I have never seen 100 octane for sale anywhere in where I live. Are you mixing additives or is there a place where you can buy 100 octane gas. I live in St.Louis and have seen 110 on a race pump with a lock on it at the one station. Do you need to be a "registered race car driver"?

I would love to run higher than 93 but can not find it anywhere. <img src="confused.gif" border="0">

 

[quote]Originally posted by GTR 993:
<strong>I would love to run higher than 93 but can not find it anywhere. </strong><hr></blockquote>

Rather than tell you to "search the archives" (sorry, just my opinions on another subject/topic being discussed right now), here's one answer previously provided on this question. Go to <a href="http://www.76.com/" target="_blank">Union 76</a> and do a Store Locator/Search and click on the race fuel. It doesn't appear to differentiate between 100 octane or above, but at least you can locate stores that sell race fuel in your area, if they exist?

Cheers,
Dylan. <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

 

The question really is can I buy race fuel just like any other fuel. Like I posted above there is a station with 110 but they have a lock on it outside???? For what reason? Do I need a special permit of some sort to buy it? Do I even want 110 Octane or is that too high? <img src="graemlins/yltype.gif" border="0" alt="[typing]" /> <img src="confused.gif" border="0">

 

GTR:

They lock it so some clown doesn't pump $5/gallon gas and tell them they didn't notice the octane.

You don't need a permit (other than from the wife...). By the way, 110 and above is usually leaded. Look for the 100 stuff and mix it with 91 (or 93 in your area).

 

I'm not sure about the 76 stations, but ERC gas is for "off-road use only" and you can not simply pull up and fill up your tank. I buy it in a 16 gallon drum than I use to mix with 91 octane prior to an autocross. ERC RUL is the highest octane unleaded I've seen (104.5).

At Sears Point you can buy 96 or 100 octabe at the pump just like regular gas (except for the price!).

 

GTR,
Unfortunately it seems there are only certain select States that sell race gas and Union 76 does not have stations either in my area or yours, here is another alternative (albeit very expensive) I found on the net:

<a href="http://www.goemerchant7.com/index.cgi?PageToView=catalog&Department=81048&Cartid=&Merchant=highfuels n&ExpandedDepts=" target="_blank">race gas to go</a>

I don't think that you need permit for this, but beware of LEADED race gas. In Houston I can only find Unleaded race gas in one place and at the race track. Exxon makes it and it comes in two varieties max at 104 octane.
On my last DE event I filed with 104 and the engine run strong all day without running any high temps dispite the fact that the event was called "Hotter than Hell" as it was held in Houston in August... A strange after effect is that I had less fuel consumption but at the price of race gas... Great stuff but it hurts the wallet big time!

 

I'm lucky enough to have a Unocal near my house that sells 100 octane unleaded for $4.19/gal. I regularly blend a few gallons with our 91 octane California "premium" to achieve a 93-95 octane mix.

I know people who have run 100LL unleaded aviation fuel that can be purchased at the local airport for about $2.00 a gallon but supposedly its chemical composition is not optimal for cars.

Sears Point and Laguna Seca both sell a variety of high octane leaded and unleaded fuels ranging from 96 to 118+ octane. Most racetracks have similar fuel services.

Whatever you decide, DON'T put leaded fuel in your car! It will ruin the catalytic converters I'm told.

MC

 

" Steve,

I have a “map” question for you. Is it possible for the ECU to detect a condition, say outside air
temp or something innocuous like that, and retard timing by a certain amount regardless of any
knocking? This would be the prudent thing for the ECU to do if you had 93 octane gas, but if you
had 100 octane gas, you wouldn’t be getting the full programmed advance.
Tom
’95 993[/QB][/QUOTE]

Hi Tom:

Great question!

Yessir, under the right conditions of high outside air temperatures, high cylinder head temperatures and high oil temps, you might not always get the full amount of preprogrammed ignition advance IF,....IF the ECU detects any abnormal knock sensor activity.

In this case, the ECU will reduce timing in the offending cylinders until it ceases, for 720 deg of crank rotation and then start to add that timing back in. If you watch the ECU's knock sensors in real time with a scan tool, you can see this happen,....

This is why its so important to use really good gas under such conditions usually found on the track for peak performance.

 

[quote]Originally posted by Tom W:
<strong>Matt, why do you think octane is a combustion inhibitor? Octane is a measure of knock inhibition. A higher octane means the gas is less likely to ignite without the spark (pre-ignition) and cause knock. This does not mean the gas will not burn. (But, as you suggest, it may burn slower.)</strong><hr></blockquote>

Hi Tom,

You actually answer your own question above: "higher octane means the gas is less likely to ignite without the spark (pre-ignition) and cause knock."

Pre-ignition occurs when the heat generated by the compression of the air/fuel mixture is by itself enough to cause the fuel to combust -- i.e., a spark is not necessary. (Remember, when a gas is compressed, heat is generated.)

When this happens before the spark, it's called pre-ignition, and causes "knock" (as you state above), wherein the force of the early combustion jars the piston against the connecting rod before it's changed direction in the cylinder, resulting in the "knock" sound.

