There is no such thing as RON in leaded aviation fuels, anyone who mention RON or AKI in the context of leaded aviation fuel was probably living on another planet.
If you look at lead Avgas ASTM/STAN (87/88, Grade 100 and 100LL Avgas), there is no min RON or min AKI listed in their specs, only min MON is listed, there is something called Performance Number which is 130 for 100LL, something that some people confuse when talking about Avgas 100/130 being MON/RON octane rating 
RON or AKI never existed for leaded aviation fuels, once this is digested, we can talk about unleaded aviation fuels
I have not seen Type Certificate (TCDS or STC) issued by FAA or EASA that explicitly mentions min RON rating?
For anything else, I can put fish & chips oil if I judge it’s OK
maxbc wrote:
now I can’t find the reference to the RON of UL91… It’s not present anywhere in the norm
RON crept under ASTM D7547 and UK MoD DEF-STAN 91-090 for unleaded aviation fuels specs
The specs of UL91 need RON > 95, the actual testing in Total and Hjelmeco are usually around 97
See page 7,
http://www.np.co.tt/wp-content/uploads/2020/09/def-stan-91-090-issue-5-avgas-dec-2019.pdf
https://www.astm.org/d7547-21.html
I have not seen Type Certificate (TCDS or STC) issued by FAA or EASA that explicitly mentions min RON rating?
As explained in the Wikipedia link (which is in this case a pretty good reference) aviation gasoline has a similar but different anti-detonation rating system, also using two test numbers. But they are different numbers representing test performance in lean and rich conditions, which is not what RON and MON do.
The context for discussing the automotive RON and MON ratings here is that the aviation octane ratings for automotive fuel are not available at the pump, so in order to use automotive fuel in an engine designed for and certified for on aviation fuel of a certain octane, it becomes necessary to compare the two systems.
Answering your question, the original 1980s autofuel STCs reference the automotive AKI rating (average of RON and MON) as the requirement, requiring a AKI that is equivalent to US premium automotive fuel.
I’m really having a hard time finding hard data on UL91. The only thing the specification says is MON > 91. No AKI and no RON. Thus I don’t know if the aircraft manual (which is a copy paste of the engine manual for these sections) allows for UL91.
On the other hand, automotive SP98 not only has up to 5% ethanol, but its MON is 87. Here is Total claiming their 100LL has a 100 MON, also making the point that MON is more suitable for aircrafts (being at full power). So in theory, this would confirm that UL91 is better than SP98-E5 for these engines. But no official info or confirmation that it’s approved.
This SB dated from 2011 claims a 96 RON for UL91 but acknowledges that this part of the norm is pending approval. The norm that we have now says nothing about RON.
Edit :
Thanks Ibra, I missed that (although I can’t find any official source from EASA or Total linking their UL91 to DEF-STAN 90-091).
Ibra wrote:
I have not seen Type Certificate (TCDS or STC) issued by FAA or EASA that explicitly mentions min RON rating?
The manual extract above definitely mentions RON rating (for auto fuel). I don’t know if using a line that allows “automotive fuel” to take it for granted that an aviation fuel (which UL91 definitely is) is a stretch or not. Silvaire’s comment makes it a little more clear as to why maybe this interpretation should not be done (different rating methods). Although again, mudding the waters even more, the UL91 specification uses MON very explicitly (and no AKI, nor rich nor poor knock indices). DEF-STAN also provides both RON and lean.
Silvaire wrote:
Answering your question, the original 1980s autofuel STCs reference the automotive AKI rating (average of RON and MON) as the requirement, requiring a AKI that is equivalent to US premium automotive fuel.
So there is no min RON in aviation papers and fuels, why we are talking about this? I only see min AKI (for auto fuels and Mogas STC) and min MON (for aviation fuels and TCDS)
The only times RON gets mentioned correctly or incorrectly is usually in some ULM forums, manufacturers SB, aviation magazines…these have no legal value
I’m really having a hard time finding hard data on UL91. The only thing the specification says is MON > 91. No AKI and no RON. Thus I don’t know if the aircraft manual (which is a copy paste of the engine manual for these sections) allows for UL91.
