OCTANE RATING QUESTION FOR CHEMISTRY WIZARDS

Ur.. no, it doesn't. It has exactly the same "energy" as any other gasoline, 125,000 BTU/gal (with the exception of 10% ethanol gas, which is lower due to the alcohol content). The difference is in the stability of the fuel and its tendency to autoignite.

Detonating on 91 is not especially rare in a high compression NA engine, especially if the 91 in question is the sludge they sell in California. Just because you can't hear detonation doesn't mean it isn't happening, and doesn't mean the ECU isn't pulling timing. The only way to monitor that exactly is to map the knock sensor output(s) and timing advance while doing a run.

There's a lot of benefit to be had in the 89-93 octane range so far as timing advance on a NA engine goes. Not so much after 94-95, as I said.

 

Right on the money, mate! Same energy just different stability, as higher compression, and forced induction engines tend to run hotter the gas's resistance to knock is must be greater.

 

Actually SRT Mike is right and you are wrong. Gasoline does not always have the same energy content. It will depend on the chemical mix. Gasolines are made up of Olefins, Aromatics and Parrafins, each with its own BTU ranges and octane ranges. Throw in oxygenates to the mix and you have more variance. Eg. a winter blend 91 octane will have different BTUs than a summer blend 91 octane.

This is from a former refinery engineer,

"Just like octane has a minimum requirement, fuel sensitivity (sometimes called driveability index b/c driveability suffers as it becomes too large) has a maximum value, though it can vary by area. I don't have the material to tell you what the sensitivity spec for different areas is anymore, so some research on that might be in order.

A local gas survey done years ago by the fuels division of the institute where I work showed some wide variations in fuel sensitivity mainly due to the different potential blends that can be made to arrive at the same spec. The difference got even wider when you compared the blends from different seasons- the sensitivity was higher for a winter blend than a summer b/c of the components necessary to achieve a substantially higher RVP. This suggested to me that some of the winter-time detonation problems we were seeing may in fact we the result of this parameter (probably along with several others as well).

The fact that Cali has the aromatics limit set quite a bit lower than most states strikes me as further evidence of this phenomena...."

 

All things being equal, no, it doesn't always have the same energy content. But that's not what I said. I said that the octane rating of a gasoline in itself is not inductive of its energy value, albeit perhaps in not such a clear manner.

Tossing a bunch of aromatic hydrocarbons into a gasoline blend isn't going to significantly effect its overall energy value. Many other metrics do have a significant impact, yes, adding alcohol to the fuel as is common in California is one of them. However, an octane rating alone does not mean that a fuel has less "energy" per se.

One needs to go into significantly more depth to answer the question properly, I agree, but I stand by my original contention.

 

On that we agree. Octane is an independent variable. It has nothing to do with BTUs, or flamefront speed (burning speed) or all the BS attributed to it. It is simply a number that quantifies a fuels resistance to detonation. Nothing more, nothing less.

 

I think one of you chemical gurus needs to write a treatise on octane the way AEHass did on oil so we have all the info in one place.

Birdman

 

All I know is that my Honda S2000 went faster and used slightly less fuel when using 98 RON rather than 95 (UK).

 

Yes, your ECU knows.

Have someone capable read out the knock adaptation.

(This is of course, assuming your ECU(s) have knock sensors and
adaptive knock control)

If your ECU is pulling timing based on spark knock, you can benefit from
a higher octane fuel. If not, you won't.

If you don't have knock sensors, the only way to do this is to instrument
your engine with a cylinder pressure sensor and watch the traces.

Jim Conforti
(ECU Tuner)

 

And I stand by mine. I never said the difference is significant, but there a difference in favor of lower octane. I am too lazy to go find the articles but we had a discussion about this with a chemical engineer from Exxon and the result was that the components that lead to higher octane ratings have less stored energy. On motorcycle engines, you generally get a smoother idle and rev on lower octane (if you can run it) vs higher octane too. How many stories did we hear when we were younger about someone putting avgas in their car and it was so fast they could barely control it.

