87 octane versus 91 octane - Mazda 6 Forums : Mazda 6 Forum / Mazda Atenza Forum
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post #1 of 19 (permalink) Unread 04-30-2019, 12:28 PM Thread Starter
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87 octane versus 91 octane

Can anyone who uses both for their 6 tell me what significant differences they notice when using the different octane levels?

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post #2 of 19 (permalink) Unread 04-30-2019, 02:39 PM
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Unless you're driving the turbo in a 2018+ model or have a tune specifically for that grade gas, the only difference you'll see is a lighter wallet with no perceptible performance change.


The 2.5L Skyactiv gas engine is designed for 87 grade. Going to a higher grade doesn't help (or hurt necessarily). Keep in mind there are no additional detergents or anything else that makes higher grade gas any better - unless your engine is designed to use it.
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Quote:
Originally Posted by Modzas6x View Post
Can anyone who uses both for their 6 tell me what significant differences they notice when using the different octane levels?
Some folks may like certain brands of 91 octane fuel which have no ethanol... Typically 87 octane has 10% ethanol. Ethanol can be corrosive but our cars appear to not have any corrosion of the fuel system prbs.

If you hav a tune that utilizes higher octane fuel for a benefit (say like strengthening your low rpm response a bit) - well then of course you'd use it.

Theoretically using higher octane fuel lessens potential fuel dilution of the motor oil... but folks on here have results which seem to indicate, no, this effect is not realized. The Skyactiv 2.5 does not appear to have a propensity to dilute motor oil with fuel, in general.

Some on here believe the economics are poor for upgrading fuel octane: too little benefit for too much incremental cost.

I can't say I notice any performance change in running 94 octane Canadian west coast Chevron.

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post #4 of 19 (permalink) Unread 04-30-2019, 04:31 PM
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If you're running a turbo model run the 91+, if you're not run the 87. The higher octane prevents detonation, that's really it unless you're getting into the world of custom tunes.

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post #5 of 19 (permalink) Unread 04-30-2019, 04:36 PM
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Quote:
Originally Posted by dasrider View Post
Unless you're driving the turbo in a 2018+ model or have a tune specifically for that grade gas, the only difference you'll see is a lighter wallet with no perceptible performance change.

The 2.5L Skyactiv gas engine is designed for 87 grade. Going to a higher grade doesn't help (or hurt necessarily). Keep in mind there are no additional detergents or anything else that makes higher grade gas any better - unless your engine is designed to use it.
In fact if you are in Colorado or other higher-altitude and "regular" is 85 run it -- even with an 87 tune. I've made a few trips out there, have seen no misbehavior, but what I HAVE seen is a material difference (higher!) in mpg.
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post #6 of 19 (permalink) Unread 04-30-2019, 07:23 PM
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I have to run 91 in the winter in the 04 V6 as they put much too much ethanol in the lower grades. It's getting harder to find no ethanol premium grades now too. I get no performance boost of any kind. It does dramatically improve cold startups. As soon as the temperature gets over frerezing I stick to 87.

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post #7 of 19 (permalink) Unread 05-01-2019, 01:55 AM
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Some folks have said that Shell (in Canada) has no ethanol in their 91 grade. I never thought to check.

Many years ago I had a '78 Chev Belair - 305 2V if memory serves... and it would lightly ping on much of the Shell fuel... versus NOT on the same indicated AKI / Octane for other brands... like Chevron (which we get on the west coast). Shell, it seems to me, was always stingy when it came to octane.

Having said this (and this was long, long ago) - Shell lubricants and fuel are I believe among the highest quality products in the market.

I like, though, the fact that I can get 94 octane Chevron... for my air-cooled Guzzi... and so I tend to patronize Chevron, all other things being equal.

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post #8 of 19 (permalink) Unread 05-01-2019, 02:19 AM
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Quote:
Originally Posted by Cdn17Sport6MT View Post
Many years ago I had a '78 Chev Belair - 305 2V if memory serves... and it would lightly ping on much of the Shell fuel... versus NOT on the same indicated AKI / Octane for other brands... like Chevron (which we get on the west coast). Shell, it seems to me, was always stingy when it came to octane.
My first 2 cars were 78 & 79 Impalas (US Impala = CA Bel Air) and I can vouch that the 305 was finicky when it came to marginal fuels. Car #3 was a 70 Caprice (ooh... I got chrome this time) with the 350/4bbl which never had issues with bad gas.


As for my Mazda, I switched to Shell exclusively and my MPG is currently about 10% better than the other readily available brands
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I should try the Shell then. Thx.

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Check any pump you are not familiar with to make sure it is not posted as containing ethanol no matter the grade. They are sneaky about it.

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