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Old 08-28-2018, 05:27 PM   #1
alschnertz
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Default Does octane affect gasoline density?

Just wondering if lower octane 87 E10 is more dense than higher 93 E10.

Couldn't find anything about density in my google search.
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Old 08-28-2018, 06:42 PM   #2
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I'm no expert but with the ability to make more power with the higher octane. Then there should be a bit more density per volume. A simple test would be having the same amount of each fuel in the same container. The higher octane fuel should weigh more.
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Old 08-28-2018, 07:57 PM   #3
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I would shoot an email off to Uncle Rob, he seems to know a bit about gasoline...
https://youtu.be/IuYiAhDanY8
I think gasoline even changes density with temperature, that's why you get more gas when you fill up in the morning when it's cooler.
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Old 08-28-2018, 09:20 PM   #4
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Originally Posted by VB242 View Post
I would shoot an email off to Uncle Rob, he seems to know a bit about gasoline...
https://youtu.be/IuYiAhDanY8
I think gasoline even changes density with temperature, that's why you get more gas when you fill up in the morning when it's cooler.
not really, the tanks are underground and are at steady temp.
https://www.consumerreports.org/cro/...n-it/index.htm
not worth the effort to even waste time thinking about it.

OP, the density of different brands of gasoline of the same octane likely vary as much as bewixt octanes, is there any practical reason you ask?
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Old 08-29-2018, 04:03 AM   #5
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Are you tuning for caburetors?

https://www.sunocoracefuels.com/tech...gravity-matter

"The specific gravity of pump gas will typically range from about 0.720 to 0.770. As you can now guess, this wide range is a reflection of the wide ranging composition. Pump gas composition varies by octane, by region, and by season."

Octane does affect gasoline density, but so does the weather and region.
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Old 08-29-2018, 05:25 AM   #6
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Originally Posted by apachechef View Post
is there any practical reason you ask?
What's driving this query is an issue with my '01 S40.
It has a slight pinging issue under a medium load for a second or two right after it shifts gears.
We've always used 87 octane E10 and I've noticed the issue happening more often lately.

Ran a tank of 93 octane E10 through it and the issue went away.

In an effort to run a higher octane than 87 and to try and save $ by not using 93 all the time, I tried making my own cocktail of 90 octane my running the tank down to half and then filling with 87, running that down to half, fill with 93, repeat etc.

So what I'm finding is that the car will ping after the "half fill" of 93 and won't ping after the "half fill" of 87. This led me to think the two different octanes are sitting on top of each other instead of mixing, but I'm having a hard time believing that myself.

I figured the two octanes would mix inside the tank on their own, but it seem like they stay separated.
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Old 08-29-2018, 08:55 AM   #7
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Quote:
Originally Posted by alschnertz View Post
What's driving this query is an issue with my '01 S40.
It has a slight pinging issue under a medium load for a second or two right after it shifts gears.
We've always used 87 octane E10 and I've noticed the issue happening more often lately.

Ran a tank of 93 octane E10 through it and the issue went away.

In an effort to run a higher octane than 87 and to try and save $ by not using 93 all the time, I tried making my own cocktail of 90 octane my running the tank down to half and then filling with 87, running that down to half, fill with 93, repeat etc.

So what I'm finding is that the car will ping after the "half fill" of 93 and won't ping after the "half fill" of 87. This led me to think the two different octanes are sitting on top of each other instead of mixing, but I'm having a hard time believing that myself.

I figured the two octanes would mix inside the tank on their own, but it seem like they stay separated.
They mix completely during the turbulence created by filling the tank.
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Old 08-29-2018, 09:43 AM   #8
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I don't think there's much of a difference between the gasoline part of the gas. The octane is modified through the addition of various other chemicals, partly the ethanol, partly through other additives.

Most of them say 'E10', but that just means that it can contain 'up to' 10% ethanol, in practice, they're using the amount of ethanol to fine tune the octane, since some of their past favorite octane boosting chemicals have ended up being banned (lead, MTBE).

Since the additives probably don't have the same density as pure gasoline does, they probably alter the overall density of the mixture. But not in any significant way. And if there was an additive that wouldn't go into a solution with the gasoline, it wouldn't be a very good additive, and it wouldn't be used (since it would settle or rise out of the mixture as it sat in the gas station tanks, as well as in the car's tank).

