their
clutches all the way out because of a decrease in belt speed which means an increase in
transmission efficiency. That is their bottom line.
For oval racing, the small benefit you may achieve in top end speed would probably be lost by the
loss of acceleration on the start and out of the corners on a tight oval circuit.
This holds true for cross-country and snow crossers also. Top speed is not as important as quick
acceleration out of the corners and ditches.
05-40
ENGINE CLUTCH
EFFICIENCY
7800
8000
8200
8400
8600
8800
9000
9200
9400
9600
9800
10000
83. IYo
81.470
78. IYo
97.5
96.5
95.6
94.6
93.6
92.7
91.8
90.0
89.8
88.9
87.9
86.9