The 53 / 51 starts at a 53 degree angle and progressivly changes to 51. That's assuming you have a progressively cut helix. Some are straight cuts that change without a smooth shift. The one on my 02 sno pro starts at 62 i think and is straight until it hits something like a 58 / 48 progressive cut. That 53 degree cut will allow your secondary to open a bit quicker than the 51. Just a bit quicker.
Allowing the secondary quicker will make the sled load up the motor a bit more which in theory will make the sled quicker under certain conditions. The reason cat chose the helix they did is cause it's best under a wide variety of conditions. If you went with a 57 / 50 progressive say for example the motor would load up more on low end while keeping the top end essentially the same. You would probably see a bit of decrease in RPM down low and it would pick up to where it should be on top end. This is not necessarily bad because usually the peak torque of an engine is at an RPM below the peak horsepower. Torque is accelleration, horsepower is top end. Confused yet. Clutching is tricky.
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