What happens if I mix 95 and 98 gasoline?


The debate about refueling 95 or 98 gasoline is one of the great topics as far as cars and motorcycles are concerned, and not only among the “most knowledgeable”. There are those who affirm that the only difference is the label, others that only feed their urban 1.2 with 60 atmospheric CV the best, 98 octane, and a third group uses it as a home remedy: “every four deposits of 95, one of 98 to clean”. This last sector is the one we are going to focus on, and try to give an answer to those who wonder what can happen if they mix 95 and 98 octane gasoline.

What are the differences between 95 and 98 octane gasoline?

To explain whether both types of gasoline can be mixed and explain what could happen, we must first know the differences between the two and what it means to use a lower octane fuel than necessary. It is something that we have already talked about in Diariomotor in depth, but we can summarize it by saying that 98 gasoline has a higher octane number, and some cases add certain additives They help keep the engine clean and reduce carbon buildup. From there we only have to carry out a chemical analysis of it or consult the official technical sheet provided by the oil companies, in which we can see that they calculate the values ​​of each component for both octane ratings, even at maximum sulfur content: 10 mg/kg.

that major octane number implies that it has a higher anti-knock power, which reduces the risk of knocking or self-knocking problems, phenomenon that consists of a spontaneous gasoline explosion before its time as a consequence of high pressures and temperatures in the upward compression stroke. This explosion, which produces a very particular sound (connecting rod knock), suddenly hits the piston head, preventing it from rising and generating a series of stresses and tensions in the connecting rod and other elements, without forgetting the corresponding deformation of the head itself. of the piston.


A factor closely linked to this self-detonation phenomenon is compression ratio, which refers to how much the air initially filling the cylinders is compressed. For example, a compression ratio of 10:1 means that the air has reduced its volume 10 times. Thus, and according to the laws of thermodynamics, this compression ratio is one of the key parameters that indicates the maximum temperature and pressures reached during the cycle, factors that are responsible for the self-detonation we were talking about. With this in mind, and in general, It is recommended to use 98 gasoline in cars with a compression ratio greater than 10.5However, it is the manufacturer of the same who has the last word.

The big difference between 95 and 98 gasoline is the greater anti-knock power of the latter, which is why it should be used in cars with a high compression ratio (usually greater than 10.5).

Internal section of a cylinder and combustion chamber of a BMW gasoline engine

Can you mix half 95 gasoline and half 98 octane gasoline in the same tank?

Now we would be in a position to clarify what happens if I mix both fuels, finding ourselves in two different situations. The first case is those vehicles in which it is not necessary to refuel 98 gasoline, a situation in which nothing would happen, since its propellant is not capable of making use of the greater anti-knock power of that fuel and extracting more power reaching higher temperatures and pressures. It should be clarified that the extra power is precisely due to those higher temperatures and pressures, since 95 and 98 gasoline have a similar energy capacity.

On the contrary, In those engines specifically designed to run on 98 octane gasoline, we will experience, first of all, a loss of performance and increased consumption, possibly followed by more lazy and rough behavior, as well as higher vibrations. This decrease is caused by the readjustment in the operating parameters that the ECU performs when the knock sensor detects that a lower octane fuel is being used.

Volvo direct injection system diagram

Among these readjustments we can find an increase in the amount of gasoline injected, so that when it evaporates it reduces the heat inside the cylinder, or a readjustment of the injection and ignition point. The only question that would remain in the air, “looking for the three feet to the cat”it would be what happens if the mixture has such a proportion that the knock sensor is not capable of detecting a lower octane number, but micro-self-detonations do occur.

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So, In cars that accept 95 gasoline, you can mix both octane ratings in the same tank without any consequence in terms of reliability, and only a small decrease in performance and a slight increase in consumption in those in which the manufacturer gives preference to the 98. However, in mechanics where you can only refuel 98 you should not mix both types of fuel.

Nothing happened in engines designed to use 95 gasoline, on the other hand, in those designed for 98, we would experience worse performance and higher consumption.

Petrol pump

Is it possible to alternate a tank of 95 gasoline and another of 98 octane gasoline?

Another of the big issues is the one related to alternating full tanks of 95 and 98 octane, a behavior that responds to the argument of “Every three deposits of 95 I fill it with 98 to clean the engine”. Regarding the gain in performance and reduction in consumption, as well as suitability, the answer is identical to that stated above.

Now, here we are presented with another interesting front: does 98 gasoline contribute to cleaning the fuel/injection system? As well, In general terms we could say that 98 gasoline does not clean the engineat least theoretically, and that is that this field is covered by oil companies by adding different additives to their fuels, such as antioxidants or detergents, so that with the same additives, they should be just as clean.

Nevertheless, to somewhat justify the higher price of 98 gas they do tend to reserve some of them for that premium fuel, so in practice it will produce a cleaner combustion. How much more? It is somewhat difficult to quantify without carrying out an empirical study whose results will depend on the different propellants tested and the brand of gasoline used.

On some occasions, 98 gasoline is “cleaner” for the engine, not because of the higher octane number, but because the oil companies reserve the best additives for them as justification for a higher price.

Laboratory samples of different gasolines and additives

What gasoline should you use?

We are left to talk about what fuel you should use in your car. Lines above we mentioned that it is recommended to refuel 98 gasoline in vehicles with a compression ratio greater than 10.5, although this is a general rule, the important thing is to use the octane number recommended by your manufacturer. To do this, inside the fuel tank cover (in other cases on the driver’s door sill) there is a label with these recommendations. So, if it only supports 98 octane, this is the one you should use exclusively, and In the case of recommending 98 but accepting 95, you should keep in mind that using the latter means not getting the most out of it. to their benefits.

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Finally we have those vehicles where it is recommended to refuel 95 gasolinewell, pay attention and fill the tank with it, otherwise the only benefit you will get will be spending more money to travel the same kilometers and with the same benefits, since the design of the engine itself is incapable of reaching those extremes where the greater anti-knock power of a higher octane gasoline can be used. Along the same lines, alternating tanks of 95 with 98 to achieve greater cleanliness of the feed-injection circuit does not make much sense either (it will always depend on the quality of the gasoline consumed), and at most, the effect will be similar to that of the typical additive.

Tank cap of an Audi TT specifying which gasoline to use

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