As for detonation, if you're getting it on 91 to where performance is hurt, then IMO there is a problem. Yes, the CA gas is worse in this regard but if any of my cars were detonating on it I'd be looking seriously into a solution. Its unlikely that the kind of detonation that would cause a noticeable performance loss would be solved by going up one point, unless there was something else about the gas in question other than the octane rating.

My .02

 

I find this thread to be very interesting.

My father ran a refinery during World War II in Shreveport LA and was part of a team that led the military efforts to extract more energy from fuel. He was an expert on octane. They focused on increasing the power of gas used by the Air Force. They were also, as he told it, responsible to bringing down the octane of civilian gas by blending gasoline with previously worthless byproducts (I think he said "hectane") in order to dilute it and help ease the shortage of fuel (and send more to the war effort). According to him, prior to their work, gas was relatively plentiful and there was no experience with blending down the octane, so they were constantly experimenting with "how low can we go". He told a funny story that they decided they had blended it down too far when they got a call that all of the trucks in Texarkana were stuck and would not run.

Back to the topic. Based on his 1940's expertise, he always told me that octane "blends up". Specifically, that half a tank of 100 and half a tank of 90 would not blend to 95, but rather it would blend to a higher number (such as 97.5). For those of you who are more current in the industry, is this still considered to be true for modern fuels?

He also insisted that the best performance was to have octane that was high enough to not cause detonation, but that any higher was a waste (again, might not apply to cars with chips that can make use of the higher octane to recalibrate the settings).

Finally (not related to my Father), I thought I read an article a few years ago that the 91 vs 92 vs 93 rating issue had more to do with what the station was guaranteeing as a Minimum octane, and that in fact a lot of the fuel is actually higher. The 91 issue in California (per the article that I read) said that it had to do with a major west coast pipeline not willing to guarantee that "premium" being sent though the pipeline would not be mixed with the remnants of other fuel with an octane as low as 91, therefore even though your fuel might be 93, they would not guarantee it (since they did not want to purge the pipeline after pumping a load of 91). Does anyone else remember this, was the article accurate (cannot believe everything you read), and is it still the case?

 

same here in NV for the last three years. Stable at $5.00 a gallon. It is all I run in the car and it makes a big difference.

 

Odd. Normally the pipeline transfers diesel, heating oil, or gasoline for days on end. When they do change from one fuel to the other they mix in radiation treated fuel. Radiation detectors go off at the interface and valves for output disposition are switched automatically to appropriate interim tanks. That small amount of fuel at the 'interface' fuel is sent to heating oil. Gasoline from the refinery is one grade only but may have ethanol added. The gasoline is modified with additives to determine the octane mix, not by the refinery. I.e. distributors all get the same gasoline from the pipeline and modifies it with their secret cleaner, octane booster etc. A few years back Shell added the most gunk and Richfield the least.

In Oregon we have one pipeline from the Puget Sound area and one grade of gas is transported but may contain ethanol depending on calendar state mandate. Six distributors for the entire state all fill trucks from the same spigot and the driver pours in a concoction for the destination station. Heating oil etc are distributed in a separate pipe.

 

I run 110 octane in my 87 TR.........$6.50 a gallon.............runs great and the performance difference, to me is well worth it...........even if half the "performance increase" is in my brain........Kerry

 

Av gas octane is rated differently than auto gasoline...110 is probably close to auto gas 100. Also av gas contains lead additive for upper cylinder lubrication, long banned for auto use.

Interesting aviation article, dangers of using auto gas in planes-
http://www.eaa.org/education/fuel/autogas_vs_avgas.pdf

 

This isn't AV fuel..................It's from a gas pump at a small country store in Virgil, NY........... that is 2 miles from a local good-ole' boy Saturday night stock car track..........racin' gas...................Kerry