No matter the choice of mass or weight, the resulting quotient of "velocity" or "time" usually doesn't correspond directly to an actual velocity or time. Due to various losses in real engines, the actual exhaust velocity is different from the "velocity" (and for cars there isn't even a sensible definition of "actual exhaust velocity"). Rather, the specific impulse is just that: a physical momentum from a physical quantity of propellant (be that in mass or weight).

Units

The most common unit for specific impulse is the second,

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