question bout underneath model cars?

allright some of you guys know that i am working on a kit right now made by tamiya......its a 959 Porsche.................when i put the engine in if i flip the car over so that you can see the bottom.......you see the engine..............i have seen some of your guys kits and then underneath seems whole............and i was just wonderin is that a custom thing or is it only on some kits...................

thanx in advance for you help......

Some kits come with underbody panels such as The GT-R R34