I created both the shapes separately however i need to connect the two shapes as there are sharp and smooth edges to the overall engine case. As you can see in the image below the engine case as the back is a very smooth curve between the two parts.



This by the way is a reattempt I did start this ages ago and gave up because i just couldn't get the topology to work properly. Here is the previous attempt and it's very poor topology so i just deleted it and started again rather than try and sort it out it just seemed easier to redo it but trying to maintain good topology.



And this is the wire frame without smooth preview on:



But this was really bad topology and needed a lot of work to clean it up properly. I just can't figure out how to mix the sharp and soft edges of the engine casing and also not get any unwanted pinching or creases etc. Is there any free tutorials out there that explain in detail about topology in 3D modeling?