93 or 100 octane

[steven choi]

I just found a place in woodside, ca, a 76 gas station that sells sunoco race fuel 100 octane. Filled up my 2004 boxster S. And definitely feel the differences in performance accoding to my butt.
It was $4.39/gallon... a dollar more than 91 octane.

my question is since porsche recommends 93 or higher, will 100 octane gas deliver more performance than 93, or not.
My plan is, each fillup, to put four gallons of the race fuel (100 octane) and 12 gallons of the premium gas(91 octane). That comes to around 93 octane combined.

thanks for any input.

Hey Steven, don't they have 93 octane where you live? You really felt a difference with the 100 octane stuff? If so, pretty cool!

[Brucelee]

No, there is no power difference in a gas rated at 93 vs 100.

Sorry, your butt is lying to you!

Save you rmoney for Mobil 1 in your next oil change.

Good luck!

[Rail26]

91 octane is only available in El Paso (Chevron). Should I be boosting the octane with the stuff off the shelf?

[steven choi]

Hi Bruce lee..

you misunderstood.
I've meant there is definite difference between my usual 91 octane versus 100 octane. 93 octane is not available where I live. That is why i am going to mix 91 and 100 from now on.

Thanks for the replies everyone..

[steven choi]

oh and bmusatti,

The difference between 91 and 100 is very much noticeable specially at 70mph or above in top gear..

Pulls much more easily..

[Porschekid]

Our Texacos offer 93 and 94, I normally fill up with 94 but i'm not sure if it helps with performance any. Hell an extra 10 cents a gallon these days is nothing anyway.

[MNBoxster]

@Steven Choi,



Sorry to tell you but, Octane rating and Energy Content (Calorific Value) are two totally different things.

The Gas you're buying is a waste of money as any percieved power increase is merely a Placebo Effect. Also, mixing various Octane rated fuels rarely works because these have different densities and don't easily mix. Park your car for just an hour and the various fuels separate.

The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites, it has nothing to do with how much energy it contains.

When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can severly damage an engine because the pressure built-up from this explosion creates a shock wave that can actually shatter internal components. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.

The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel or it will knock. The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight -- that is what makes the engine "high performance." Using a higher rated gasoline than your engine calls for is wasted because the engine never reaches a compression which might trigger knock. You should use the lowest Octane fuel which your engine can smoothly run on without knocking.

Gasoline is what is called a Aliphatic Hydrocarbon meaning that it is composed only of Carbon and Hydrogen molecules arranged in chains of varying lengths. These different lengths display different properties when burned.

The primary components of Gasoline are: Heptane and Octane. These are molecules of Carbon and Hydrogen and get their names from the number of Carbon Atoms chained together in the molecules (7 for Heptane and 8 for Octane).

Heptane handles compression very poorly. Compress it just a little and it ignites spontaneously. Octane, on the other hand, handles compression very well -- you can compress it a lot and nothing happens. Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13-percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane). It spontaneously ignites at a given compression level, and can only be used in engines that do not exceed that compression ratio.

In Simple terms, the octane rating of a fuel is determined by running it through a very special Test Engine costing in excess of $300,000. This Test Engine is a single cylinder engine in which the cylinder volume can be varied to create different pressures (variable compression). Two fuel sources drive this engine. One is pure Heptane, while the other is pure Octane (or an Octane mixed with other Hydrocarbons). These fuels are simultaneously introduced at varying combinations until knock is induced in the engine. Then, the ratio of Heptane to Octane is recorded and this is the Octane rating.

So, the Octane Rating is the % of Octane contained in the fuel. For example: Octane makes up 87% of the fuel in 87 Octane gas while Heptane makes up approx 13% and so on.

To prevent fouling, lower emissions and types of emissions and to promote smooth burning, other HydroCarbons are also used such as an additive named MTBE.

MTBE is short for methyl tertiary butyl ether, a fairly simple molecule that is created from methanol. MTBE gets added to gasoline for two reasons:
First, it boosts the octane rating, Second, it is an oxygenate, meaning that it adds oxygen to the reaction when it burns. Ideally, an oxygenate also reduces the amount of unburned hydrocarbons and carbon monoxide in the exhaust. Gasoline can contain as much as 10 percent to 15 percent MTBE, substituting it for Iso-Octane.

Now the bad news, because most Gasolines use Hydrocarbon Molecules with roughly the same Carbon to Hydrogen ratios, the Calorific Values (Energy Content) are pretty much the same. Some additives, such as Toluene can increase power, but these are restricted by Law as to the amount the fuel can contain because of their toxicity or polluting qualities.

Also, using very high Octane rated fuels in a modern engine can confuse the ECU which is set for specific parameters. Thus, if it reads a Sulpher, Oxygen or CO² content in it's sampling which is outside it's parameters, it can actually reduce HP or Torque by limiting fuel, advancing/retarding the timing or intake air.

All you're really doing is probably polluting the air more and spending more money. The best thing you can do is stick to the Octane Ratings suggested by the Manufacturer and buy the fuel from the busiest Station you can to insure both it's freshness (yes - gasoline ages and Octane can drop as much as 2-3 points) and it's purity (because busier Stations empty their tanks on a more regular basis reducing the amount of corrosion, contaminates and moisture the gasoline absorbes). Hope this helps...

Happy Motoring!...Jim'99

[Brucelee]

The above post is what I was getting at with my earlier comment.

Nice job on the research!

