[Qfrank]

Happy new year to all.



Ok, I just drove back to Tampam from Melbourne Beach this morning, and stopped at a Chevron to fill up the family SuV, and i saw a pump with racing fuel, 100 octane. I inquired inside and the attendant said tons of airboat folks, motorcycle guys, drag racers etc load up on it.



I bought a 5 gallon can and came home with 5 gallons at 6.50/ga.



Now I have read some posts here and it seems I may have wSted some money. I have 93 octane readily available, Nd onlymuse chevron or shell, and I use Tex Ron pretty routinely in all my cars.



I have air box mod, K&N, stage 1 RSBarn chip, and RS barn cat back exhaust.



So now what. Do I just use 1-2 gallons mixed in with 93 to get rid of it. I drive it once a week or so, and autocross monthly.



What do you think?

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[Cloud9...68]

I'm not an expert on the topic, but I remember seeing some dyno runs on what I believe was a relatively stock 968 on standard pump gas (don't remember the octane - probably 91 or 93) compared to E85, which has an equivalent octane rating much higher, probably around 100. I was surprised to see that the engine produced several more horsepower on E85, so you may see a small boost from the 100 octane stuff. But with only 5 gallons, the thrill won't last too long, unfortunately.

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[tamathumper]

IIRC, an "octane rating" is not a measure of how much energy is available in a given fuel, but rather a measure of how resistant the fuel is to "knock"?

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[JWahlsten]

Yes, the octane rating for a given fuel is the resistance to pre-ignition. Your car will be able to run more advance with the higher octane fuel, but it only makes a difference if the DME will call for more aggressive timing.



Maybe Flash or Pete can weigh in with how our DME can adjust for a higher octane fuel. I'm guessing to get to the most benefit you'll have to program for it.

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[Cloud9...68]

Yes, that's what I thought as well, but based on my recollection of the results with the E85, there appears to be a bit of advance left "on the table" with the stock DME programming, which a higher octane fuel is able to exploit.

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[Qfrank]

Oh well. I will just add a gallon here and there until all gone, or give it to my neighbor who drag races.



Good to know these cars run so well on 93 octane. My Mercedes is the same way, hates everything but Chevron.

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[robadams56]

My recollection is that E85 is ethanol (up to 85%). That being the case, I also thought that ethanol was not as efficient as gasoline and therefore would not produce as much power. Maybe we have a petro engineer who could weigh in on this.



I go to MotorSport Ranch from time to time and have been tempted to put some VP racing fuel in the tank. I have been told however, that you shouldn't put a full tank in, but rather mix about half and half with V Power.

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[Cloud9...68]

Rob,



Yes, E85 is 85% ethanol, and while it's true that ethanol doesn't have nearly the total heat content as gasoline (thus cutting your mileage by over 35%), it does have a much higher inherent "octane rating," which, as Joel points out, reduces the fuel's propensity toward pre-ignition, and thus allows the use of more advance (in the case of an N/A engine), and significantly higher boost with forced induction. So, it's a great fuel for performance, not so great for mileage. But I cited my recollection of the dyno results with E85 as a possible piece of evidence that our engines may produce a bit more peak horsepower given a higher octane fuel. But, as is the case with my inability to remember how to put that piece of air ducting back in my car in another thread, my memory may be faulty here <img src="/forum/images/smilies/968/smile.png" class="smilie" alt="" /> .

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[robadams56]

Selective memory?

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[Rap]

I took a driving course at Pocono speedway this summer and couldn't resist putting 100 octane in. 9 gal $73. Can't say I noticed much difference. Had a ball driving car ran well in 96 degree heat but nothing noticeable other than $73 lighter!

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[Ceenit]

Running the straight higher octane certainly won't heart anything (except your wallet). You can average down by mixing in with some standard or midgrade octane fuel as well. Won't get your money back, but could make your psychy feel better. We run 100 octane on a dedicated track car, that is tuned to require it.

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[Langley968]

I always gas up at Chevron because here in Canada they have 94 octane with NO ethanol added to that fuel blend. The main reason I avoid the lower octane ratings is becuase they can have up to 15% ethanol.

This ethanol content can cause two problems for the 968:

1) it will break down gaskets and rubber components that are not specifically manufactured to handle ethanol (based on when our cars were manufactured, I think that can be a problem)

2) ethanol has a percentage of water content which can cause corrosion in a fuel system that was not manufactured to handle ethanol (because of the water content). I think here of the fuel tank, injector nozzles, etc.

I lived in Brazil from 1972 - 1982 (8 - 18 years old) when the Pro-Alcohol program was initiated (most vehicles run on a mixture of 85% ethanol / 15% gasoline). I also lived there from 1989 - 1999 as an adult, and remember being stranded because the fuel line filter in my vehicle became so corroded that it caused the vehicle to shut down (of course at midnight on a deserted stretch of highway!). All vehicles in Brazil are now "Flex" fuel, which means they can handle any mixture ratio of gasoline and ethanol.

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[Lear35A]

Some racing fuels might also contain lead to get that higher octane. Not so good for your cat.

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[craigawoodman]

Depending on how the fuel is made, the octane rating will drop dramatically over time. We used to have 100 Octane rated fuel over here for a while, but a fuel company employee told me that if you don't use the fuel within a couple of weeks, it loses it's octane rating as the components which raise the octane rating simply evaporate off. As per above, it does not really make that much difference in a N/A engine, but in a turbo engine with high boost, it sure does.

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[flash]

turbos are indeed a very different animal when it comes to octane, as knock is a much bigger factor there



on a normally aspirated engine, if you have the ability to alter timing, the old school rule of thumb is that 1 point raised in octane will increase power one point. so, in theory, a couple of hp are available.



the stock ECU is tuned for 93 octane (measured as RON+MON / 2 not RON). you will achieve maximum power with that. anything more is a waste of money, and can actually increase your combustion temps.



on the stock chip, and many other chips that do not alter timing much the gains aren't there. for example, on the racer x, which adds a boat load of timing, you'll get some. i ran some dyno tests back in the chip shootout and saw a couple of hp with that chip going from 93 to 95. chips like the rs barn chip which primarily alters fuel mix and doesn't really mess with timing, you won't see gains by raising the octane.

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[ds968]

Once or twice a year, just for the heck of it, I add 5 gallons of 100 to the other 14 ( 15 ? ) gallons of 91 on a fill up. I don,t feel any difference whatsoever in performance ( well, duh.. who would feel only a couple of extra hp even if they were there.. ) , but one thing that is distinctly different is the sound at full throttle. And it's a major difference, at least to my ears. I can only describe it as considerably louder and "angrier" .. Not sure why this is, but it's consistently there. Incidentally, I have a Racer X chip and a BB catback, in case either of, or the combination of the two with the higher octane cause the sound to change.

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[PorscheG96]

Dan, that could be ignition timing you're hearing. I've never noticed any difference in feel between 100 and 91 octane fuels but I still mix with 100 for track days to help resist pre-ignition from extra heat.

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[flash]

i noticed a bit of a difference, but it was more throttle response than anything else, and it could easily have been the timing of the racer x chip no longer being pulled back, due to the increase in octane.

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[ds968]

timing makes a difference in sound ?!

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[flash]

it can. timing is what determines the incident of fuel burn. altering the timing changes when and how long it burns. think of it in terms of opening a door in the wind. if you do it slowly, it sounds one way. if you do it quickly, it sounds another.

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