Gas/Octane Question

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Another stupid question of mine. I've been reading about Octanes in gasoline, and according to around 90% of websites there is no real benefits from a higher octane. Is there any real benefits? Or difference?

posted by  triggerman

There is absolutely no benefit to using a higher octane gasoline than the engine requires. Using a higher octane than needed can actually reduce the efficiency of your engine, cause you to get worse gas mileage and put out less power.

posted by  vwhobo

There is absolutely no benefit to using a higher octane gasoline than the engine requires. Using a higher octane than needed can actually reduce the efficiency of your engine, cause you to get worse gas mileage and put out less power.[/QUOTE]

Alright thanks, well how do you get worse gas mileage? I've been told using a higher octane gas helps the engine if it's a "high performance engine" What exactly would or could be considered a high performance engine?

posted by  triggerman

Alright thanks, well how do you get worse gas mileage? I've been told using a higher octane gas helps the engine if it's a "high performance engine" What exactly would or could be considered a high performance engine?

posted by  triggerman

1. As I said in the previous post, decreased efficiency.
2. More correctly you need higher octane in a higher compression engine. I can't give you a specific number because it depends on factors such as engine design, materials used (aluminum or cast iron) and the powertrain management system. Generally speaking the newer (technolgy wise) the engine, the higher compression it can run before an increase in octane is required.

posted by  vwhobo

Ok im a little confused, cause my car only requires 91 octane, but when i put 100 octane in there it re-adjusts the timing and runs faster smoother and i get better gas mileage, so when you say no gains do mean once you reach the optimal octane for your engine ( which in my case is way above pump gas ) or when you go above what your car manual says ( which in my case is 91 ).

posted by  VrrmPshh

In your case what I'll say is you have a great imagination. If your car requires 91 octane, that means it is optimized for 91 octane. Running 100 octane is a waste of money.

posted by  vwhobo

don't most vehicles have programming built into the 'puter that allows lower octane through various tricks?
ie: retarding timing
leaning out the AF mixture
(if Forced Induction) lowering boost

it's just that they get the optimum power curve at X octane, but they can survive on Y octane

posted by  asa67_stang

You're right, although it depends on how smart the PCM is. Most cars get by with a knock sensor which retards the timing when detonation is present, some are a little more advanced then that. However that's a short term solution, if you regularly run gas with too low an octane rating there will most certainly negative long term results.

posted by  vwhobo

good thing i told my mom she's been runnin too low of an octane on her Passat then, eh?
i was suitably impressed by that 1.8T and how well the rest of the car worked together, very nice

posted by  asa67_stang

I wish i had more then just my "butt dyno" to prove that running a higher octane in my car was increasing its efficiency and performance, but i dont so i wont even argue the point as it is all goin off my word. I did however look in my manual to see if there was anything on running a higher octane and only found that it said use "at least 91 octane" which leads me to believe that perhaps 91 is not my car's optimum octane rating its just what subaru feels is the lowest you can go without any negative side effects. Now im not sayin 100 octane is my car optimum rate either but im going to do a little more research ( gradually increasing octane until i can no longer feel a difference ) to figure it out...i guess i could also just call subaru and ask them..but wheres the fun in that

posted by  VrrmPshh

where do you get 100 octane gas?

posted by  SuperJew

You can read some "official" data here:

...but who really trusts the government anyway...


posted by  BavarianWheels

some gas stations sell GT-100, but i use 91 octane then add toulene acorrdingly heres an article on it that pretty much explains it all

it also explains that newer cars (like mine) do benefit from higher octane gas

posted by  VrrmPshh

toulene is very harsh on rubber, dunno what effects it has when dilluted through gasoline

posted by  asa67_stang

gasoline is pretty harsh on rubber too though so im not worried about it....

posted by  VrrmPshh

Running toulene for an octane boost is nothing new, my father was doing it in the '50's in his '49 Merc lead sled.

If we're going to do something stupid and dangerous, why don't we go all the way. Let's run xylene (117) as an octane booster, after all it has an even higher octane number than toulene (114). Of course some of the short and long term effects of having either in your fuel system are rotted fuel lines, damaged injectors they can even decompose your plastic gas tank. And if you run them in high enough concentrations you'll damage your catalytic converter(s) and your oxygen sensor(s). Cheap parts to replace I'm sure you'll agree.

