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Is there any evidence that premium and/or high-octane fuel (petrol/gas for cars):

  1. improves performance; and/or
  2. improves engine longevity; and/or
  3. improves mileage so that money is saved on fuel, even when taking into account the extra cost of it

I'm somewhat sceptical of the claims of fuel companies, especially given the price of premium fuel. Have independent tests been performed?

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    I was under the impression that higher octane fuel only suppresses combustion, so you can run at a higher compression ratio (and thus get more energy from the same fuel) without causing pre-ignition (engine knocking). If an engine runs on low-octane fuel without knocking, I would be skeptical that it would run any better on high-octane fuel. – Mike Dunlavey May 18 '11 at 22:29
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    @MSalters: Ummm... wait a minute. You don't save fuel by turbocharging, or by injecting more fuel. To prevent knocking you have to inject the fuel into the cylinder (as in a diesel), not spark it later. High power-to-displacement engines get that way by having a higher compression ratio, thus need higher octane, regardless of turbocharging. High volume ratio provides more power by making the pressure volume curve larger, for the same chemical energy. – Mike Dunlavey May 19 '11 at 18:03
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    @Mike Dunlavey: turbocharging etc does not directly cause a performance increase with premium fuel. Instead, it's the underlying design. When your design goals are to maximize power per volume, you end up with both a turbocharger and a high compression rate. The latter in turn requires premium fuels. So there's indeed a correlation. – MSalters May 20 '11 at 8:49
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    I've never heard fuel companies claim that premium fuel is good for cars. They often claim that THEIR fuel has additives that are better for your engine, but don't make a distinction between the grades. – JohnFx Jul 10 '11 at 1:41
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Seems to be a yes based on this:

http://www.thorneymotorsport.co.uk/tuning/ford/focus-rs/focus-rs-fuel-testing.shtml

Remember this is completely standard car, no mapping no toys just as it came from the dealer. Peak power went from 272bhp to 293bhp – 20bhp for doing nothing more than putting better fuel in the car. MPG also went from an average (dash display so hardly that accurate) 22.3mpg to 24.8mpg and that only took three tanks, its now on 25.6mpg now that its modified. Its a free lunch, it really is. But don’t trust us, try it, just run three tanks through the car, we can guarantee that the car will feel better, you’ll get more power and cover more miles, try it.

Fuel Comparison

  • ThorneyMotorsports worked on my car, transforming it from 217bhp to 370bhp, with the parts I suplied. However, these people replaced 3 abs sensors, charing me for them each time, until they realised the ECU was firing up failed ABS warnings as too much power/Air was going through the MAF sensor, and the ABS sensor warning was the first one on the 'get the car into limp mode' list, so it simply fired that, rather than create a new mode for jesus H, how much air are you trying to force through me... In short, I am not 100% convinced Thorney know much about what they are doing. – Hairy May 19 '11 at 13:42
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    In the USA, "Premium Fuel" is usually 92 or 93 Octane, with "regular" being around 87 Octane. (That's one reason why fuel is relatively cheaper in the US, and also a reason why some fuel-sensitive sports cars aren't marketed here.) I wonder if the difference between 93 vs. 87 is as pronounced as 99 vs. 95? – ESultanik May 19 '11 at 13:46
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    Note that US 91 = UK 95 octane. UK uses RON number, US uses AKI. – horatio May 19 '11 at 14:52
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    1 UK Gallon = 4.55 Litres 1 US Gallon = 3.79 Litres (Obviously approx) Also our Pints are bigger than US pints so anybody coming over here and having a few pints, beware you may be drinking more than you expect :) – Ardesco May 24 '11 at 15:52
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    Even if their numbers are accurate ( could be for a specific engine ), and generally applicable ( not going to be ), it is no "free lunch" since the higher octane fuel costs considerably more. – psusi Dec 9 '14 at 4:19
6

"Car and Driver" did a test which concluded that cars designed for premium fuel (which in general are higher-performance cars) do run better on premium fuel.

So, at least one of the claims can be confirmed. With respect to the longevity claim, Car and Driver offers an opinion: a dirty engine will increase knock, which can be ignored by using premium fuels or solved by cleaning the engine. The reason is that soot deposits act as ignition sources.

  • +1 Right. Cars with higher compression ratios need higher octane fuel. I had the impression the question was "All things being equal, is it better to use premium fuel?". Like my jalopy runs fine on 87, & I doubt it would run any better on 93. But, if the only change to the car was to get the same power from a smaller displacement engine running 93, would it save enough gallons to pay the extra price per gallon? I guess I don't know. But I agree with what you say about running clean. – Mike Dunlavey May 19 '11 at 18:22
  • Higher compression engines run hotter, which does improve the thenoretical efficiency limit (Carnot efficiency), and also practical efficiency (no surprise, given the importance of engines to high-performance cars). But monetary efficiency? High-performance cars are generally owned by people who are less price-sensitive, and AFAICT premium fuel prices reflect that. – MSalters May 20 '11 at 8:42
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No. Higher octane fuel is not "better", it simply burns at a higher temperature/pressure. The advantage that high octane fuel has is that you can build the engine to use a higher compression ratio which allows you to get more fuel, and hence, more power, into and out of each stroke. If the fuel is too low octane for the engine, you will get pre ignition ping, otherwise known as knocking, because the fuel explodes too early. If the octane is too high, then it explodes too late or won't burn at all. These days the electronic control systems can play games with the valve timings to compensate somewhat so that you don't get knocking, but it still isn't doing the engine any favors to use the wrong fuel.

So the bottom line is: use whatever fuel the engine was designed for, no higher, and no lower. For most non sports cars, that means 87 octane.

See:

http://en.wikipedia.org/wiki/Engine_knock

http://ask.metafilter.com/128168/Does-high-octane-gas-matter

http://www.toyotaperformance.com/fuel_octane_vs_horsepower.htm

https://mechanics.stackexchange.com/questions/39/what-are-the-benefits-of-premium-high-octane-petrol

There also is, of course, the owners manual of pretty much any car that will tell you what octane fuel to use, and they don't specify X or better. Some manuals, like my Toyota Prius, explicitly specify NOT to use higher octane fuel.

  • We expect users to back up any significant claim with a reliable source. Please edit your answer to include the references on which you based your answer. – Oddthinking Jul 1 '11 at 8:21
  • @Oddthinking fine, there's a few – psusi Jul 1 '11 at 14:17
  • Hopefully with your edits, some people will retract their downvotes, this is now by far the best answer here, though it would be even better if someone could find some truly independent, ideally scientific, studies of the issue. – Mark Booth Sep 27 '11 at 10:41
  • Interestingly, mine does say "X or better." -- My Mazda 3 (not speed edition) says: Regular unleaded fuel 87 [ (R+M)/2 method] or above (91 RON or above) – ckittel Dec 9 '14 at 1:33

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