87 vs. 89 vs. 91 vs. 93 octanes... What's the difference??? (1 Viewer)

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i posted this in another forum that i frequent, but thought some of you may find some value in this since your 100's "require" 93, but can run on 87. (oh, and as far as my credibility is concerned - i'm a chemical engineer by degree...)

Lesson in Octane

they will burn the same. no assertainable difference in carbon buildup within the same brands. however, compare shell or chevron to an off brand and you can and will have more buildup w/ the off brand b/c they tend to have less detergents and poorer quality detergents in them.

now as far as power goes, you probably won't feel the difference. if the engine was meant to run at it's optimum on 93 octane and you put 87 in it, then it will simply retard the timing enough to prevent detonation. in doing so, you loose some power, but typically less than 3 or 4%. that's about 6-8hp and for the everyday driver, you'll never feel the difference. however, you will more than likely have a decrease in your efficiency (MPG's) if you drop from a recommended 93 down to 87 due to the engine retarding the timing. this is more noticeable on automatics than on manual transmission equipped vehicles due to the downshifting at cruising speeds from the loss of the top end power.

for fun, let's walk through a quick & dirty calculation with some assumptions:

15 gallon use of fuel. 93 octane is $2.50 and 87 is $2.30 per gallon. you normally get 17MPG with 93, but only 16MPG with 87. which grade is more economical to use?

for 93 octane:
15 gallons * 17 MPG = 255 miles, 15 gallons * $2.50 = $37.50 to fill up
255 miles / $37.50 = 6.8 miles per dollar -OR- 14.71 cents per mile

now for 87 octane:
15 gallons * 16 MPG = 240 miles, 15 gallons * $2.30 = $34.50 to fill up
240 miles / $34.50 = 6.96 miles per dollar -OR- 14.38 cents per mile

add that up for somebody who drives 20k miles per year, and the difference is an whopping $66 per YEAR...


the reason that they require 93 is due to the high compression ratio of the engine. higher ratios means that during the compression cycle, the piston moves in the upward direction so far that the air inside is compressed. compression causes heat, and that heat can cause premature detonation of the fuel without the spark plug ever firing. this is detonation. the engine has a knock sensor and will detect pre-detonation conditions and will adjust engine timing accordingly to compensate.

also, if you have an older engine, you're more likely to get pinging (detonation). why is that? it's because older engines typically have some carbon buildup on the valves and on the piston head. those carbon deposits can get heated to very high temperatures while the engine is running and will actually glow (think of charcoal in a grill). those glowing embers will then ignite the fuel and cause detonation. using something like BG44K or Seafoam on a regular basis will remove the carbon deposits and therefore will reduce your chances of pinging.

why does 93 not ping when 87 octane will? it's in the octane rating. 93 is "harder to ignite" than 87. it doesn't burn any cleaner, or have more energy than 87 octane does. however, it requires more energy to initiate combustion, and therefore is less prone to detonation during the compression cycle or due to hot embers in the combustion chamber. also, like metioned before, 93 octane allows the engine to run at peak performance by allowing the timing to be as far advanced as possible.

on the contrary, for an engine designed to run on 87 and then you put 93 in it, you are simply wasting your money. the engine will still run the same and you won't notice a bit of difference in power or performance. absolutely ZERO difference. as long as it is well maintained w/ low amounts of carbon deposits that would normally cause detonation and lead to a retarted timing, then using 93 octane won't do anything but make you poorer.

as far as Mid-Grades go. it's really a waste.

ok class. back to work...

:rolleyes:
 
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Professor,

Since I can't buy 93 octane gas in California, how many gallons of 100 octane gas can I mix with 91 octane gas to get a full tank of 93 octane?
 
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bamachem said:
.......93 is "harder to ignite" than 87. it doesn't burn any cleaner, or have more energy than 87 octane does

Do I read this correctly, there is no energy difference in any different fuel octane, 87, 93, 98, 100, 130........no energy difference?
 
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Like mentioned, the fuel rating is all about resistance to burn. 93 octane isn't more "powerful" than 87, it takes more energy (heat) to get it to ignite.

