www.radioshack.com
Typically these snake oil vendors are selling a 20 cent resistor, that will skew the readings of your IAT/MAT values, so the computer sees colder air going into the motor. Some computers will command higher advance and/or richer fueling; others won't change it a bit; these conditions may or may not give better hp. They may or may not hurt your motor as well. It's hit or miss.
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I think they are great, the 'module' from Advanced Timing Technologies, along with my blow off valve, and Tornado™ intake gave me some great seat of the pants improvement on my full size chebby pickup.
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