Some non-scientific data:
- Total improvement - taking fastest qualifying lap of each team - from Australia 2017 to Australia 2018 is about 1 second across all teams.
- Mercedes-powered cars have improved by about 0.8 seconds per lap.
- Ferrari-powered cars have improved by the same margin as Mercedes-powered cars.
- Renault-powered cars have improved by more than 1 second per lap (about 1.5 seconds).
- McLaren have improved the most, about 1.75 seconds a lap.
- Toro Rosso have improved the least. On the basis of qualifying, they're about as fast as last year.
At this point one could suggest that, because Renault-powered cars are 1.5 seconds faster, and Toro Rosso is no faster than last year, the Honda engine is 1.5 seconds slower than the 2018 Renault engine.
However, there are other mitigating factors such as:
- Toro Rosso making a late switch to Honda power.
- Toro Rosso running a new, and highly inexperienced driver lineup.
- The difference in improvement between Red Bull + Renault vs. McLaren is relatively small; McLaren improved by 1.75 seconds relative to Red Bull + Renault improving by 1.5 seconds.
It is difficult to quantify the true influence of the above, especially considering Renault's times have also improved (on average) thanks to the addition of Sainz, which has detracted from Toro Rosso itself.
However, I think it is safe to conclude two things:
- I'd expect McLaren to show further improvement in future races. The difference in performance between Honda power and Renault power is certainly larger than 0.25 seconds, and chassis integration of the power unit should unlock some additional time with optimisation.
- The gap between Honda and Renault power (2018) is likely less than 1.5 seconds on this track. As per the above reasoning, Toro Rosso have almost certainly had to compromise their layout to begin with. More potential should evidence itself as the situation improves.
- Making some assumptions about the driver quality, and relative experience, I'd put the deficit to Renault at about 1 second a lap relative to the Melbourne circuit layout.