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..

[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.

[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