ckyguy

is it better to have higher octane or lower in an s4? i was thinking of starting to use higher octane fuel cause i thought it was better for racing, burns cooler n all, but i heard that since its a little different density or something-it gets too much into the engine and not enuf air? im confused.

J-Rat

iTrader: (2)

There are TONS of threads on this subject. Ultimately, you want to run the LOWEST octane you can, SAFELY. Lower octane burns faster, and (believe it or not) cleaner.

What does this mean? Low octane for N/As and High Octane for turbos.

Jarrett

gergrx7

do you have a n/a or turbo. if its a n/a then use the lowest you can find like 87. but if its a turbo use highest you can find like 93 octane. i use 87 in the n/a and 93 in the turbo 2.

Evil Aviator

The octane rating is simply an anti-knock rating. It has nothing to do with anything else, including power, detergents, emissions, cool burning, burning speed, bling, mojo, or game. You need to contact the fuel vendor to get these other statistics, although the last three may be a bit subjective.

Yes, different fuels may have different densities. They may also have different optimal mass air-fuel ratios. This is why it is easy to run the engine lean if you do not know what you are doing.

Race gas usually has both a higher octane rating and a higher energy content, as well as other desirable characteristics.

As for pump gas, lower octane fuel may or may not have more energy content than high octane fuel, but it all depends on the blends used by the manufacturer. Even if there is a slight difference in power, you will probably not even notice it.

If you are looking for something to give you an edge in racing, try driving lessons. Yes, I am serious.

J-Rat

iTrader: (2)

IIRC, octane is a measure of the ratio of the actual octane molecule added to the fuel.

Here is some interesting reading:

The name "octane" comes from the following fact: When you take crude oil and "crack" it in a refinery, you end up getting hydrocarbon chains of different lengths. These different chain lengths can then be separated from each other and blended to form different fuels. For example, methane, propane and butane are all hydrocarbons. Methane has a single carbon atom. Propane has three carbon atoms chained together. Butane has four carbon atoms chained together. Pentane has five, hexane has six, heptane has seven and octane has eight carbons chained together.

It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously. Octane 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).

More:

A high octane rating ensures that it takes a REALLY hot ignition source to ignite the fuel (such as a spark plug or the flame-front itself) and not just the rise in pressure & temperature that's a result of normal combustion. Note that the thermal rises in the cylinder are in direct proportion to the compression ratio of the engine (more below). The higher the compression ratio, the higher the octane of the fuel that's needed.


Octane rating is in no way correlated with engine power or efficiency. There is more potential energy in a gallon of diesel fuel than a gallon of gasoline, yet the diesel fuel has a much lower octane value (more on that below).

VietFC

makes me kinda wonder now.... if it's the percent of octane ratio... how do they get 100+ octane, such as at japanese pumps or in jet fuel? i'm confused

rxcutioner7

for an N/A use regular gas(87). i heard that octane levels really dont matter on N/A so i tested it out. one weekend i went to the track on regular 87 oct gas. I ran a 16.1 my first time down, 15.89 my second time.
*NOTE: this is with an exhaust and intake no other mods.
The next weekend i put 93 in plus a bottle of octane booster. First time down the track i ran a 16.2, second time i ran a 16.08 so octane really doesnt make a difference. my car actually ran a little slower. the only difference i can tell is that when i jacked the octane up my N/A ate more gas than Mother Love at a Golden Coral buffet. i get VERY crappy gas mileage with high octane.

Evil Aviator

You are misinterpreting that article. The octane/heptane model is the reference for the rating, not the actual content of the fuel. It's similar to how the measurement of a "foot" may or may not apply to the length of your particular foot, but we still use it as a standard (at least in the US, lol). The second section in this link describes the relationship better than the source you quoted:
http://www.refiningonline.com/Engelh...ep/TCR4_29.htm

The octane rating is usually determined by standard testing as defined by ASTM D2700 and ASTM D2699.

It's not the percent of octane ratio, it is simply the anti-knock rating, and that's all. Anything over 100 octane means that it has greater anti-knock qualities than 100% iso-octane (2,2,4-trimethyl pentane).

Japan, and most other countries, use a different octane rating than the US. This link will explain:
http://www.btinternet.com/~madmole/R...RONMONPON.html

Jet fuel (Jet A, JP-8, etc.) is simply high-grade kerosene, much like diesel fuel. Also like diesel fuel, it is usually defined by a cetane rating rather than an octane rating. The octane rating would actually be rather low.
http://www.madsci.org/posts/archives...2182.Ch.r.html

... and still it confuses people. Therefore, I continue to participate in these threads.

beefhole

iTrader: (29)

Dang! That's a good bit of info, Evil Aviator.

BigTone

I don¡'t have the kick *** sites that Evil Aviator has, but this is briefly what I have learned about gasoline in engineering, perhaps someone can tell me if what they are teaching me is correct.

