Zenki and Kouki, the Quick Answer
The difference between Zenki and Kouki is just in the front look of the Nissan 240sx also called (Silvia S14). This picture may help you to see the difference especially the headlamps and curves on the hood:
- rounded headlights on the Zenki,
- the Kouki features sharper, aggressive headlights.
When you enter the world of drift-cars enthusiasts, Japanese words "Zenki" and "Kouki" may arrive in your ears. If you don't speak Japanese, you may ask yourself what is the meaning of theses.
The Nissan 240sx, a detailed explanation.
The Nissan 240SX is a sports car, build between 1989 and 1999. This car is called Silvia in Japan (a small blue bird in Japan).
From 1989 to 1995, the Car was an S13 model (S stands for Silvia) and (13 stands for the Mark model ), From 1995 to 1999, The Car was an S14 model which is called Nissan 240sx outside Japan (this is not 100% true for purist)
This is a very famous car in the drifting world, a "must-have !", gained in popularity and got praised by drifters people. they suddenly started to speak Japanese.
Japanese lesson :

- Zenki is for "zenki-gata" : prior period.
- Kouki is for "Kouki-gata" : latter period.
Basically, it's an automotive term to distinguish cars before and after the car facelift also called the mid-generational refresh. (a.k.a fix minor bugs and boost the car sales)
The great thing is: it sounds great and it even sounds JDM (Japanese Domestic Market)!The cherry on the cake: it can be used for Japanese cars but also for the other car.
-Mike : " Hey, Look at this "New Beetle Kouki"
-Jim : "Dude ! you're wrong it's a Zenki Newby !"
If Nissan wasn't Japanese?
Just for fun, imagine if Nissan was from another country, we would have a translation of the bird name and the translation of facelift ase well :
English: The Nissan "Warbler", before and after Facelift.
German : Das Nissan "Grasmücken", Vor Modellpflege / Nach Modellpflege
Spanish : El Nissan Sylvia, Fase I / Fase II
Polish : Nissan Jarzębatka, po lewej / po prawej
French : La Nissan Passereau, Phase 1 / Phase 2
Here it is for the story of the Kouki and Zenki, and if you want more stories about the Nissan Silvia 14S, i've done a compilation inside the Silvia drawing below. Explanations are here.