I'm sure that's exactly what's being done Marc. The manual specifies 93 RON for some BMW engines (European rating - an older system with flaws) which is 89 AKI (North American rating - a better method of classification). I have never seen any volume produced car or motorcycle in NA specify 93-AKI. The RTW calls for 95-RON which is 91-AKI.
This is starting to look too much like a bad oil thread.
Please remember, a little knowledge can be a dangerous thing, and, just because someone else said something or wrote something on the internet does not make it so. Also, do not think that what applies to generalized statements cars can blindly be applied to the specific requirements of motorcycle engines.
Unfortunately, far too many folks with too little depth of understanding on these matters are willing to provide pearls of wisdom and confuse matters significantly.
First, without sufficient information from BMW engineers or running detailed tests on these specific engines, we can only go by what information we do have from BMW and what is KNOWN about octane and engine management.
So, let us look at what BMW has published. BMW specifies that their maximum HP rating for the R1200RTW is achieved using 95-RON which is 91-AKI and that is the fuel they suggest. If we were talking about the previous generation 1200 (1170cc) engine, than things are different. IN that situation BMW provide two RON grades 95 and 98 (AKI 89 and 91) and identify that maximum HP is obtained using 91 AKI (98-RON).
That absolutely does not mean that the R1200RTW will produce any more or less HP with anything higher than 91-AKI. In fact it would indicate (not state, but indicate) that there would be no gain, otherwise, BMW would have followed their existing practice of identifying this.
Will the R1200RTW run on 89-AKI? Yes.
However, it will not run as optimally designed. The ability to operated in an acceptable manner on 89-AKI is a compromise due to poor fuel quality standards in the US and elsewhere. For some markets BMW used to specifically de-tune engines ECU (different mapping, part #, etc.) so they'd run on the typically inferior quality gas prevalent in those areas.
Will doing so provide any benefit whatsoever? Unlikely and you may reduce your long-term relaibiliy if you keep the bike.
The 3% (approx) cost savings between the two grades of fuel has as much or more to do with the differences in the "Additive Package" to the different fuel grades as it does with the octane difference. So, even if the lost of fuel economy by running 89-AKI is say 2% (I don't know the actual figure, but do know that it will exist) than there is a 1% savings in cost per mile ridden for the fuel expenditure, but, through that running/savings there has been an inferior additive package being used that in most cases is not as effective at reducing/ removing carbon deposits, gunk and wear. For instance, Shell's 91-AKI, V-Power NiTRO+ Premium Gasoline (which I use when practicle), "removes an average of 60% of performance-robbing gunk on intake valves left behind by lower quality premium gasolines".
Does a higher compression ratio require a higher octane fuel? No, not necessarily. The better the combustion chamber design is the lower octane it can run without any lose of power, fuel-economy, etc. I have seen engines with CRs of 9:1 that required 91-AKI to avoid detonation and others like my R1200RTW that have a 12:1 CR and are optimized to run on 91-AKI. It is a matter of design, and can not be blindly referenced from unrelated writings about any other engine.
As an aside, one of the reasons I run Shell V-Power is that indepentently of each other, I have had two top motorcycle engine builders (multiple National Championship winning) tell me that they tuned street bikes using Shell V-Power exclusively as they found their motorcycle engines made 1-2 HP consistantly using it vs any other readily available pump gas. YMMV
Just so you know, whenever I buy a vehicle from someone, I casually ask them what fuel the vehicle takes. If they admit that they run a below spec grade, I automatically reduce my maxium I'm willing to pay for it due to the "possibility" of issues. It isn't worth the possible 1% fuel cost saving only to end up with a top-end valve job perhaps 10% sooner - that is far more expensive. Would that happen? I don't know anymore than much of the unfounded speculation that has been posted here on the benefits running a "permissible but below recommended" octane for extended periods of time in this specific engine.
I do know that there is far more difference between Top-tier 91-AKI and 89-AKI than just the octane rating and not taking that into consideration has it's own costs. For me, I run what the manual states. Now oil is a different beast, anyone want to go down that worm-hole?
Only partially done the worm hole... First, there is no direct correlation between AKI and RON. AKI is actually a combination of methods averaged. Second, just because a station carries some "branded" gas does not mean you are getting that gas. Watch gas stations when they are getting filled. Look at where the tanker trucks are coming from, then where they go. You cannot be sure of what you are actually getting. We won't even get into the mixing of fuels on the trucks, and the slightest contamination can have significant effects on the actual RON/ROZ/ROM/AKI of the fuel. Distribution, at least is the US, is fairly centralized by region. Lots of mixing going on outside the sight of the consumer.
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