As for different types of gasoline refusing to mix in the tank - I'm pretty sure that's not going to happen.
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Old 08-29-2018, 09:54 AM   #9
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I'm thinking ambient temp has more effect on the density of gasoline.
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Old 08-29-2018, 09:56 AM   #10
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You can't just buy 89 at the pump?

Or is CT that backwards?
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Old 08-29-2018, 12:24 PM   #11
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The fuels will intermix. If the new fuel is cold (underground storage) and the fuel in the tank is ambient temperature there could be some initial separation due to thermal density difference. This would depend on much mixing / agitation occurs during the fill process. I would expect the separation to be minimal.

There is no guarantee that the octane of the resulting blend will land between the two octane numbers in proportion to the blends (i.e. half way between the two numbers with a 50/50 mix). Mix the two and depending on the chemistry you might end up with an octane rating that was closer to the bottom number.

Interesting fact. When leaded gas was still available, you could mix some brands of leaded regular and un leaded regular together and end up with a fuel blend that had an effective octane rating much higher than the individual leaded and unleaded regular. It was referred to as octane bump. It didn't work with all fuels because it was sensitive to what technique the formulator was using to get the octane rating in the unleaded fuel.

Also, consider the fact that you are never running on a 50:50 mix. If you start with a 1/2 tank of 50:50: mix and fill up with 93, you will have a tank of 25% 87 and 75% 93. Fill up with 87 and you have 25% 93 and 75% 87. Given the weird world of octane chemistry, the 25:75 mix might be better than the 75:25 mix.

If you want to go mixing fuels to get an octane blend you better bone up on your chemistry.

Last edited by 142 guy; 08-29-2018 at 12:43 PM..
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Old 08-29-2018, 12:34 PM   #12
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Originally Posted by Redwood Chair View Post
You can't just buy 89 at the pump?

Or is CT that backwards?
Pretty much every place I can remember getting gas at has three grades.
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Old 08-29-2018, 01:09 PM   #13
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OP it may be helpful to apply a decarbonizing cleaner to the engine on the off chance it is pinging due to an issue other then gasoline.
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Old 08-29-2018, 04:59 PM   #14
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Quote:
Originally Posted by Redwood Chair View Post
You can't just buy 89 at the pump? Or is CT that backwards?
Oh, you're quite correct in that CT is very backwards.
Yes, 89 E10 is available at most places. However, I usually tank up at BJ's or Costco since they run about $0.35 / gallon less. And they only carry 87 E10 and 93 E10.

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Originally Posted by 142 guy View Post
This would depend on much mixing / agitation occurs during the fill process...

There is no guarantee that the octane of the resulting blend will land between the two octane numbers in proportion to the blends (i.e. half way between the two numbers with a 50/50 mix). Mix the two and depending on the chemistry you might end up with an octane rating that was closer to the bottom number.

Also, consider the fact that you are never running on a 50:50 mix. If you start with a 1/2 tank of 50:50: mix and fill up with 93, you will have a tank of 25% 87 and 75% 93. Fill up with 87 and you have 25% 93 and 75% 87. Given the weird world of octane chemistry, the 25:75 mix might be better than the 75:25 mix.

If you want to go mixing fuels to get an octane blend you better bone up on your chemistry.
Good points.

I'm not sure there is as much mixing and agitation during filling as expected, but there has to be some.

I thought I read somewhere that it is a simple ratio to achieve the mid-grades at the station. In other words, the station just buys 87 and 93 and if you select 89, the pump will give you 60% 87 and 40% 93 or however it sums up.

Interesting comment about the octane chemistry.

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OP it may be helpful to apply a decarbonizing cleaner to the engine on the off chance it is pinging due to an issue other then gasoline.
That's completely possible. I do notice the pinging more on hot (above 85F) days than cooler. However, those are also the days when the windows are open and it's easier to hear.

Thanks to all for the input.
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Old 08-29-2018, 08:51 PM   #15
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In my area of Indiana I was told that the ethanol content may be lower for higher octane ratings, but, I was also told that the actual ethanol level could be higher or lower than 10%.
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Old 08-30-2018, 10:29 PM   #16
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i think you have to measure the specific gravity of the fuel in question yes fuels will blend, i need 100 l.l. with 93 for my car's
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