Solder (60/40) or any other lead-based sort has little mechanical strength. And it decreases as heat goes up. Furthermore, some of the wires used in various places don't work very well with field soldering (such as Litz wire). On the other hand many of them don't work well with mechanical fastening, ie, crimping. Depending on the wires you're working with, I'd prefer to crimp, solder, and maybe even use a heat resistant wire nut on top of it all. And I would be certain to strain relief the join, so vibration won't cause a mechanical stress or strain and pull the joint apart.
Sometimes it makes sense to (more or less) braze a connection in the case of something whose environment is quite hot (ie, near exhaust manifolds).
But you haven't said which sort of engine you have -- I'm merely assuming it's some sort of gasoline automotive engine.