For sure yes.
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Some fuels are better (new spec ETS, VP, etc) and some worse (WRC Panta), and some engines cope better with worse performance fuel (less prone for knocking, etc.). So for sure this will make a difference. On the other hand all manufacturers have mappings for few different fuels but not every mapping is done same way, some are done 100%, some let's say 90% because some customer wanted to use this fuel, so they made mapping for it and offer it also for everyone else, but it's not done with so much attention, etc. So if they know they all need to use some XXX brand fuel everywhere in the world for example they focus on mapping for it and then it's more even with performance, but still some differences, as some fuels favor more some engines, etc.
Other thing is that in R5 main concern is pop-off valve so strategy for it is most important, as in ideal conditions one engine can be super strong but then will have many issues with pop-off so will loose most... so all is about finding proper balance. Better to loose some 0,1-0,2b boost (as most R5 cars allow to do manually or automatically by software itself), so of course loosing hp/Nm then but still better than loosing 0,8b boost when pop-off opens. Similar rules with ALS strategies, etc.
Homologation jokers:
Fiesta engine
Fabia engine x2 and fuel circuit
It's in Czech but quite interesting article based on ewrc-results database regarding R5 cars statistics. Likely not 100% but in the amount of data it can be fairly accurate. https://www.ewrc.cz/clanek/31611-f-f...spolehlivosti/
Particularly interesting for me is percentage of retirements from technical reasons:
Fabia 7,18%, Fiesta 9,84%, 208 15,91%, i20 17,11%, DS3 18,31%, Mirage 27,27%
Aren't these retirement stats for the R5 Fiesta going to be biased against it as it was the earliest R5 ?
Of the total number in use, many Fiesta's are going to be older cars and have done more mileage and events.
They were also cheaper to buy originally.
I mentioned only stats for 2017. Fiesta had pretty good numbers (not that good in 2014 when it was new).
I don't think that the age of the rally car makes it worse in relation to reliability. That is more about particular team preparation and maintenance and I think that we can assume that all cars get in average similar maintenance. How old is the bodyshell is imho not very important (see Loeb's Xsara WRC stats).
On the other hand we may ask whether older cars actually aren't generally more reliable because their technical solutions are proven. The new cars often tend to be unreliable because some issues are very hard to find and solve during testing.
I don't know what You're trying to say. Old bodyshell means nothing. Fiesta R5 has much better reliability stats in 2017 then it had in 2014 so what's Your point? Both Fiesta and Fabia reliability stats show that the cars became more reliable with every following year, i.e. they had the worst reliability when they were new and since then they become gradually better and that is perfectly logical.
By the way Loeb used to drive 4-5 years old bodyshells of Xsara WRC and it didn't stop him from winning WRC titles.