The bikes are old carb'd bikes.. 91 was good fuel back in the day! America were still using 85!
Unless the bike has been super fine tuned (or has fuel injection managment to alter the timings etc to accomodate for fuel) which most old bikes have neither it's just a waste.
Leaded 4 star is 98 RON (it's also £1.60 a litre from a very small number of garages
) plus the bike doens't need leaded fuel.. hmm
One of our users commented that using higher octane fuel than your engine requires actually gives no benefit and may be a waste of money. This is because virtually NO engines require 98 RON over 95, and the market for 'super' fuels seems to be based on people's misunderstanding of octane ratings and the placebo effect of filling up with 'more powerful' fuel - making motorists think their engine is running better in some way.
What do the fuel companies say then, to justify the "increased power" claims for the super grade fuels? Some companies say that while all fuels contain cleaning additives, 'super' fuels contain more or better detergents to keep the injectors cleaner than standard fuel. Others say the fuel is a few percent denser which gives slightly more power per litre. These benefits may be marginal though in comparison to the extra cost involved so it is worth ensuring that your engine will actually benefit before filling up.
Some engines actually do need higher octane fuel, such as race engines with very high compression and some turbocharged engines, such as the import version of the Nissan Skyline. Also, a few vehicles, such as the new BMW K1200R motorbike, can sense knock and adjust their engine tuning to take advantage of higher grade fuels. Another user commented that the 2004 BMW 330 also does this, according to the driver's handbook it makes 231 BHP on 98 octane and 221 BHP on 95. This ability is apparently widespread amongst German performance cars using Bosch / Siemens electronic engine controls.
For further information on the major fuel companies fuel options see their individual information pages:
I did a double blind test a few years ago using a mini fuel tank (2 litre bottle of coke) that was filled with 99 RON 'Super' Unleaded, and another one filled with '95 Regular'
Three guys rode my bike a short blast around some A-Roads, and only one of them got it right but admitted it was a guess as they couldn't tell, I just use normal grade.
Over in the US of A you can still buy 85 / 86 Ron at a lot of places.. and until very recently 91 was 'Super'
Most people mis-understand how the RON system works, and therefor think that a higher number has 'more power'
The higher the number, the more knock resistant it is (won't 'explode' but will burn in a controlled manor) as you compress the fuel more, burn it later (things done to improve power) ie super-charges, higher compression ratios.. you need a fuel that will resist burning so soon.. these are where the RON's come into play.
If your engine is old and has a relativly low compression, look an old big block chevy, you can put cheap 85 gas in the tank as it will burn ok.
If you put 85 gas in a VFR you'll find it 'pinging' all over the place, especially at higher revs as the fuel isn't stable enough to run in the engine.
These bikes were designed to run on 91, so anythign higher is fine. 95 is more than enough and a lot cheaper :)