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60

There is absolutely no reason to use higher-octane fuel unless your car explicitly requires it. The higher the octane, the more compression/heat required to combust the fuel. High-performance engines (turbo-charged, high compression cylinders, etc), a higher octane fuel is needed so the fuel doesn't combust prematurely (knocking). If you put this fuel in ...


28

tl;dr: They do. It's just harder to tell how much. The longer answer is that they do and that effective compression is failing you as an approximation for actual effects. Think about detonation (AKA premature ignition of the fuel-air mixture). Normally we consider two causes: compression (the change in the space enclosed by the cylinder as the piston ...


24

Use the recommended gas for your car. Going lower than the recommended may reduce fuel economy as the engine may have to retard timing to avoid detonation. Going higher than recommended won't help as your engine is unable to take full advantage of it, as well as the fact that higher octane fuels actually contain slightly less energy (they just offer a more ...


20

Realistically, if the manual and the manufacturer are stating you should use 87 octane, that's really what you should use. If you purchase more expensive 89, 91, or 93, you are just wasting money. The higher the octane rating, the harder it is for fuel to burn (or ignite). If the vehicle was specified to use 93, then that's what you should buy or you risk ...


19

High octane is required for some cars: e.g., a turbocharged car will likely require higher octane to reduce risk of detonation with higher charge temperatures. High octane is also required to get the optimal power out of a tuned car: properly advanced timing will enhance power and economy (obviously, not at the same time). Some cars are capable of ...


15

Some aspects of this question have been covered previously. In my previous answer, I pointed out that using octane that is too low will lead to retarded timing and higher emissions. With respect to fuel economy, you can never trust someone's casual anecdote about fuel economy. As Mark points out, they're almost certainly going to be operating under a case ...


14

So-called "octane number" is a measure of how much the air-fuel mix is resistant to detonation. The higher the "octane number" is the more resistant it is and the more is can be compressed without exploding. The higher the pressure is at the point the air-fuel mix ignites the more efficient the engine works. Modern engines are designed for some rather high ...


8

The main benefit is to reduce knocking in higher-compression engines.


7

Octane rating is a representation of how much fuel can be compressed before before it explodes on it's own. The action of the fuel igniting spontaneously due to excessive pressure causes what mechanics refer to as knocking or pinging. Knocking has the ability to do some real damage. So avoiding it at all costs is a very good idea. Sometimes, knocking can be ...


7

It means "at least". You will not cause any damage by using the higher octane fuel. The reason for the RON 95 rating in the first place, this is the lowest octane rating you can run before you might start hearing pinging or pre-ignition.


6

I have heard anything from it does absolutely nothing to help your engine to it actually hurts your engine. While I don't believe it could actually hurt your engine, unless your engine needs it, then I believe it won't do anything to help. If you hear your engine knocking during normal driving you should try a tankful of the next grade fuel to see if the ...


5

Mixing 95 octane gasoline with 92 is perfectly fine. In fact, this is what many gas stations do in order to offer their mid-grade product. For example, many gas stations in the United States offer 87, 89, and 91 octane gasoline. The 89 octane gasoline is a 50-50 mix of 87 and 91 octane gasoline. Mixing grades of gasoline will not harm your engine as long as ...


4

As far as I know premium fuel is (or was) better suited for engines with higher compression ratios (10:1 or more), since lower octane fuel is more likely to combust before the sparkplug fires.


4

Premium will last longer sitting in a tank. Chevrolet recommends premium for the Volt for this very reason. According to Volt Vehicle Line Director Tony Posawatz (as quoted by gm-volt.com): If people are not using the extended range capability a lot, the premium fuel does last a little longer,” he said


4

One of the reasons that a turbo setup with the equivalent effective compression is more forgiving of low octane gas than than a static compression setup is that you're not at that compression ratio all the time. Take that honda, for example. At 9:1 static ratio, you can run 87 octane all day as long as you don't push any boost at it. When you do start ...


4

In addition to good answer by @Bob: There are some tricks that can be used to ease the problem: A knock sensor for detecting premature detonations (and adjusting boost pressure). E.g. Saab APC allows safe use of lower octane fuels. Injecting water to cool the combustion chambers (instead of excessive fuel) Per cylinder exthaust thermometers (and ...


