This article is from the Star Trek Tech FAQ, by Joshua Bell inexorabletash@hotmail.com with numerous contributions by others.
There's no reason to think that a Newtonian drive (Impulse) would
augment a non-Newtonian drive (warp). Also, consider that the maximum
velocity attainable with a Newtonian drive is c. At Warp 2, which is
~= 10c, this gives you a whole 11c at maximum (overloading, fuel
wasting) impulse. Warp 2.1 is about 12c anyway, so overloading the
impulse drive doesn't get you much.
 
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