Solved

Understand DNF reason

Hello,

"You should always repair your car every race and swap the engine at least every second race."

In my opinion, I would switch the engines and parts in this section - the pace lost by not repairing your parts is much less than not repairing your engine.
If you can afford to, I would recommend repairing your engines every race as the race pace lost by having old engines is substantial. 

Hello,

The time loss effects of not repairing your car are widely considered to be zero, however, I wrote that it's "much less", as if there is an impact, it is very small. 

You may as well renew engines every second race otherwise you're wasting your "free issue" engines.

You get free issue engines every 20 races. In two car leagues (this is what you are in) you will be awarded enough free engines to make your stock up to 20. So you will be issued with 20 free engines if you have zero in stock but only (example) 14 if you have 6 remaining at the time of issue. The trick is to make sure your engine stock is fully depleted and you can do this by renewing engines every second race.