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Suppose a production fuel-injected engine is tuned for 93 octane fuel. As I understand it, the engine can still run on lower-octane (i.e., less knock-resistant) fuel, but it will pull timing to avoid knocking. In effect, on lower octane gasoline it has to run at a lower compression ratio. This is practically the definition of reduced fuel efficiency.

What I'm looking for are any sample numbers to indicate how significant the timing change can be on fuel economy and/or engine power. To keep it "simple" we could stick with a naturally aspirated engine. Holding air density constant, suppose we fill it with 85 octane. It pulls timing (changing valve overlap and/or spark firing) to avoid knocking.

What would be a plausible effect on fuel economy? Or output power? Or both? Feel free to pick whatever constraints necessary to give realistic numbers. (E.g., "I only know that for my car when warm at 5000rpm with load X the effect is Y" is still helpful!)

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I can't tell about precise numbers, but have some experience with my cars and multiple gas types usage: if a lower octane is used in an engine tuned for higher octanes, the engine looses power a lot. If you normally climb a moderate hill on 4th gear, with lower octane you will need to go up in 3rd, for instance, in a manual shifted car.

It runs and won't get damaged but it won't be efficient like with what it is setting for, besides it will tend to run more hot, more noisy and probably more smokey. To somehow fix that, you need to change the timing, retard it a bit until you get a better performance which will be almost the same as if you use a higher octane. The problem is that, independent of engine age, design, etc. low octane gas usually burns with more carbon residues, specially bad if the car is not carburated.

As for fuel economy, there is a trade: using lower octane (in an engine tuned for it) may be a good thing, however you loose power and you will need to gas it more if you are driving with some load, or in hilly towns. Likewise, if you use higher octane gas in an engine tuned for lower quality gas, it will "drink" the gas quite fast!

I personally, sometimes mix gas :) run always on lower octanes because I'm carburated and old engine, and spike the tank sometimes with a few higher octanes liters. It works wonder.

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It's obviously not like: 10% lower octane is 10% less timing advance, 10% less power, and 10% more fuel consumption. I don't think it's even possible to calculate beforehand how much you will benefit or not from lower or higher octane fuel, even if you precisely know how the ECU will react. And that's the thing: It heavily depends on how the ECU deals with the situation.

Ideally, your ECU would change its spark advance just enough to cope with the lower octane fuel, while still giving you the best performance possible. Just like with higher octane fuel. But in my experience, it's only the top notch ECUs (read: real expensive cars) that do that. In that case you'd have to test it on the dyno to find your performance and fuel economy gain, I don't think there is another way to determine it. Keep in mind that every ECU can react differently, and results of other people may not apply for you.

It's possible your ECU just firmly pulls down the timing advance, like 10 degrees, when it detects ping. All the profit is lost then I think.

Not even all cars have knock sensors. If an engine is designed for a specific octane rating, I don't think you can improve things with higher or lower octane fuel. It's certainly not worth the great time investment of adapting the engine for that. As far as I know higher octane fuel is only used in sports cars that have their engines designed and optimized for it. (Higher being 98RON, lower 95RON, the only ones widely available in the Netherlands.) Next to spark advance, there's a lot of additional design parameters that you'd have to change to really make use of more spark advance.

In short: Your average Opel Astra engine is in all probability not susceptible for adaption to really benefit from different octane rated fuel. Lower grade just results in loss of power and increased fuel consumption if you don't adapt. Higher grade will probably have no effect.

I don't have any sources or hard evidence to back this up, but I'm speaking from personal experience.

  • The question is more along the lines of: We have a lot of cars on the road designed for "premium" fuel. I wonder, if a lot of those were filled with regular octane fuel, how much that could affect gasoline consumption? – feetwet Dec 13 '16 at 19:00
  • @feetwet I think that the risk of damage you're taking is of more importance than the chance that you're saving a little money, if at all. If you're unlucky, and your engine has a compression that is too high for non premium fuel, it will cause damage. If that's not the case you may hope that your ecu notices knock and retards the timing, or there will still be damage. As i explained in my answer, the engine behaviour and thus fuel consumption, depends on how your ecu deals with the situation. You can't say anything about all cars or one car until you know how the concerning ecu is programmed. – Bart Dec 14 '16 at 7:57
  • @feetwet Look at Aram's answer below, he explains that his car loses a lot of power when filled up with the wrong fuel. That's probably because his ecu notices the knock and retards the timing to maybe like 0 degrees to be sure there won't be any knock. Consequently the cars towing power now equals that of a dead horse. He also told that it runs hot. That's because your exhaust gas is much hotter when the timing is retarded. Especially your exhaust valves certainly won't like that. And keep in mind that higher octane only holds more potential if used right, the energy is the same. – Bart Dec 14 '16 at 8:07
  • The question is about the case where the ECU retards timing to avoid knock. My rough understanding is that for purposes of preventing knock this consists of leaving the exhaust valve open longer, which effectively reduces compression and cylinder volume, both of which reduce power, and the former of which reduces fuel economy – but by how much? I'm talking about modern production cars, so pick a plausible factory ECU program and let's try to work out the efficiency consequences of a "bad" low-octane scenario. – feetwet Dec 14 '16 at 13:50
  • @feetwet I very much doubt it your ECU reduces compression by playing with valve overlap and such. Let's put it like this: Maybe, maybe you can save money if you properly retune your engine to make use of lower octane. You make use of its full potential then. But you ECU is not a magician that exactly knows how it should change its management to run just right on lower octane. – Bart Dec 14 '16 at 16:36

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