Cars retire if their parts aren't replaced and they break (most common one is suspension). To combat this make sure you replace your parts, I see that you have been playing for a couple of months now so I'm sure you know these basic things. I'm also trying to find my perfect engine but the only way to do this is to find it yourself. There are too many factors to keep in mind, such as the engines weight, what tyres you have and which manufacturer, the type of driver/s you have and what your trying to get out of your engine, parts development and of course the competition you're competing against.
My advice is just to experiment a bit and find one that suits you. I'm currently with Murk and have been for quite some time, however I think I'm going to give Tifosi a go because they offer better fuel economy (saving weight on fuel), slightly lighter power unit, and still produces quite a bit of power.
I'm not 100% sure but I think that the reliability stats are indicators on how hard you can push the engine, if you're pushing hard for the while race for instance then the engines power and response (i.e what makes the engine good) decreases massively and you end up going slower.
One indicator could be the Average Points an engine supplier has. Murk, Beep and Donelots has the highest Average Points; but that could be caused by other factors.
