Idk for sure, I don’t follow these things close at all but I think the surge models are suppose to drive things like screws into wood faster and stuff like that. They are more popular with construction trades than with mechanics.Picked up a Milwaukee Fuel Surge 1/4 Impact Hex driver today. The chuck on my old Milwaukee non-Fuel brushless 1/4 Driver exploded and I needed a replacement in a pinch, and this one came with a free 5aH battery, so I went with it. I don't think it hits as hard as my old one, likely on account of the hydraulic drive but that may be a good thing for reassembly, considering I mostly use it for small fasteners.
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I could be way off thou.