Premium Gas vs. Lower Grades

[lovcom]

Additive packages and octane aside, what are the differences between premium, mid grade, and regular gasoline for a given brand (Chevron, Shell, Mobil, etc)?

I was under the impression that the base gas was the same across all grades, but that the additive packages and octane levels are different.

Anyone know that can tell me what the truth is on this?

Thanks!

 

[Auto-Union]

Premium contains more and different additives and likely less water. It does not contain more thermal energy though. There are plenty of good threads on this subject.

 

[mechtech2]

OF any grade of a Mfr., premium will have the most good additives.
Some use the same amount in all of their grades, however. Chevron used to.
Of course, higher octane fuel is more resistant to spark knock. Some cars need it.
Not much to it!

 

[lovcom]

Ok, but is the base gasoline the same? I realize premium will have more octane and perhaps a better additive package, but I really want to know about the base gasoline.

Anyone?

 

[Tones]

Thats what I have also heard is that the tankers for the different grades all fill up from the same spigott. Then they pour in a little extra additive in the premium gas. Thus, there is no difference in the gasoline other than the additive.

 

[mfisher1967]

There is a difference between the grades. I work at one of the large oil refineries located along the Houston Ship Channel. We make two grades of gasoline, 87 and 93 octane. Typically our gasoline is ~87.5 and ~93.5 octane, as it is better to be slightly above target than to be too low. Detergent additives are added at the rack at the time the tanker trucks are loaded. 87 and 93 is mixed 50/50 to fill the 89 octane mid-grade tank at the gas station.

To answer your question, the gasoline blending process is different for 87 and 93 octane. I have been told that 87 octane grade contains more light hydrocarbons (short chain) than 93 octane (long chain). Short chain hydrocarbons are less resistant to detonation, which can be an issue in cars that require higher octane and/or do not have knock sensors. There is a lot of information about this on the net, so I will not try to post a thesis. I hope this helps.

 

[mechtech2]

I know that some stations Mfrs. simply use additives to make different octanes available for sale.
Shell may use a different base for it's V Power premium, though.
It tough to get a straight answer on the internet!

 

[Shannow]

mechtech2, I don't know of very many additives (except TEL, MMT, MTBE) that can be added in useful enough quantities to make a decent change in Octane rating.

Toluene needs to be added at large number of whole numbers of percentage points to make the differences between regular and premium...it would be an ingredient rather than am additive.

mfisher (1967 was a good year)...I think it's the other way 'round (in general) the shorter chains have a highr Octane rating than the longer. Shell make a high density premium which has higher energy density as well as octane.

 

[mechtech2]

Shannow - Right. I would tend to agree with you.
But I have done a bit of digging and found that octane is increased by additives, and also by base stock.
If my uncle was alive, I'd ask him [Chemist for Standard Oil]!

 

[blackdiamond]

Good information. This makes sense to me.

 

[mfisher1967]

The way that it was explained to me is as follows: Long chain hydrocarbon molecules are more more difficult to break apart. This improves their ability to resist detonation, which improves octane ratings. Short chain hydrocarbons break apart easily, yielding less stability and lower octane ratings. Anyway, I am quite possibly remembering this wrong, but I believe the above to be true.

As for the gasoline that we manufacture at the refinery where I work, octane is controlled at the plant and not the tanker loading rack. There is no magic "foo-foo" juice that is added to gasoline to improve the octane rating of gasoline as it is being loaded into tanker trucks for delivery that I know of. Octane "improvers" are added at the refinery during the gasoline blending process from what I have been told. Perhaps this is where the confusion comes from?

 

[Gary Allan]

No matter how it's done, there aren't enough refineries to service all of the (mostly former) name branding sales outlets. The gas in the pipeline is generic and (typically) any "identity" is added at the terminal ...which also may be an "all brand" transport service. I don't see too many Exxon or Sunoco trucks anymore.

 

[Shannow]

mfisher, it's sort of the other way round.

Long chain stuff like engine oil is less resisitant to detonation than the short stuff.

Methane (a single carbon atom) is far more resistant to detonation than normal heptane (7 carbon atom chain).

Octane, although having 8 carbon atoms isn't longer than N-heptane, as it's a compact molacule of 5 carbon atoms long, with three single atom branches off the side (2,2,4 trimethyl pentane).

The octane rating for any fuel is how it compares to a blend of x% iso-octane versus (1-x) of n-heptane. x is your octane rating.

Ratings over 100 are by extrapolation, or by charts which have stuff like TEL added.

 

[Dad2leia]

So, if I'm reading all of you correctly, premium isn't really all that much different or better unless the design of the engine warrants the extra "kick" so to speak of premium, and those of us that are experimenting are basically wasting time, effort, and green stuff...

 

[Big Jim]

There is no "extra kick".

Higher octane permits the engine to be tuned more aggressively. The only "extra kick" isn't from the gasoline, it is from the engine configuration.

 

[blueiedgod]

Most of the higher octane gasolines have more ethanol. We are mandated in NY state to have up 10% ethanol. Ethanol has higher octane rating than gasoline, but it contains less energy per cc (2 carbons in ethanols vs 6-8 carbons in gasoline).

Unless you have a high compression engine (10:1 and higher) you are wasting your money buying premium gasoline. Higher octane will not let a low compression engine produce more power, and if it does, you need a tune up.

 

[Win]

Originally Posted By: Shannow
mechtech2, I don't know of very many additives (except TEL, MMT, MTBE) that can be added in useful enough quantities to make a decent change in Octane rating.


Agreed, you can adulterate a higher grade fuel down to a lower grade, but you can't additize a lower grade motor fuel into a higher grade. The additive packages added at the terminal are enhancements to whatever the base fuel grade may be, and if the additive package is not sufficiently distinctive, markers may also be added that allow the majors to be able to sample a station's tanks to prove their gas is or is not being sold under their brand.

I sell generic unleaded and premium at my stations, but I won't use it in my cars - they get Shell V-Power only.