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Government The Almighty Buck Transportation Politics

Is E85 Dead Now? 556

twdorris writes "With a stoichiometric ratio far lower than that of gasoline (much lower than the price difference), buying the E85 ethanol fuel blend instead of gasoline was already hard to justify. Unless you raced your car on a track where E85 provided a great alternative to race fuel, it really didn't make financial sense. And there are other reasons not to buy E85, too. Like the impact corn-based ethanol is having on food prices or the questionable emissions results (PDF). So, now that the ethanol subsidies provided by the U.S. federal government are scheduled to end this summer, it's going to be even harder to justify E85 (at least in the U.S.). This change will basically make a gallon of E85 cost the same or slightly more than gasoline. With so many things working against it, are the days numbered for readily available E85 at your local gas station? And should it have ever even been made available to begin with? How much did all that government-backed R&D and tax credits cost us for something that was pretty clearly questionable to begin with?"
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Is E85 Dead Now?

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  • Kinda sucks (Score:2, Informative)

    by Anonymous Coward on Monday January 16, 2012 @04:15PM (#38717036)

    A lot of the racers are putting turbos in their cars, running e85 and getting great horsepower AND gas mileage. It works great for them. However, most americans hate it because they get no increased hp in their car, and the price offset doesn't justify the worse gas mileage. Then theres the whole CORN IS FOOD. To which I say, there's enough corn in my food already

  • by james_van ( 2241758 ) on Monday January 16, 2012 @04:17PM (#38717072)
    Even though it cost less than standard gasoline, it came at a reduced gas milage. I did the math and at the cost in my area, it was more expensive per mile than regular. Maybe in other areas that was different, I dont know.
  • by Anonymous Coward on Monday January 16, 2012 @04:17PM (#38717078)

    For engines that can handle it, E85 is a nice alternative to gasoline because it does give a tad more horsepower. However, even with the included subsidies, it was still not worth using because of the MPG difference compared to plain gasoline.

    However, being forced to use gasoline with ethanol in it results in more energy lost in making of replacement engines and parts than it saves.

    Ethanol is an enemy of small engines. It is hygroscopic, which means the engine has to deal with water sucked in, and gas + water makes a nasty acid (nicknamed "gacid" by mechanics) that destroys engines. Of course, this stuff is not covered by any warranty, so your new car that gets ethanol damage, the owner is stuck with the bill.

    Of course, you can add Sta-Bil to the gas tank to help combat the ethanol's effects, but gas additives get expensive.

    I just hope that ethanol goes away except for the occassional E85 pump, just for the sake of lawn mower, generator, motorcycle, and boat owners everywhere. The carbon savings from not having to keep purchasing new engines will more than make up for the difference in pollution.

  • Re:10% Ethanol (Score:5, Informative)

    by smpoole7 ( 1467717 ) on Monday January 16, 2012 @04:17PM (#38717086) Homepage

    For the record, I know of only one location that sells E85 in this area. Doesn't mean there aren't others, but if there are, I haven't seen them.

    One of the talk shows on our station is a good ol' boy who talks auto repair. He insists -- vehemently -- that ethanol lowers mileage so much that whatever you saved on emissions, you lose because you're burning more fuel as a result. The callers to that show seem to echo that sentiment.

    I know in my own car (Nissan Altima, and I LOVE it), I seem to get a bit more mileage when I'm burning pure gasoline -- about 5% more.

    YMMV (literally, in this case) and that's hardly scientific, but there you go. :)

  • by Chemisor ( 97276 ) on Monday January 16, 2012 @04:19PM (#38717104)

    Let's not forget that ethanol fuels destroy engines [], lower gas mileage, and drive farmers into bankrupcy.

