Hi bro, welcome to the 400D club!
Your first pic is great!
However, some of the other pics are not focused porperly or are
blur..
Try doing the following:
1) Use manual focus point selection, and select the middle to be
the focus point for the autofocus. Then u can choose which area to
focus, instead of letting the camera choose for u.
For eg, for the engine shot, u can focus on the front part of the
engine, which will be the first place the viewer will look at. But
u focus in the rear instead.
2) Use a tripod to elimate blur.
3) If the camera cannot focus, dont force it to take the picture.
Reposition the camera further away frm the model..
I feel that the f-stop of 11 and above is ok, as the higher the
number (smaller the aperture), the more the whole car will be in
focus. 
Also u can improve on the framing more.. Try to have an area of
interest.
For eg in ur 7th pic, there is no area of interest. If you like to
showcase the rims, u can zoom in more, as other parts of the cars
are 'disturbance'..
You set the colour saturation to maximum? Colour is rich, which is
nice! haha..
My $0.02 worth of crap