The octane number of gasoline is obtained by comparing its knocking characteristics with those of "Isooctane" (2,2,4-trimethylpentane) and heptane. Isooctane is assigned an octane number of 100, whereas heptane is assigned 0. Gasoline with the same knocking characteristics as a mixture of 90% isooctane and 10% heptane is rated as 90 octane.

The octane rating of gasoline is also increased by adding certain "antiknock agents" or octane enhancers. Tetraethyl lead was the principal antiknock (C2H5)4Pb agent until mid 1970s. Some current antiknock agents are aromatic compounds suck as toluene (C6H5CH3) and oxygenated hydrocarbons such as ethanol and also methyl t-butyl ether. If I am correct these are the some of the agents that would allow you to have greater an octane rating of greater than 100 because these agents have a higher knocking characteristic than 100% isooctane.

J-Rat

iTrader: (2)

I am not really misinterpreting the article, but its pretty clear that octane is the rato of the 8 molecule chain (Octane) and whatever is used to dilute it (MTBE, Hexane, ****, whatever).

*edit* so your saying the above model is only an example of how octane rating is derived?

But whatever. The point here is (and I think you can agree), if your car isnt meant to burn a higher octane gas, then your just wasting money.

*edit #2*

That article just verified my original statement to a point (and I see the caveat at the bottom):

The octane number measured is not an absolute number but rather a relative value based on accepted standards. By definition, n-heptane has an octane number (RON and MON) of 0, while iso-octane (2,2,4-trimethyl pentane) is 100. Linear combinations of these two components are used to measure the octane number of a particular fuel. A 90%/10% blend of iso-octane/n-heptane has an octane value of 90. Any fuel knocking at the same compression ratio as this mixture is said to have an octane number of 90.

andrew lohaus

AHHHHHHH its chemistry class all over again. damned hydrocarbon isomers!!!


makes me wonder if anyone has tried running a rotary on kerosene.

J-Rat

iTrader: (2)

Maybe we can archive this or something, so we dont have to go through it every 2-3 months...

BigTone

agreed.

3RotorRocket

Sunoco 94 octane!!!

edmcguirk

Actually, I get higher MPG on high octane. Not enough to pay for the difference but more MPG than with low octane.

All that really proves is that the average MPG for the average RPM and the average HP and the average ignition advance of a whole tank is a little better with high octane. (rxcutioner7 has better proof than me)

For that reason, I race with high octane. But clearly the driving skill has much more influence on whether I win or not.

ed

Radial GT

iTrader: (16)

for the money, I'll stick with the old 87 octane. Mazda says that's OK, and so does my wallet

rxcutioner7

87 is all i need to beat the ricers

maxpesce

the Octane RATING in and of it self has NO effect on Mileage - MPG is dependant on the Energy Density of the fuel, Different Mixtures can have different Energy densitys w/ the same Octane RATING. Both Methanol and Toluene can be used to boost octane ratings, however Toulene has about twice the energy density of Methanol so using Toluene produces a fuel with greater MPG potential than a Methanol blend. In CA the change from an MTBE blend to a methanol blend made a 5-10% reduction in energy density of the fuel and a corresponding reduction in MPG potential.

Blake_91

Thus... Higher performance/more HP= higher octane petrol!
Your n/a "stock"13b does not create enouph power to ignite 87 octane petrol before the spark does!

-Blake

domesttuner

I run Shell 93 Premium on my NA, and the only thing I have been able to tell about the difference in MPG or anything else like that, is that I can shoot flames with 87, but i can't with 93. Besides, really good fuel burns cleaner IF you maintain your engine, regardless of rotaries or pistons. Why do you think Porsches and BMWs run with premium?

Or am I totally wrong and just need to shut up?

andrew lohaus

true, good feul does burn cleaner. but this does not directly corrolate to octane #. crapy 93 octaine can burn dirty than a good 87. and you realy are wasting your money if you run more than 87 on your n/a. IIRC mazda was able to get n/a's to run perfectly fine on as low as 80 octaine. n/as dont have a high enough compression or the forced induction to justify premium. besides-----dont you want to throw flames??

BigTone

Its not that hard. Your octane rating will not affect you MPG at all. If you have an N/A there is NO reason to run anything but 87, unless you have money coming out of your ***(and most N/A owners dont have that issue)

BigTone

Porsches and BMWs run premium becasue: A) they have high compression N/A engines or B) they have turbos. Just becuase an expensive car has or does something different then you is no reason to suddenly copy them. Throwing the extra money into higher octane gas will not help you at all.

domesttuner

Well, I guess I have been using 93 out of habit. My old car ran much better on 93 than 87, and old habits are hard to kill. I wasn't copying expensive cars simply because they were expensive. I knew they used 93 because of the engine design and turbos. I simply use 93 because it makes my car run better. Question - if the previous owner of the car ran premium for the entire ownership of the car, will suddenly using 87 cause poor performance, or does that not matter? I thought rotaries were high compression anyways.