As you likely know, the purpose of the modulator is to tell the transmission when to shift based on engine load/vacuum. Engine vacuum is dependent on atmospheric pressure. The only time you'll really notice a difference in shift points is when you're driving at high elevations such as the Colorado Rockies. The original modulators were color coded based on application and where the car was being sold to; cars sent to Florida would get a different modulator than those sent to Utah. This is why Cadillac had an enormous modulator on their transmissions. That big can was adjustable and designed to compensate for changes in atmospheric pressure. The dealer would do the final adjustment just before the customer would take possession of the car. If you really want an adjustable modulator, go with GM# 3027843 or 3027823.