Tutorial: Intercooler
Adding an intercooler can be pretty easy..
       
Intercooler
 
Difficulty - ***
In this case we are gonna add a 'cooler and the pipework to a sporty Punto. You'll need a car that has a fair amount of large vents. If not, you could make some vents yourself.

 

 
Select the vents with the lasso tool and fill with black, with the fill tool.

Now find yourself a intercooler image.. select the intercooler and then paste it on the image you are working on. Hit Ctrl+T to adjust the size and skew the 'cooler, until it fits the middle hole nicely.

 

Paste it onto the Corsa and then press CTRL+T and begin skewing it and altering it until it fits the car well.. Just like so.  

Next, grab the eraser tool (E) and begin to rub out the edges of the 'cooler. It'll begin to look like shadowing, and the 'cooler will look part of car. Rub out the sides too, so it looks like the pipework is going back into the bumper.  

Now its time to add the pipes, select the pipe and then copy and paste it onto one of the corresponding side vents. Again, you may have to use the old Ctrl+T to transform it to the correct size. Use the eraser tool to rub out top sections where a shadow would be formed.  

Round off the end of the pipe with a nice smooth polygon lasso selection, I've added a really small black line where the bumper meets the pipe as a bit of reflection.

Do the same on the other side too, and it should look something like that!

 

With a bit of effort, it could look like this. I decided to add some carbon inserts to the outer vents just to fill up the large gap, you could easily add mesh here etc.