Better yet, I can get you a really good deal on some MTBE (118). Any takers?

posted by  vwhobo

actually, there's a guy at FSP (my inline six forum that i hang at) that flirted briefly with all of those...
notice i said briefly, he knew what it could do to the various parts, he just wanted to see what he could get away with without pinging

posted by  asa67_stang

I run 116 octane in my car, and it works just fine, I don't get whats so stupid about it, as a matter of fact, if I don't run it diluted with a premium, my car gets minor detanation, and runs lean, my RPM will increase by about 400, and my intake sounds like it's a vacuum. Of course if you don't have a car that will handle the fuel properly, and you think you're some kind of racer when you aren't, don't run it, because it will harm your engine, I designed my engine specifically to run at a higher compression.

posted by  Logicalbomb

Octane basically says how much compression it can handle without knocking. In "high performance" engines, the compression ratio is higher, so low octane fuels won't be very good as you might get spontaneous combustion (ie explosion without the spark).

posted by  windsonian

As silly as it sounds, octane ratings are about how slow the fuel burns. I think, but don't quote me on this, that 91 octane under x pressure at y volume takes 91 milliseconds to combust. 100 octane would take 100 milliseconds under the same conditions and so on and so forth. For most street cars produced in the U.S. they are set to run on like 87 or 89 octane and too much more than that is indeed a waste of money. As for adding chemicals to increase octane ratings, be very careful and remember that all of the previous batch has not been run out. :2cents:

posted by  srober32

Octane rating is a measure of the "anti-knock" properties of gasoline, and doesn't have much to do with combustion speed. It is not measured in milliseconds, either. Consider what would happen if you used 98 octane gasoline in an engine with huge pistons (many liters per piston). Would this gasoline combust in the same amount of time as 98 octane gasoline in a normal cylinder (0.5 liters)? Obviously not. The flame front would move at a similar speed, but because of the greater size, the combustion would take longer to complete in the larger engine.

Here is the real story, and the first eight hydrocarbon names:

1 Methane
2 Ethane
3 Propane
4 Butane
5 Pentane
6 Hexane
7 Heptane
8 Octane

CH3---C---C---C---CH3 <-- Iso-octane

Iso-octane, otherwise known as (2,2,4-trimethylpentane) is very smooth-burning, and is assigned an anti-knock rating of 100. Heptane, on the other hand, is very susceptible to knock, and is assigned a rating of 0. The gasoline you buy at the pump is rated on a scale from heptane to iso-octane. An octane rating of 93 means the gasoline has the same anti-knock rating as a mixture of 93% octane and 7% heptane. An octane rating greater than 100 indicates that the fuel is even less susceptible to knock than pure iso-octane.

You can see a sticker at the pump that says "calculated using the (R+M)/2 method". The R is the Research Octane Number, from an engine running at 600 RPM. The M is the Motor Octane Number, from an engine running at 900 RPM. The two are averaged to provide the total octane number you see at the pump.

I hope this dispels some of the myths floating around.

posted by  lithiumdeuterid

Well it sure shut the thread down. I guess now Satty will come along and lock it. :banghead:

posted by  vwhobo

I heard that higher octane burns either faster or with more temperature and can help clean the valves.

posted by  newyorker

Once again you heard wrong. What a surprise. All you had to do was read the thread and at least one of those is covered.

posted by  vwhobo

Better do some reading on that subject. There are plenty of research articles the confirm that running gasoline with an octane rating higher than required actually decreases power output.

posted by  vwhobo

In a completely stock car you are better off switching from the cheap stuff to the premium fuel if you are selling it. If someone is test driving it and puts the pedal on the floor it is less likely to rattle if something doesn't go just right and you might only have the one chance on a test drive.

If you run high blower, or turbo boost, or have a performance chip and find that your car preignites at high load use higher octane gas or ethanol
(if you can get it). If you raise the compression above the stock compression ratio then you will need higher octane gas or ethanol if it preignites under load. Check your base ignition timing as well as your advance curve before stepping up at the pump. Additives cause crappy deposits. If you are worried about rubber seals then step up to Viton as it is easy to get and holds up to gas additives and ethanol both (sometimes better than aluminum).

Octane is strange, as is the Cetane rating of diesel. Research the BTU ratings if you switch fuel types. Ethanol doesn't preignite in the real world and you can always add more but the lower BTU means you need to fatten it up (nearly 40%) to get the same amount of power. To get even more power with ethanol you can run much more compression and boost, then watch your oil turn green. It shouldn't cost as much to run ethanol as 116 octane gasoline.

If the 100 octane mentioned is blue then suspect it is a low-lead aviation fuel and don't run cats or you will cook them.

I have a diesel car I run with biodiesel and regular diesel. The cetane is about the same in each but the BTU in the biodiesel means much worse mileage and it costs more. Used to pitcrew for alcohol dragsters and can not wait until that stuff is available on the street. Nice complex topic...

posted by  electrodes

Too many guys no too little about cars

posted by  Wally

You're 110% correct on that. I usually run on 87 octane in my T-bird with 16* base timing and about 38* total...from what everyone else says they've experienced, I shouldn't be able to do that without some detonation...probably because I'm not running the E7TE heads, but rather the larger chambered E6SE heads. It runs slightly better on Sunoco 94 at the track.

Just a few months ago, I went to Atco Raceway and ended up making twice as many runs as I had planned...leaving me in a slight fuel crisis, didn't think I could make it home on what I had. I went to the pumps and the lowest octane I could find was like 104...I bought $30 worth...about 5.25 gallons, just to get me home. After that, the car ran like absolute sh!t, started surging real bad at idle and didn't want to make power. After about 5-6 fill-ups of 87, it was back to normal. Car still surges slightly...but that's due to needing a new TPS sensor.

posted by  Sick88Tbird

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