Thanks for the reply!

What's Shell?

Ok so I figured that part of my problem was because I wasn't using solid objects (for the exhausts?)

And I figured out that another of my problems was that I was trying to boolean some things together when I should have been joining.

So you boolean intersecting solid objects, and you join touching lines.

Here's my new version :D

engine3.3dm

Maybe someone could check it out and see if it's done right?