The simple answer is the 2s will provide more improvement over a larger rpm scale. The 1s is only correct one time during a full load run. Since the 1s only delivers a set amount of water in can not increase as rpm and load increase.
The 2s can be programed to deliver a correct amout water at about any boost or rpm.
Tuning the 2s can be time consuming at first. It can be done with out a dyno if you are sensitive to your cars performance. One of the things you will notice when the water flow is correct is that the engine is quiter.
The other problem with tuning the 2s is you well get it correct and then fiddle with it and have to start over.
Depending on the application the 2d is a good compromise if the car has electronic fuel injection.
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Brad
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