Water meths
On the 748 water meths was used as a power boost for short runway t/offs, and to restore power on warmer days.
From memory power was increased by some 10%, from 2050 ish HP to 2280?
However I always thought that the psychological effects were greater than the physical, as indicated t/off torque increased from 300 to 500 lbs, and the noise level increased in a similar proportion!
I also flew the Trident 2e, but never used the rather unreliable demineralised water system fitted on that a/c.