[airbox1]

nice job mnboxster. however, don't be discourage in using higher octane gasoline, steven choi. since boxster engines are high compression engines, they benefit from having higher octane gasoline. yes, it's true that higher octane gasoline doesn't directly "give" your car more horsepower, but it definitely indirectly effects how much horsepower your car is getting. in other words, with a higher octane gas, your chances of premature detonation decreases. therefore, with a performance enhancing chip, like the revo or giac, you can delay the timing beyond its normal parameters, thus, giving the engine more horsepower. again, please keep in mind that it only works if you have the chip and higher octane gas working in conjunction.

[larez2]

Wouldn't an engine that knocks be less powerful than an engine that runs smooth without any knocking?

We can't get 93 in most places in California. I've never seen it at any pumps in San Diego. I have to put 91 in, and i think my engine does knock a bit as a result.

Not sure about MN and other places, but in California, the highest octane gas I seem to be able to get is 91, not 93 like the Porsche recommends.

Anyone ever use any of those octane booster additives?

Larez2

[steven choi]

Thanks MNBOXSTER.
That must been a real effort to type.
Yeah..You are probably right. however in california we can't get anything higher than 91. I just don't like to put anything less than what Porsche recommends.

[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

[RandallNeighbour]

I've always been fearful of premature detonation

[MNBoxster]

Originally Posted by airbox1

nice job mnboxster. however, don't be discourage in using higher octane gasoline, steven choi. since boxster engines are high compression engines, they benefit from having higher octane gasoline. yes, it's true that higher octane gasoline doesn't directly "give" your car more horsepower, but it definitely indirectly effects how much horsepower your car is getting. in other words, with a higher octane gas, your chances of premature detonation decreases. therefore, with a performance enhancing chip, like the revo or giac, you can delay the timing beyond its normal parameters, thus, giving the engine more horsepower. again, please keep in mind that it only works if you have the chip and higher octane gas working in conjunction.

Hi,

Thank you for your kind words. I think there are a couple problems with your arguements.

First, the Boxster is a higher compression engine, but it benefits from Gas which has only so much of an Octane rating. Any more than this is not needed and you derive absolutely no benefit from it, although it usually costs you more.

Second, your car only gets it's power from releasing the Heat Potential of the Gas, that is, how much energy is released during combustion, and how efficient the engine and drivetrain is at using that energy. The most efficient IC engines on the market today only contribute about 25% of the energy released to actually turning the rear wheels. The rest is lost to mechanical inefficiencies and heat. There are no chances of detonation as you suggest, either the detonation threshold has been reached, or it has not. Some things which can affect reaching this threshold are:

Ambient Temperature - Air (any Gas really) is heated when it is compressed. Roughly, the increase is 1°F for every 1 PSI of pressure. If you start with 70°F Ambient Air and compress it 100PSI, it's temp will increase to 170°F, but if you start with 100°F Ambient Air and compress it 100PSI, the resulting temp will be 200°F and so on. On very hot days, this can exceed the combustion threshold of the A/F mixture. Note, to simplify I used the example of 100PSI compression. In reality, most high compression Engines achieve cylinder pressures of 150PSI-190PSI, so you can see it can make a big difference.

Cooling System Efficiency The more efficient your cooling system, the less leftover heat remains in the cylinder to heat up the incoming charge before the Piston compresses it. Similar to the example above.

Spark Plug Heat Range The Heat Rating of your Spark Plugs has NOTHING to do with how Hot a Plugs burns. It is a measure of how much heat the Plugs extract from the cylinder and pass through the head to be carried off by the coolant between cycles. The difference in one rating of the same type plugs (for example a 6 to a 7, or a 6 to a 5) is equal to about 70°C-100°C (158°F-212°F) of heat it can remove from the system. Again, the less heat retained in the cylinder, the lower the intake charge will be when compressed.

Most ECU's will compensate 1 or 2 points in Octane rating, but performance may decrease slightly (usually unnoticable unless at Top End or under extreme load) as the ECU switches to a less efficient MAP.

If you cannot find 93 Octane fuel in your area, there are a couple things you can do:

First, switch to a lower Heat Rated Plug (the Boxster is a '7' rated Plug, try switching to a '6')

Next, make sure your cooling system is in top order - no debris clogging the radiators and if in a temperate climate such as California, run a coolant mix of 60% water/40% Anti-freeze (water has greater Thermal Conductivity than Glycol), you'll still have plenty of anti-corrosion and lubrication protection, but you'll be removing more heat from the engine.

Finally, there are octane boosters. But, be careful with these OTC types. They will raise the Octane a few 0.1 points and they're usually sold in 12oz. containers for maybe $2.99 (that's equal to almost $32/gal. !!). The primary ingredient of these is Toluene (Octane Rating 117), mixed with some Dye and Light Oil.

You can buy Toluene from any Sherwin-Williams Paint store for approx. $4/gal. and make you own. Mixed 10% with 91 Octane Fuel will raise the Octane Rating to 93.4 Octane. Care must be taken when storing and handling Toluene as it is extremely flammable and somewhat toxic to breathe, but for little more than a gal. of fuel (which you'll be replacing with the Toluene) you can achieve the Octane Rating recommended. Adding amounts greater than 10% are not recommended because of excess wear and tear to the Fuel System, but 10% is just fine.

Here is the formula for making your own Octane Booster:

This is the basic formula to make your own octane booster. (128 oz = 1 gal):

100 oz of toulene for octane boost
25 oz of mineral spirits (cleaning agent)
3 oz of transmission fluid (lubricating agent)

Diesel fuel or kerosene can be substituted for mineral spirits and light turbine oil can be substituted for transmission fluid. Note, the final 2 ingrediants are also added to ordinary pump gas as well, so don't be shocked.

Hope this helps...

Happy Motoring!...Jim'99

[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