4

Just to add to what ManiacZX covered in his answer: On modern turbocharged cars, the ignition computer will prevent knocking, which will prevent damage. You will end up with a bit less power, and a bit less fuel mileage. I have never heard of a car being damaged by using a higher octane fuel than needed - in fact, many gas companies try temp people to do ...


4

Being in the midst of summer, I may have erred on the side of caution and gone with the 93 but I'm sure the 89 hasn't been harmful to your vehicle if the manual states you can put 87 into it. Since you mentioned knocking, it sounds like you have already done some research. Still, to cover it, the higher rated octane fuel is essentially "harder" to ignite ...


4

In my experience the difference is marginal at best. Given that I have known many people who swear by the huge benefits of high octane fuel, I've tested the hypothesis on my own vehicles several times. Each time I've tried the experiment (first with a 125cc motorbike, then with a 1100cc car, then with a 650cc sports bike and most recently with a 600cc ...


4

In the case of topping off, it would make absolutely no difference other than costing you more at the pump. Octane is a rating which would indicate how hard it is for the fuel to burn. The higher the octane rating, the harder to burn. If anything, leaving gasoline for longer periods of time is going to make it harder to burn, thus effectively raising the ...


4

This claim is my opinion. I have no facts to back the claim. This could even be urban legend I've read a plethora of articles recently, due to this question, regarding this subject. It seems to be the consensus of some somewhat reputable sources that manufacturers will do testing with the lowest grade fuel to ensure that the vehicle does not ...


4

There is no such thing as an octane sensor. Octane is analyzed in two ways. Analysis of the chemical composition, they call this Research. They place the fuel into a test engine and run it until it pings, they call this Motor. If you look at a gas pump it will usually show for example, 87 octane (R + M)/2. This is an average of Research and Motor. A modern ...


4

The acid test is if you try a lower octane fuel, if you hear any "knocking" sounds under hard acceleration (perhaps up a hill), then you definitely need a higher octane rating. As Paulster2 mentions, there will not be any performance improvement, in spite of what a lot of people say. The octane rating is just a measure of resistance to knocking. I have a ...


3

I can get you halfway; here's the formula for the effective compression ratio of a forced induction motor: Effective Compression Ratio (ECR) = sqrt((boost+14.7)/14.7) * Static Compression Ratio (SCR) So, for 5psi on your 9.9:1 SCR motor: ECR = sqrt(19.7/14.7)*9.9 = 11.46:1 However, I have had quite a difficulty trying to find a good way to get ECR to ...


3

Generally, you're best off using the fuel recommended by the manufacturer. Octane rating is a measure of how resistant the fuel is to ignition. The higher the octane, the harder the fuel is to ignite. Fuel that is harder to ignite allows higher compression ratios and more agressive engine tuning. If the fuel is too low octane, modern engines will detune ...


2

Here's the deal : higher octane means less volatility, and this is meant for engines with higher compression so that the fuel doesn't ignite too early in the cycle . Lower octane fuel is more volatile and is meant for engines where compression rates are lower, so that the ignition in the cycle not occur too early . In all cases, follow the manufacturer's ...


2

My wife's car, though designed to run on 99 RON or above can run on lower Octane numbers, but if you do use lower Octane fuel the engine de-tunes itself dramatically, and the process to tune it back up again is difficult to do yourself - so be aware of the particular tuning requirements for your car. Because of this, my wife's car now runs on 98 RON and has ...


2

It's like this... Higher octane fuels allow the compression ratio of the engine to be higher, and the higher the compression ratio, the more efficient the engine - this is the principle behind diesel engines, in fact. That said, the compression ratio of most engines is not allowed to change significantly, so if your engine has a 10:1 compression ratio, ...


2

If the specs call for 91 octane, use 91 octane. Using higher octane fuel will not give you better performance. Only engines which need the higher octane will see better performance from the higher octane. In most cases, engines which should use the lower octane will get worse gas mileage from the higher octane fuel.


2

To answer your question directly, adding the 95 to the 92 in your tank will not cause any real problems, in the short term. Once you have used the tank full you can carry on filling up with the recommended 95. Modern engine ECU programming will adjust the timing of your engine, knock sensors, to minimise or prevent engine 'pinking' caused by the lower octane ...



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