Tag Info

Hot answers tagged

22

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 ...


14

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 ...


12

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 ...


8

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 ...


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

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

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 ...


3

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 ...


2

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 ...


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

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 ...


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 ...


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.


1

Your vehicle requires 95 or 97 octane fuel. Any lower rated octane fuel will give you the engine pinking and knocking you describe. When your mechanic gives the vehicle his/her attention, ask that the engines 'knock sensor' is checked for correct operation. Also that the 'engine temperatures' are within specification. In a simple description, lower octane ...


1

It sounds like you have carbon buildup in your combustion chambers. A thorough intake tract cleaning should take care of it. You can get it done at the shop, or there are treatments you can do at home (ie: Seafoam). What happens is the carbon buildup creates hot spots in the combustion chamber which pre-ignights the air/fuel mixture and causes the ping or ...


1

Most likely put in whatever the manual is saying, else if you are going to service, you might have some trouble over there. Also it is for the sake of durability of engine internals.


1

I have a 9.1:1 compression car that I normally boost to 16 psi as the intercooler maxes out there on hot days. We've run it up to 18 psi no problem on a normal day. At 21 psi the intercooler is totally saturated in a hurry and the ECU starts to flatline the timing due to knock. I've also got a 9.6:1 NA car that knocks on anything less than premium ...


1

It may be worth trying out a tank of higher octane once, paying attention to how long it lasts over a tank of what you've normally been using. I noticed my 2001 Lancer went further on a tank if I used higher octane. It wasn't a lot, but enough that it was obvious: I routinely use the trip meter to measure how far a tank goes, and don't normally fill it ...


1

Well the first thing that you should do is re-read your manual. If the manual specifically says to use the lowest grade use that, the maker knows best. However if it says that the cars were made for 87 or higher your car may be able to take advantage of the higher octane, such is the case with many subarus. However the only way that you will truely be able ...


1

Your manual states 91 octane fuel (I checked). Filling up with a lower octane fuel will cause knocking and the computer will retard timing to prevent engine damage. You won't hear it, the knock sensor is more sensitive than your ears. Running in this retarded state isn't a normal operating parameter and not recommended. Go with the 93, the .20 more per ...


1

I'm not sure whether this is relevant, but the lowest octane rating you can buy in the UK is 95. I recently filled up with 99 octane by accident, and haven't noticed much difference. There is commonality of quite a few models across the pond, so whatever's suitable there should be suitable here, unless they come with a different ECU configuration. It's ...



Only top voted, non community-wiki answers of a minimum length are eligible