The ending relationship between Scuderia Toro Rosso and Renault Sport has become sour just 2 weeks before the end of the season. Renault's Cyril Abiteboul and Toro Rosso's Franz Tost have made it no secret who they believe is to blame for the situation.
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I think they run detuned engines. Maybe they are really running out of PU elements therefore they can't risk any more damages. It's important for them to take part in the race.
My guess is that they are out of mileage on their planned and used pool of PU components, so they are having to run the PU in "safe" modes to stand any chance of finishing. Renault is not prepared to give them any new PU components because (a) it is the end of the road for the 2017 PU spec (b) they want to make sure the works team beats TR to 6th place in the constructors' championship to get up to $12m more prize money (c) Toro Rosso pissed Renault off by writing contentious press releases alleging inferior treatment, to the point where Helmut Marko had to issue a damage control press release from Red Bull.
This is the 2nd time in as many years Red Bull have snatched or traded away Toro Rosso's star driver mid-season, the team is being treated as pawns to develop the Honda engine to the point where it might be worth putting in a Red Bull, and in the meanwhile STR is clearly getting 3rd best treatment from Renault who are struggling to produce enough new components for themselves and RB, let alone a customer team who is coincidentally fighting them for championship position.
You can't really blame STR management (distinct from RB management) for being somewhat bitter over the whole situation.
Yes, the performance has been pants. Hartley was always going to be starting from the back anyway, due to the 10 spot grid penalty, so his focus has probably been on race setup rather than qualifying performance.
Supposedly the updates didn't produce late in the year. They also took quite a few grid penalties. The last few races were used as training for the new drivers from what I could see.