6 ms·

News flash: use the octane rating your automobile requires. (period) Higher octane is for high compression engines. It's a waste of money to use in in most car

News flash: use the octane rating your automobile requires. (period) Higher octane is for high compression engines. It's a waste of money to use in in most cars.

dghughes 11y ago
You can even use a lower octane than recommended since modern cars with computers can compensate.
- modoc 11y ago
  Please don't. You force the car to retard the timing, get less power, and burn fuel less efficiently. If you really don't want to pay for higher octane fuel, buy a car that runs on lower octane fuel...
- mehrdada 11y ago
  "Modern" is a quite vague here. Direct injection (usually coupled with turbocharging) is a key rule of thumb. If your car has a direct injection, turbocharged engine AND it requires ("recommending" is cheap) high-octane, pay for it. Otherwise, it is probably an overkill.
- sliverstorm 11y ago
  You can but that doesn't mean you should. Most cars go into a "limp mode"- they still run, but nowhere near as well as they should. Additionally the most common way to "detect" low octane fuel is with an engine knock sensor, so any engine that tries to optimize itself instead of using a "limp mode" will have to flirt with knock.
- jacquesm 11y ago
  You're one flaky knock sensor away from a very expensive repair if your car has a knock sensor. Knocking, aka uncontrolled (usually premature) detonation can be detected but by then it has already happened and with aluminum pistons it doesn't take much to blow a hole in them depending on when in the cycle the fuel detonates. The higher the compression ratio of the engine the earlier in the cycle this would happen the bigger the chance of a very expensive repair (if not a write-off...). Don't use a lower octane than what your manufacturer recommends, that compensation is there as a safety precaution, not as a means to allow you to shift the standard octane your engine really needs. Note that running your engine on fuel with lower octane number than what your manufacturer recommends is also a way to void your warranty (and since the damage is quite specific and the evidence is right in your tank there is a very good chance that will kick in).
  - lutorm 11y ago
    This is true, but in practice you are one flaky knock sensor and a lead foot away, because detonation really is only an issue at high power levels. I'd wager most people don't really ever go full throttle and if they do, for a few seconds at most, so I suspect the potential for damage is not that great. If you habitually floor it, though, then yes.
    - jacquesm 11y ago
      That's only partially correct. If your engine 'knocks' regularly you're stressing it in a way that is detrimental to the engine's life-span. If you do it whilst under high power then the risk of damage increases, but doing it over a longer period at lower power level is just as damaging (due to the repeated stressing of the engine materials, specifically the piston heads, the cylinder walls, the gasket material and the valves as well as the head). An engine that knocks regularly will die, no matter what the power levels at which it takes place simply because it isn't designed to deal with that stress on a regular basis, engines are designed with some leeway but not so much margin that they could take such abuse and still manage the same life-span they'd achieve with normal use.
      - lutorm 11y ago
        You might be right. My thinking was this: peak cylinder pressure basically is proportional to torque, so at lower power levels cylinder pressure is overall lower. Even if the engine knocks, which will cause a spike in cylinder pressure, it starts from a much lower baseline, and there's also much less air/fuel mixture in there. Thus peak cylinder pressure for a knocking engine at low torque may not be much larger than that of a non-knocking engine at full power. But this is just speculation on my part, I don't have quantitative data. It's also possible that it is the sharp pressure spikes that do the damage rather than the overall pressure level, in which case what I'm saying would be less applicable.
  - dghughes 11y ago
    I disagree but people can do what they want but octane ranges are not as bad as people think. Just consider many countries sell gasoline with vastly different octane ratios. You buy a Honda Accord in the US or Australia or Canada and the choice of fuel is quite different there is no standard for premium. There is a post of reddit where someone in Germany said regular is 95 octane and premium is 100 octane versus the US with regular 89 and premium 95. So there's a big discrepancy there how can the same cars in two different countries operate with such vast differences in octane?
hugh4 11y ago
Some newer Fords, at least, claim to detect and adjust for the kind of fuel you're using, producing less power but avoiding knocking when running on lower fuel grades.
gweinberg 11y ago
I suspect there are a lot more people buying lower than recommended octane than there are buying higher.
SeanLuke 11y ago
> Higher octane is for high compression engines Generally yes: but it's not a requirement. For example, Mazda's Skyactiv engines run at 13:1 (unusually high compression) and consume regular gas.

