Solved

Building engines for myself

I’m sure this has been asked before. 

I’ve become an engine manufacturer but I can’t see how I’m making engines for myself 

What am I missing please?

Hi David,
Engine manufacturing unlocks the ability to set your own custom engine as your supplier in the 'engine' slot. This is unrelated to the "cars" tab's engines, which can also affect car performance but not your DP into any particular stat
For a guide to engine manufacturing I'd recommend checking out this guide by Kevin Bissell.