Benefits of High Octane at High Altitude?

[MikeM.]

Get bent Mike. My dealer in Lakewood Co said 85 was OK too.

No need to get personal when discussing technical matters. You disagree, that's fine. The fact remains, the engine doesn't know or care how many Mazda dealership employees spread misleading info, the engine will continue to follow the laws of physics and the internal programming assumes the fuel meets the minimum octane spec which is 87 regardless of the barometric pressure.

 

[MikeM.]

It's been said time and time again by professionals that 85 is the equivalent to 87 in higher elevations (due to thinner air causing lower compression).

OK, a little digging and I've found some more definitive info on the subject of using lower than recommended octane at altitude in cars with barometric pressure sensors. The Society of Automotive Engineers published a paper on this very subject back in 1987 when electronic fuel injection started becoming more popular.

Past tests of vehicles show that their octane number requirements decrease with altitude. As a result, gasoline marketers sell lower-octane-number (ON) gasoline in the mountain states and other high-altitude areas. The current ASTM specifications, which allow reduction of gasoline octane of 1.0 to 1.5 ON per thousand feet, are based on CRC test programs run on 1967 to 1972 model vehicles. However, many new vehicles are now equipped with sophisticated electronic engine systems for control of emissions and improvement of performance and fuel economy at all altitudes. Because these new systems could minimize the altitude effect on octane requirement, Amoco Oil tested twelve 1984-1986 model cars and light trucks. We found their ON requirements were reduced on average about 0.2 ON per thousand feet on an (R+M)/2 basis (RMON/1,000 feet). We expect octane demand on gasoline suppliers in high-altitude areas to increase as these new cars make up a larger part of the vehicle population, and this could raise the cost of gasoline.

Link to the Abstract: http://papers.sae.org/872160/ (you can read more detail without paying $25 for the full article by clicking on the "preview" on the right).



Keep in mind that engines falling into the Skyactiv family are much more sophisticated than the 1984-1986 models that these tests were based on. But even with the relatively crude EFI systems in the day, it was found that altitude did not decrease octane requirements but 0.2 octane numbers per thousand feet. Which, on these old EFI cars, meant that 85 octane would be under-spec even at 5000 feet (by a full octane point). On a Skyactiv engine the difference in octane reduction requirement due to altitude is likely much less than these old tests found.


This is exactly what I was saying in previous posts on this subject. Bottom line, run 87 octane even at altitude due to the self-compensating function of Skyactiv engines.

 

[flyfisherdon]

WOW, MikeM that is a lot of work to try to help those guys, but as my father used to say " When ignorance is bliss, tis folly to be wise"
Don

 

[MikeM.]

WOW, MikeM that is a lot of work to try to help those guys, but as my father used to say " When ignorance is bliss, tis folly to be wise"
Don

Sorry for the late reply, just now noticed this.

To clarify, I don't do all this work to help those who are willfully ignorant but plenty of people visit this forum who might not know the answer but who are open-minded and capable of learning something new. I don't like to see them misled with false information propagated by the former.

The Society of Automotive Engineers publish lots of useful, peer reviewed studies (like the one mentioned in my previous post) and these can help dispel long running myths (like even modern cars are just fine on below spec fuel at altitude). The paper above explains why this us a fallacy and this can be used to decrease ignorance or can be ignored (as the individual prefers).

 

[V8toilet]

Mike, are you running out of new stuff and bringing back 2 year old threads now? On a serious note that is great information you provided.

 

[MikeM.]

Mike, are you running out of new stuff and bringing back 2 year old threads now? On a serious note that is great information you provided.

Ha! You think 2 years is old?

The research paper I linked to is 28 years old!

What amazes me is we've known the old rule of thumb that cars octane requirements dropped by 1 octane number for every 1000 feet above sea level did not apply to cars with barometric pressure sensors for 28 years but people are still claiming it's true. Unbelievable!

 

[Surfsup]

It's been said time and time again by professionals that 85 is the equivalent to 87 in higher elevations (due to thinner air causing lower compression). I've been told by my techs it won't void my warranty. I'm going to keep using it. There is no reason for me to pay more so I'm not going to.

Have you ever noticed any valve pinging? I doubt any tech could tell if you were historically using fuel with 2pts less octane

 

[MikeM.]

Have you ever noticed any valve pinging? I doubt any tech could tell if you were historically using fuel with 2pts less octane

With fuel below the minimum approved octane, the knock sensor will prevent destructive knock and you will likely not notice except to the extent that you will be considerably down on power and the engine might throw an error code. In this case, the tech could see the error code but there would be no easy way to conclusively prove that the fuel was below spec, just that detonation was detected.

Unlike some cars that are designed to constantly monitor detonation and use that information to make an educated guess as to the effective octane in each tank, the car then uses that info to load maps appropriate to that octane, the CX-5 makes no attempt to be that sophisticated. Knock detection triggers a fault which loads a lower power safe map. Unlike the previous example, this is not considered normal operation but fail-safe protection, limp home mode. It is simply not designed to run on octane below the minimum spec (although I'm sure there is a certain margin of error built in).

The possibility of loss of warranty coverage is not the best reason to always use the minimum specified octane (or higher). The best reason to use at least the specified octane is because that's the only fuel the engine is designed to run properly on.