93 or 100 octane

[MNBoxster]

Originally Posted by xavier405

Excellent post, and thank you for your detail

Some questions:

- You mentioned that MTBE additive increases the octane rating, but also that octane rating is purely the ratio or percentace of octane in the 100% mixture? Are you saying that MTBE PLUS the octane percentage add up within the 100%?

- I read somewhere that higher octane also leads to slightly higher fuel mileage. From the previous essay, we can rule out higher energy content, so there may be something about the efficiency of combustion? Anybody have comments on that?

- I guess when I see a "110 Octane Racing Fuel" that it is a ploy, since apparently octane rating peaks at 100. Is that right?

Thanks very much
- X

Hi,

Thank you for your kind words.

To get to your questions, the intricacies of the subject matter are VERY complicated, so I'll simplify as best as I can and still pass on the basics. This post will still, by necessity, be somewhat lengthy, but here goes...

Gasoline is a very simple substance in it's components which are mainly Carbon and Hydrogen , but very complex in the way these two elements are first combined into over 500 different molecules, and then mixed in various quantities to form the end product - Gasoline.

That sounds a little confusing, but if you think about it, Gasoline must have many qualities at once, to meet with the dynamics of the environment and driving conditions. Just a short list includes it's ability to flow, it's ability to atomize, it's volatility (through a range of different altitudes and barometric pressures), it's ability to remain a liquid at Ambient Temperatures (or it would all evaporate from your tank, or not flow through the fuel rail into your injectors- vapor locking), and it must not readily freeze. Add to that it must be clean burning both in terms of efficiency in the Engine (so it doesn't muck-up the engine) but also with respect to post-combustion products (environmental pollution), non-corrosive, and maybe most important, it must readily burn and contain a lot of energy/vol. And, this is just a partial list of all the qualities it must possess.

Also, a gal. of gasoline weighs approximately 6.25lbs. at STP - Standard Temperature and Pressure (72°F and 1 ATM of pressure). But it is mostly made up of Carbon. In fact about 5lbs. of Carbon per Gal. ! Since elements do not readily decay, this means that every time you burn a gallon of gasoline, you are really throwing the equivalent of a 5lb. bag of Charcoal into the atmosphere as most of the carbon is released as CO²! Maybe those GreenHouse Gas people are on to something.


Gasoline comes from petroleum (Crude Oil), and contains more than 500 different Hydrocarbons. There are basically 3 categories of Hydrocarbons in Gasoline. They are:

Saturated Hydrocarbons ( aka paraffins, alkanes )

- stable, the major component of leaded gasolines.
- tend to burn in air with a clean flame.
- octane ratings depend on branching and number of carbon atoms.

Unsaturated Hydrocarbons

Their Characteristics include:
- Unstable, are the remaining component of gasoline.
- Tend to burn in air with a smoky flame.

Oxygenates

Oxygenates are just preused hydrocarbons :-). They contain oxygen, which can
not provide energy, but their structure provides a reasonable antiknock
value, thus they are good substitutes for aromatics, and they may also reduce
the smog-forming tendencies of the exhaust gases . Most oxygenates used
in gasolines are either alcohols ( Cx-O-H ) or ethers (Cx-O-Cy), and contain
1 to 6 carbon atoms. Alcohols have been used in gasolines since the 1930s. The relative advantages of aromatics and oxygenates are environmental-friendliness and low toxicity octane-enhancers. MTBE is one of these.

To get the right mix of qualities, one of which is the Octane Rating for higher compression Engines, the Octane component(s) are often a mix of different molecules from all 3 categories above. Remember not to confuse Octane Rating with Octane a Hydrocarbon molecule, one of which is 2,2,4-trimethylpentane. Once this Mix is determined, it is this mix which is combined with the Heptane to achieve a certain Octane Rating.

It is possible to have an octane rating greater than 100, but the mix making up the gasoline will never (cannot) exceed 100%.

Take Toluene for instance. It has an Octane Rating of 117. Twenty years ago, when Formula 1 Race Engines were forced (Turbocharged), the rules limited the Fuel to 105 Octane. The primary component of the Fuel was Toluene (Octane 117), but it was mixed with Heptane to lower the Octane Rating to the required 105. This mix allowed very high Boost Pressures to be run, producing lots of HP.

Could you run pure Toluene in your engine? The short answer is Yes (with some modifications), but you'd get lousy mileage, pollute like crazy and have a very inconsistent running and starting Engine depending upon environmental conditions (Temp, Humidity, Barometric Pressure), and your maintenance schedule(s) would be measured in the 100's of miles instead of the 10k+ miles. All-in-all, not a good all-around fuel.

With respect to your 2nd question about higher Octane fuels making more power, I think you may have misread whatever it was. With a higher Octane Fuel, you could increase the Engine's Compression Ratio allowing it to produce more power from the same displacement, but it is the changes to the Engine, not the Fuel which makes more power. You'd be feeding More Fuel to the Engine, hence more power (This is why the Corvette gets worse Gas mileage than say a Civic - it burns more Gas/mile) . And, combustion efficiency isn't primarily a factor of the Fuel at all in this case, but rather Engine Design.