See screenshot above RON > 95 for UL91 and 91/96UL specs (you can download the testing by Total & Hjelmeco which show RON 97)
Again, I am not aware of an aircarft manual that states min RON (as confirmed by Silvaire post above, at least we agree on this)
The manufacturers SB, I tend to overlook these, they are mostly CYA and badly written without lot of data or physics behind, the most serious and interesting stuff with lot of testing and data tend to comes in the form of an AD or STC
Ibra wrote:
See screenshot above RON > 95 for UL91
Yes we cross-posted, I saw it after (thanks)
Ibra wrote:
So there is no min RON in aviation papers and fuels, why we are talking about this? I only see min AKI (for auto fuels and Mogas STC) and min MON (for aviation fuels and TCDS)
Look at the screenshot I posted above. Min ROZ for auto fuel, no mention of AKI (and straight from the manual).
Or take a look at Rotax operator manual which mentions both RON and AKI.
If you have references for both Total / Hjelmeco and Rotax own testing, I would be interested as well (couldn’t find them).
Ibra wrote:
the actual testing in Total and Hjelmeco are usually around 97
Actually for Hjelmco it shows more than 98, which is also their own minimum.
Silvaire wrote:
Saying that something is “completely useless” and then providing an example to the contrary is not helpful to making whatever point is trying to be made here.
Sorry for not being a walking and talking Wikipedia able to remember everything 100% correct from 40-50 years ago The point is, the “MON trouble” was fixed in Europe in the 60s. From then on every car could run WOT all day long on the Autobahn using pump gasoline specifying RON only, as had been done decades before. It was fixed in the standards for gasoline, and was probably a thing most people knew nothing about. It’s only through aviation I have heard of MON.
It was no issue before the 60s because the engines were low compression, low power. It was no issue after the 60s because it was fixed in the standard. The only issue was the RON value, because filling low octane on a car that required high octane (RON) would damage it. Exactly what the relation of RON-MON was, I have no idea. But it obviously worked. You could run all day WOT on the Autobahn (using large amounts of fuel for sure). At least all German and French car were designed to do it, quite contrary to what is the common conception of what a car engine can endure. Today a normal gasoline car can easily go 200+ km/h, due to much more power. If this car can run all day WOT, I’m not so sure though, but it probably will. It’s more a matter of being able to do it with today’s traffic, and 200+ km/h is awfully fast.
maxbc wrote:
But from the aircraft’s manual
Yes, aircraft manuals… Then you should read what Rotax repeatedly has said about aircraft manuals in relation to their own engine manuals. I cannot be bothered to find that right now, but I found one of the service letters about octane level (RON)
There it is. Running a 912 S/ULS WOT on anything lower than 5500 rpm on RON95, and it will detonate (at ISA condition). With RON98 you can run WOT all the way down to 4000 rpm, making detonation possible in theory, but not much of an issue in practice. UL91 is RON95. This also makes this new Warter fuel a bit odd with RON97. Why 97 instead of 98?
High compression 912s are 912 ULS and 912 S (certified variants). All the “large” displacement variants. Low compression variants are the “small” displacement variants, the 912 UL, 914 UL + all the certified versions. As for the iS variants, I don’t know.
maxbc wrote:
Or take a look at Rotax operator manual which mentions both RON and AKI.
In certified private GA, the docs from manufacturers like (SL, SB, SI) are ‘for information only’, only docs & numbers from FAA & EASA matter (AD, STC, TC, CS-STAN) for legalities. In uncertified private GA, you can put what you wish if you deem it fine, cheap and available
Airborne_Again wrote:
Actually for Hjelmco it shows more than 98, which is also their own minimum
I may saw that somewhere, I think Total/BP are RON97 for their UL91 testing
None of these go down to SP95 RON levels that Rotax are worried about, it’s Mogas95 !
Also we are talking about 3min or 5min max rpm for takeoffs…
