93 or 100 octane

[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 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

[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