As you indirectly point out, the higher octane gas has more tolerance to heat. In essence, the higher octane is "inhibiting" the combustion at a lower temperature than that created by the spark.

<img src="graemlins/drink.gif" border="0" alt="[cherrsagai]" /> &lt;-- drinking a glass of 100 octane

-Chris

 

[quote]Originally posted by H20NOO:
<strong>Steve,

Thank you for your well-reasoned (and much expected) reply! However....... I disagree with your statement regarding using "very best octane fuel you can afford".

Since octane is a combustion inhibitor, it's my understanding there comes a point where too much octane will actually hurt performance. For example, 100 octane would likely allow my 993 fuel and ignition mapping to run at, or near optimal performance levels. But, 118 octane would actually slow flame travel enough to result in some raw fuel finding its way past the exhaust valves. One cure for this would be to advance ignition timing further to take advantage of the higher octane fuel but I doubt the Motronic has an infinite ignition advance map, nor does my 993 have compression levels high enough to take advantage of 118 octane.

My question to you, the true GURU of 993 ignition/fuel mapping is, at what approximate octane level are all engine systems optimized for peak performance? My laymans guess is that it's not much higher than 94 or 95 octane.

I guess my point is that an unmodified 993 would not likely benefit from fuel over 100 octane. In fact, the opposite might be true.

Thanks as always!

MC</strong><hr></blockquote>

MC:

LOL,.....There is a LOT more to the chemistry of race gas than there is space to discuss this here. For sure, Unocal's website has some excellent information as well as many other books on this subject and are enlightening and educational.

Racing fuels, unleaded or otherwise, are not "combustion inhibitors", they are formulated to burn at an extremely controlled rate when cylinder pressures and temperatures are beyond what is normally found in street engines and under high-load conditions.

Each and every engine's design, configuration and operating conditions require fuels of differing octane requirements and using a fuel that is higher than you need is certainly a waste.

My point was that in most cases, 91-93 octane pump fuels are insufficent to get peak performance from the 993 engine in stock form, much less with any modifications, especially on the track. My deepest apologies if I failed to make myself very clear on that,....

For a stock naturally aspirated 993, anything over 104 octane race gas is a waste unless its been heavily modified and yes, 118 would not be a good idea for many reasons,... Porsche A.G., mapping these cars for a world-wide common denominator, has specified 98 RON (93-94 R+M/2) for best performance. They will run on less octane but with substantially reduced output.

I would further tell that you should not generalize about all high-octane racing fuels burning slower or hotter. They are all very different and each manufacturer's fuels even of the same octane cannot be fairly compared. The blend of RON & MON compounds vary and all you usually see is the average (R+M/2). N/A and turbocharged engines require different ratios of RON & MON fuels even though the posted average might be identical. In short, you should try several 100-104 octane fuels (Unocal, VP, ERC, Sunoco, etc) to see which ones work best in your car.

One other thing to address some other conversations here; EPA-legal race gas is normally 100 octane. Other unleaded fuels can be as much as 104, but these do not meet EPA requirements and are more difficult to find.

Most fuels over 104 octane are leaded and should never be used in any car with a catalytic converter and OEM-type oxysensors otherwise, you will soon spend some big money.

Boy, I hope this clears up some confusion.

 

Thanks Steve, you are a scholar and a gentleman!

<img src="graemlins/bigok.gif" border="0" alt="[thumbsup]" />

Behold the enlightened (that would be me...)

MC

 

[quote]Originally posted by H20NOO:
<strong>Thanks Steve, you are a scholar and a gentleman!

<img src="graemlins/bigok.gif" border="0" alt="[thumbsup]" />

Behold the enlightened (that would be me...)

MC</strong><hr></blockquote>

ROFL,...We are in very good company; there are so few of us left,....

 

Well, now that the holidays are over, I finally found the time to head down to the 76 station off Almaden in San Jose that has 100 octane, to see for myself if it makes a significant difference over the anemic 91 octane that I am usually forced to fill 'er up with...

30 miles later, I can report that it DEFINITELY feels smoother and more urgent under hard acceleration, and the bit of "grumble" I've been used to when starting from a standstill in 1st is much diminished; I think the knock sensors aren't getting much action anymore.

So here's the real question: Tom W was going to do some tests with a G-Tech to find out what the measurable difference was with the higher octane vs. 91 crap... Since I only have a "butt dyno" to work with, do you have any numbers yet Tom? <img src="graemlins/wave.gif" border="0" alt="[byebye]" />

Curious!

-Knute

 

I'm working hard on the project (thanks to some nice weather)

I've made 10+ runs in horsepower mode and got reasonable reproducible results. Made 10 or so in 0-60 mode today with more variability than I'd hoped.

Tomorrow I hope to have the time to add some high octane gas to the tank and make the next round of measurements. I'll file a complete report once the process is completed.

 

How many gallons of 100 octane are you guys adding to arrive at your 93 octane? I presume it's a "linear" mix. For example:

(5 gal/20gal tank)x100 = 25
(15 gal/20gal)x91 = 68.25

25 + 68.25 = 93.25 octane overall

My nearest Union 76 w/"racing fuel" is about 20 miles away, in Newport Beach. Think I'll make the trek this weekend to get a feel for the difference.