Chalk it up to not understanding the etymology. I have seen the form "sup up" written on the internet a number of times and never really had the occasion to question it.
I guess that's what makes this place so much fun It seems people who use this form are under the impression that basically means "super charged"; however it appears they (and I) may be mistaken:
http://www.etymonline.com/s9etym.htmsoup (2) - "increase the horsepower of an engine," 1921, probably from soup (1) in slang sense of "narcotic injected into horses to make them run faster," influenced by supercharge
which suggests "sup up" may not be completely incorrect; however there isn't much other argument for "sup" (v.), which is first attested 1876.
http://www.tes.co.uk/staffroom/thread.asp?staffroomSearchStr=&id=18112&threadPage=1&threadID=697577&messagePage=3"To soup up" an engine relates to increasing its horsepower, and is thought to relate to the practice of injecting narcotics into a horse to increase its speed for a race.
http://www.merriam-webster.com/cgi-bin/dictionary?book=Dictionary&va=soup up Main Entry: soup up
Function: transitive verb
Etymology: soup (drug injected into a racehorse to improve its performance)
: to increase the power or efficiency of <soup up an engine>
Merriam-Webster's Collegiate® Dictionary & Thesaurus
Main Entry: soup up
Function: transitive verb
Etymology: soup (drug injected into a racehorse to improve its performance)
Date: circa 1933
: to increase the power or efficiency of
NB: Soup is also a term for nitroglycerin. Certain race cars are fueled with "nitro" fuels and are therefore souped up.
American Heritage College Dictionary 4th Edition
Soup up – phrasal verb: Slang. To modify (something) so as to increase its capacity to perform or satisfy, esp. to add horsepower to (a vehicle).
Z-Dub