If you are running high compression (leave diesels out of this example) or high boost in a forced induction engine, there is so much heat produced in the combustion chamber that you need a fuel that can burn slower (at a higher temp) so that you don't detonate.
 
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100 TD said:
Do I read this correctly, there is no energy difference in any different fuel octane, 87, 93, 98, 100, 130........no energy difference?

Doesn't a higher octane fuel require more refining? That means more oil is needed to create one gallon of 91/93 vs 87. And if it's refined more, doesn't it mean there are less impurities in it? IMHO, if one is so concerned about paying a few extra bucks for a tank of chevron 91 vs. no-name-brand 87, one should NOT even consider buying/driving a LC/LX. Or eat out one less time per tank of gas or skip one Starbucks cappucino and you'll easily save enough money for the high octane.
 
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Fortunately I don't skimp on fuel as diesel is diesel here!!

I'm still interested in the fuel octane info and how it all works, I will have to add it to the list of things to research! IIRC there used to be 2 different octane ratings, RON and MON, but have forgotten all about them as well. In OZ we have 91 (standard grade) and 95 (Premium) but Mobil services centres on the mian highways are replacing 95 with 98, which may be specified for some sports cars or WRX's or something.
 
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a990dna said:
Professor,

Since I can't buy 93 octane gas in California, how many gallons of 100 octane gas can I mix with 91 octane gas to get a full tank of 93 octane?
Toyota recommends 91 octane, so anything more is a waste of money and will perform at less than optimal.
 
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firetruck41 said:
Toyota recommends 91 octane, so anything more is a waste of money and will perform at less than optimal.

Yea, I realize that. I don't how the author of this thread determined our 100s "required" 93 as was stated in his first sentence... and since he's a chemical engineer, I gave him a math problem to solve.

I know the ratio to achieve 94/95 octane from 91/100 octane gas... I simply contacted 76 Union for the formula. Its 25% 100, 75% 91. just need to convert to gallons based on the tank size.

I was willing to offer him a tech ops job up to the point his math was off... could potentially kill someone in the business we're in.
 
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a990dna said:
Professor,

Since I can't buy 93 octane gas in California, how many gallons of 100 octane gas can I mix with 91 octane gas to get a full tank of 93 octane?

:rolleyes: simple ratio.
100 - 91 = 9
94-91 = 3
3/9 = 1/3 (ratio of 100 to 91 to get 94)
so for ever 1 gallon of 100, you add 3 gallons of 91 and you'll end up with 4 gallons of 94
but you already answered yourself.


Beretta Vertec said:
The math needs to be checked. Should be $66. Numbers are already in cents, but you converted to cents a second time at the end.

:doh: yup. got me there!
.1471-.1438 = .0033
.0033 * 20,000 = $66
thanks for catching the screwup...


a990dna said:
Yea, I realize that. I don't how the author of this thread determined our 100s "required" 93 as was stated in his first sentence... and since he's a chemical engineer, I gave him a math problem to solve.

I know the ratio to achieve 94/95 octane from 91/100 octane gas... I simply contacted 76 Union for the formula. Its 25% 100, 75% 91. just need to convert to gallons based on the tank size.

I was willing to offer him a tech ops job up to the point his math was off... could potentially kill someone in the business we're in.

yeah, if i were in the chemical industry, it could. fortunately for you, i just design food plants and breweries... :grinpimp:



BTW, the only reason i posted this is cause about once a month or so I see some noob w/ a 100 ask if they can run 87. this was just an excercise to explain the difference in 91/93 and 87 and to show them that it's actually not any cheaper to run the 87.
 
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bamachem said:
...this was just an excercise to explain the difference in 91/93 and 87 and to show them that it's actually not any cheaper to run the 87.
Assuming you actually get a 1 mpg increase with 91 vs. 87. :doh::D I can't run the different fuel and keep track of the mileage since it is the wifes car, and she never writes the miles down on the reciept like I ask.:mad: Otherwise I would be happy to check mpg of 87 vs. 91. Currently, I just have her use mid grade (89).
 

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