News flash: use the octane rating your automobile requires. (period) Higher octane is for high compression engines. It's a waste of money to use in in most cars.

dghughes 11y ago

You can even use a lower octane than recommended since modern cars with computers can compensate.

modoc 11y ago
Please don't. You force the car to retard the timing, get less power, and burn fuel less efficiently. If you really don't want to pay for higher octane fuel, buy a car that runs on lower octane fuel...
mehrdada 11y ago
"Modern" is a quite vague here. Direct injection (usually coupled with turbocharging) is a key rule of thumb. If your car has a direct injection, turbocharged engine AND it requires ("recommending" is cheap) high-octane, pay for it. Otherwise, it is probably an overkill.
sliverstorm 11y ago
You can but that doesn't mean you should. Most cars go into a "limp mode"- they still run, but nowhere near as well as they should. Additionally the most common way to "detect" low octane fuel is with an engine knock sensor, so any engine that tries to optimize itself instead of using a "limp mode" will have to flirt with knock.
jacquesm 11y ago
You're one flaky knock sensor away from a very expensive repair if your car has a knock sensor. Knocking, aka uncontrolled (usually premature) detonation can be detected but by then it has already happened and with aluminum pistons it doesn't take much to blow a hole in them depending on when in the cycle the fuel detonates. The higher the compression ratio of the engine the earlier in the cycle this would happen the bigger the chance of a very expensive repair (if not a write-off...). Don't use a lower octane than what your manufacturer recommends, that compensation is there as a safety precaution, not as a means to allow you to shift the standard octane your engine really needs. Note that running your engine on fuel with lower octane number than what your manufacturer recommends is also a way to void your warranty (and since the damage is quite specific and the evidence is right in your tank there is a very good chance that will kick in).
- lutorm 11y ago
  This is true, but in practice you are one flaky knock sensor and a lead foot away, because detonation really is only an issue at high power levels. I'd wager most people don't really ever go full throttle and if they do, for a few seconds at most, so I suspect the potential for damage is not that great. If you habitually floor it, though, then yes.
  - jacquesm 11y ago
    That's only partially correct. If your engine 'knocks' regularly you're stressing it in a way that is detrimental to the engine's life-span. If you do it whilst under high power then the risk of damage increases, but doing it over a longer period at lower power level is just as damaging (due to the repeated stressing of the engine materials, specifically the piston heads, the cylinder walls, the gasket material and the valves as well as the head). An engine that knocks regularly will die, no matter what the power levels at which it takes place simply because it isn't designed to deal with that stress on a regular basis, engines are designed with some leeway but not so much margin that they could take such abuse and still manage the same life-span they'd achieve with normal use.
    - lutorm 11y ago
      You might be right. My thinking was this: peak cylinder pressure basically is proportional to torque, so at lower power levels cylinder pressure is overall lower. Even if the engine knocks, which will cause a spike in cylinder pressure, it starts from a much lower baseline, and there's also much less air/fuel mixture in there. Thus peak cylinder pressure for a knocking engine at low torque may not be much larger than that of a non-knocking engine at full power. But this is just speculation on my part, I don't have quantitative data. It's also possible that it is the sharp pressure spikes that do the damage rather than the overall pressure level, in which case what I'm saying would be less applicable.
- dghughes 11y ago
  I disagree but people can do what they want but octane ranges are not as bad as people think. Just consider many countries sell gasoline with vastly different octane ratios. You buy a Honda Accord in the US or Australia or Canada and the choice of fuel is quite different there is no standard for premium. There is a post of reddit where someone in Germany said regular is 95 octane and premium is 100 octane versus the US with regular 89 and premium 95. So there's a big discrepancy there how can the same cars in two different countries operate with such vast differences in octane?

hugh4 11y ago

Some newer Fords, at least, claim to detect and adjust for the kind of fuel you're using, producing less power but avoiding knocking when running on lower fuel grades.

gweinberg 11y ago

I suspect there are a lot more people buying lower than recommended octane than there are buying higher.

SeanLuke 11y ago

> Higher octane is for high compression engines Generally yes: but it's not a requirement. For example, Mazda's Skyactiv engines run at 13:1 (unusually high compression) and consume regular gas.