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A Note about Steam Chuff/Cam configuration: CV133 and CV134 work together to control chuff timing:
CV133 Steam Chuff/CAM config, 128=>EXT cam, 1-127=>DRIVER dia in inches
CV134 Steam gear ratio trim, 32=100% ratio, 
Setting CV133's value from 1-127, puts the SFX064D into 'Autochuff' mode. Autochuff mode tells the decoder to simulate driver chuff timing in software. The CV133 default value is 63, approximating a real-world loco driver diameter of 63 inches. If you halve this figure you will effectively double the chuff rate for a given loco speed.
CV134 (gear ratio) also effects the Autochuff rate. The CV134 default value of 32 assumes no gear reduction. Doubling this value to 64 tells the decoder to simulate a 2:1 gear reduction thereby doubling the chuff rate. CV134 and CV133 work together to create the desired chuff rate in your locomotive.
A final note on Chuff support - Setting CV133 to a value of 128 tells the SFX064D that you want to configure an actual cam input on your locomotive. CV134-->128 tells the decoder to activate the white cam input wire on the 10 pin sound harness of the SFX064D (see diagram below)
A chuff will be triggered when a pulse of over 6 volts or DCC track voltage is seen on the white CAM input lead. This voltage has to go off (0 volts) before the next chuff can be triggered.