  • by what2123 ( 1116571 ) on Monday January 16, 2012 @04:27PM (#38717214)
    I've been wondering if Ford was already certain that the damage was too great to even risk with their vehicles. At the least, their standard non-hybrid or Alternative models all seem to be label explicitly NO-E85 or any alternative fuel other than gasoline/gasohol (10% ethanol). This is true on my 2011 Fiesta, where they state using E-85 can void the Powertrain warranty completely.
  • by msobkow ( 48369 ) on Monday January 16, 2012 @04:36PM (#38717328) Homepage Journal

    I know diesel engines have a lousy reputation in North America, but I firmly believe hemp based bio-diesel is a FAR better alternative than E85. Most importantly, hemp seed based bio-diesel is a net-positive energy solution, requiring less fuel to farm the hemp and process it into bio-diesel than you end up producing (kind of a critical point for any product to succeed in the energy markets.)

    Some go so far as to claim that hemp bio-diesel is carbon negative. I'm skeptical about that, but it would be interesting to test the theory.

    Unlike ethanol corn, hemp produces a great deal of fiber suitable for textiles and paper as a side-product, even if the main purpose of the crop is bio-diesel. Levi's jeans used to be made exclusively from hemp-fiber denim, not cotton. I've read claims that hemp based paper out produces poplar tree paper production by a factor of nearly 4:1, though again, I've not seen a study to prove that claim.

    Most important of all, hemp is literally a weed and will grow almost anywhere, allowing the use of low-grade farmland instead of taking away from food-crop acreage.

    But it's nothing new. The pro-hemp community has been screaming this "nonsense" at the top of their lungs for decades while the cannabis drug war drowned out their good points about hemp farming.

  • Re:Kinda sucks (Score:5, Informative)

    by Dahamma ( 304068 ) on Monday January 16, 2012 @04:39PM (#38717400)

    It's not just about total energy, it's about useful energy extracted.

    Turbos usually require higher octane so that there won't be premature ignition under the extra pressure. They also get more power/efficiency out of the same fuel as they are driven by reusing exhaust gases. So it's entirely possible that a lower-energy, higher octane fuel can get better mileage with an efficient turbocharged engine...

  • Ethanol problems (Score:5, Informative)

    by uigrad_2000 ( 398500 ) on Monday January 16, 2012 @04:53PM (#38717582) Homepage Journal

    Even Scientists from Ag departments of California universities have known that looking to corn-based fuels is a bad idea. Look at this report from Professor Tadeusz Patzek, A Professor of Chemical Engineering at the University of California at Berkeley []:


    Why Corn Ethanol is Unsustainable, Let Us Count the Ways:
    Approximately 99% of U.S. corn is fertilized, requiring more fertilizer than any other crop.
    Nitrogen fertilizers, herbicides and pesticides are all made from fossil fuels, as is the diesel
    fuel, gasoline, LPG, natural gas, electricity, transportation and irrigation used to grow and
    transport the corn.

    Because ethanol is a toxic and hazardous substance, its use is regulated by OSHA, DOT,
    NFPA and NIOSH. Ethanol must be handled with extreme caution because it can enter the
    blood stream from breathing the fumes, or by penetration through the skin or mouth. Exposure
    can irritate the eyes, nose, mouth, and throat. As such, protective clothing, including gloves
    and splash-proof chemical goggles and face shields should be worn by anyone coming in
    contact with ethanol.

    People are advised not to eat, smoke or drink where ethanol is handled, processed, or stored
    since the chemical can easily be absorbed. Moderate exposure can cause headaches, eye
    and skin irritation, nausea, and drowsiness, whereas higher levels of exposure (over 1000 parts
    per million over an 8-hour period) can cause shortness of breath, genetic mutations, damage to
    the liver and central nervous system and unconsciousness. Exposure to ethanol levels of over
    3300 ppm can result in death.

    Ethanol land requirements: Approximately 50 gallons of ethanol are produced per acre of
    corn. Thus 2.8 billion acres of land would be required to generate 140 billion gallons of fuel
    used in the USA annually, which is more than 5 times all of the cropland that is actually and
    potentially available for all crops in the USA.

