We all tend to think, logically, that more expensive is always better. So it is easy to reach the same conclusion when refueling at the gas station: if there is a somewhat cheaper 95 gasoline and a more expensive 98 gasoline, it must be because the latter is of higher quality.
What's more, we have all been tempted at some point to fill the tank with this higher category fuel believing that our motorcycle's engine would appreciate it.
Well, the truth is that this is not the case because the number 95 or 98 that accompanies the fuel has nothing to do with being able to generate more or less performance but rather with the octane number.
The first thing to understand is that gasoline pulverized inside a cylinder tends to explode under high pressure even without the necessary contribution of the spark from a spark plug.
And what octane or the aforementioned octane index measures is precisely the resistance of the fuel to compression detonation. In other words: the higher the octane number, the better that fuel will withstand the pressure it suffers inside the cylinder before detonating.
95 gasoline explodes at a lower pressure inside the cylinder than 98, so it is more suitable for very sporty engines that work with very high compressions - greater than 10.5:1 - and high revolutions.
In the case of motorcycles we would be talking about hyper sports models - practically registrationable circuit "Superbikes" - such as Ducati Panigale, Honda Fireblade, Yamaha R1, Kawasaki Ninja H2R, BMW S 1000 RR and similar.
No engine improves its performance by “drinking” 98 gasoline instead of 95. In fact, it has absolutely nothing to do with performance because engines are already designed to develop a predetermined pressure in their cylinders – the so-called “compression ratio.” - and perform at its best with a specific octane rating.
Thus, refueling 98 octane gasoline in a 95 octane engine is wasting money because that engine will never work under compression ratios that make it necessary.
Well, because obviously 98 gasoline contains a higher percentage of additives that increase the octane number. For years, tetraethyl lead was used but its use was banned two decades ago because it was very polluting and since then oil companies have used the so-called MTBE, methyl tert-butyl ether (a mixture of isobutylene and methanol).
Another notable difference between 95 and 98 gasoline is that the latter contains a lower sulfur index, which translates into less punishment from the catalyst. However, in a 95 engine - any engine that is not high performance - this factor is not so important either because the emission of polluting particles is not the same either.
Well, beyond having paid more, the user can rest assured because nothing will happen at all since the 95 engine, as we have repeated, will always work below the tolerances of the 98. You can even mix 95 and 98 gasoline without Negative effects.
The other way around, however, does happen. A high-compression engine will perform less well and be more likely to suffer from self-ignition problems if it uses lower octane fuel. In addition, engine performance will be reduced and both consumption and emissions levels will increase, and in the medium term it may cause mechanical problems such as crankshaft.
In any case, it is worth remembering that if you are in doubt about what type of fuel to use, you should always contact the technical services of each brand. They will be happy to advise on this.
As we have said, a 98 engine runs with a very high compression ratio inside its cylinders. When refueling with 95 gasoline - not prepared for that pressure - what will happen is that the fuel mixture will explode prematurely, anticipating the ignition management, programmed to provide spark at higher compression. The peculiar sound that this produces and the drop in performance are the identifying signs of “connecting rod knocking”.
In the original plans of the European Union - included in Directive 98/70/EC, relating to the quality of gasoline and diesel - a maximum limit of permanence in “octane 95” gas stations was established until 2013 for that would gradually be replaced by less polluting and more sustainable ones.
However, the reality is that today - and until the electrification of the sector is completed or sustainable synthetic gasolines are marketed, which could begin to reach the market within three years - the replacement of 95 is not feasible and That is why its sale is being extended.
