engine is in back nah, its just modeling without taking into consideration that the model is going to "drive around" one day basically this is the reason why its so hard to make the vehicles not spin out etc, the center of mass is in the back, or sometimes even further backwards then the back bumper (be jeep afaik) as far as i know the devs have added in "keep uprights" of some kind onto the vehicles to fix it partially but for some vehicles its still hard to fix
it depends on your scripts hl2 jeep has center of balance in the middle-back-ish if you take the code from hl2 and put the center of gravity in the middle of your model you will get weird results