As far as you last question, I think I've already answered this, but again, you can have Fuel with Octane Ratings over 100, but they don't usually contain any more energy and are purpose-designed Fuels, such as for Racing or Aviation and pretty much fail as all-around Fuels for a Street Car application. Hope this helps...

Happy Motoring!...Jim'99

[lexuspilot]

If your engine was retarding it's timing because of insufficient octane(knocking), then adding a higher octane would increase power.

The question might be "Where you really buying the 91 octane you thought you were buying?"

Sorry, but using the butt dyno means using butt logic not labratory logic. The very long post above lays the ground work to assume that you have been buying crappy 91 only to then try excellent 100. My money would say there is a power difference in that case.

[airbox1]

i totally agree with lexus. make sure you are buying gas from either chevron or unical 76.

[MNBoxster]

Originally Posted by lexuspilot

If your engine was retarding it's timing because of insufficient octane(knocking), then adding a higher octane would increase power.

The question might be "Where you really buying the 91 octane you thought you were buying?"

Sorry, but using the butt dyno means using butt logic not labratory logic. The very long post above lays the ground work to assume that you have been buying crappy 91 only to then try excellent 100. My money would say there is a power difference in that case.

Hi,

Not to Flame you, but you seem to have totally missed the point, which is: There is no more power in Gasoline of a higher octane, and that switching to a higher octane will not give your engine more power.

The energy contained in all gasoline is basically fixed. I say basically because there are some Octane Boosters, such as Methanol, which actually contain less energy than an equal volume of lower Octane Gasoline, but they have a higher Octane Rating. So, the case can be made that there is more power (energy) in Regular Gas (87 Octane) than there is in some higher Octane Gasolines, but never vice-versa.

Your arguement that switching from contaminated gasoline to gas which is not contaminated will give your engine more power is pure Pretzel Logic or, as in your words, "Butt Logic". Your statement forces me to believe that you weren't actually serious, but only stated it Tongue in Cheek.

Obviously, if you are buying contaminated 91 Octane Gas, which causes your Engine Management to retard the timing, the car will underperform. But, that is not even relevant to the discussion. What if you had faulty Ignition Wires? Do new Ignition wires contain more Power? That's essentially what you're saying.

Happy Motoring!...Jim'99

[larez2]

I don't think lexuspilot missed your point - he never said there was any more power in the higher octane gasoline. I thouhgt his points were totally relevent - most gas station in CA only offer up to 91 octane gas. If you use gas with a lower octane than recommended - 91 in our case, where 93 is recommended and your car is not performing optimally on the 91 octane, then changing to 93 octane from 91 octane will make a difference relative to what is was before - the difference manifests itself with the feeling of restored/more power. And, I do not mean it magically makes engine higher compression. True, replacing faulty ignition wires will not make your engine "more powerful", but what it will do is restore power and smoothness relative to what is was before (with faulty ignition wires). In either case you feel a difference.

The answer to the original question from Steven Choi is, 93 or 100 octane makes no difference because our cars are only rated for use with 93 octane. Also, mixing them doesn't help or work/not recommended. Most gas stations in California only offer up to 91 octane - the way to increase the octane to the recommened 93 is with octane boosters or with Toluene.

Larez2

[lexuspilot]

You got it....I didn't miss anything. If the maximum power you can get from similar fuel is X. Then with contaminated fuel you only produce, let's say .98x. If you get uncontaminated fuel and produce the maximum power of x then you would be producing MORE power. Not more power than that "maximum" but more than before. Plus, I never said contaminated..I said it may not have actually been "91". In this case the "91" not having the octane for the engine to "tune" to the power.

This is it in a nutshell(can't help but to think of Austin Powers), you may have realized your engines true performance on the 100 octane fuel because the previous fuel did not allow your car to extract the full amount of energy contained in the fuel.


MNBoxster I will turn to you for the relevant question for your noodle. What is the tolerance on octane ratings? I would guess less than 2?

It is like drinking florida beer..sep different.
Cheers

[steven choi]

Thanks a bunch Larez2.
Finally someone really understood what my question was..

Thanks.

[MNBoxster]

Originally Posted by lexuspilot

...

MNBoxster I will turn to you for the relevant question for your noodle. What is the tolerance on octane ratings? I would guess less than 2?

It is like drinking florida beer..sep different.
Cheers

Hi,

OK, now that you say it another way, I guess we're not in such disagreement afterall.

So far as tolerance on Octane Ratings, I presume you mean the ECU's (or DME's in Porsche parlence) ability to adjust to differing Octane Rated Gas.

If so, there isn't truly one answer. This is because the Combustion Threshold (when Knock occurs) isn't truly static. It is dependent on several factors such as Ambient Temperature, Barometric Pressure and Humidity, cooling system efficiency and so on. Also, if you think about it, an engine knocks usually only under Load or at high RPMs, so you may be fine on a trip to the local convenience store, but climbing a hill or shooting down a Freeway ramp, or on a very hot day, knock may appear.

And, not all gasolines of a certain Octane Rating are the same. This is because any number of Octane Boosters may be mixed with it (such as Toluene, MBTE, or Methanol, etc.). Because of all these things, I suspect most ECU's are capable of compensating for an Octane discrepency of say ± 2-3 points. Hope this helps...

Happy Motoring!...Jim'99