    Ethanol water requirements: ...8,360 gallons of water are needed per equivalent gallon of
    gasoline in the form of ethanol. 140 billion gallons of gasoline are consumed in the USA
    annually, times 8,360 gallons of water = 1.17 trillion gallons of water needed to grow and
    process enough ethanol for the U.S. economy.

  • Re:10% Ethanol (Score:5, Informative)

    by SleazyRidr ( 1563649 ) on Monday January 16, 2012 @05:07PM (#38717774)

    That's just semantics.

    It takes less energy to drill a gallon of gasoline out of the ground and deliver it to your fuel tank than you gain by burning that fuel in your engine. It takes more energy to grow corn, turn it into ethanol and deliver it to your fuel tank than you gain from burning that ethanol.

    If you were using solar powered tractors to grow the corn, and solar powered trucks to move it around it might make sense (just might, it wouldn't necessarily.) Given that most of the energy to produce the ethanol comes from gasoline or diesel, it makes no sense to use ethanol.

  • Re:10% Ethanol (Score:2, Informative)

    by BitZtream ( 692029 ) on Monday January 16, 2012 @05:08PM (#38717780)

    No, exactly the opposite.

    Ethanol in fuel is what CAUSES the condensation problem as ethanol absorbes water and then dumps it during a phase change so you'll end up with a puddle of water at the end of your tank BECAUSE of ethanol. Ethanol is one of the primary reasons boaters use fuel stabilizers, don't need it without ethanol. With ethanol you need fuel stabilizers to keep the ethanol from dumping its water and your engine sucking it into all the places that don't need it as right after it dumps its water the whole damn thing turns into one big bunch of acid that fucks up everything in your engine.

  • by netwarerip ( 2221204 ) on Monday January 16, 2012 @05:08PM (#38717794)
    For those interested, here's the wiki. [] Interesting.
  • Re:10% Ethanol (Score:4, Informative)

    by guamisc ( 1174125 ) on Monday January 16, 2012 @05:09PM (#38717800)
    Octane rating of gasoline has nothing to do with the actual "Octane" content. The simple version is that it is a pressure scale at which the fuel spontaneously combusts. You can squeeze octane to very high pressures before it combusts, heptane on the other hand will spontaneously combust at very low pressures. On the octane scale, the pure heptane value is set at "0" and the pure octane value is set to "100". Therefore a higher octane rating in your fuel will reduce the likelihood of one of you pistons firing prematurely from compression a la "backfiring". I don't really know what the octane rating of pure ethanol is. But it has nothing to do with the amount of carbons in the chain.
  • Re:10% Ethanol (Score:5, Informative)

    by Anonymous Coward on Monday January 16, 2012 @05:09PM (#38717808)

    It doesn't raise octane content, but octane rating, which is a measure of susceptibility to autoignition, indicated as the iso-octane content of a mixture of heptane and iso-octane with the same properties -- but as soon as there's anything other than heptane, the octane rating is nothing to do with octane. Ethanol raises octane rating by being difficult to ignite -- basically because it's an alcohol instead of a hydrocarbon, and they act different.

    And E85 will let you get more power, and comparable MPG, from the same block vs. gasoline precisely because of ethanol's awesome octane rating -- the only catch is, you need to increase the compression ratio to make it happen (which will boost your efficiency enough to compensate the decreased energy content of the fuel) -- but turbocharged engines (which can do that on the fly) are sadly unpopular in America, land of the big-block V8.

  • Re:10% Ethanol (Score:5, Informative)

    by Ferzerp ( 83619 ) on Monday January 16, 2012 @05:09PM (#38717812)

    The octane rating of fuel has less to do with the actual compounds and more to do with how much pressure you can put it under before it detonates (which, of course, does depend on the chemical makeup, but other things, such as ethanol can raise it, not just octane specifically). The higher the octane rating, the less likely it is to spontaneously combust under high pressures. This is why your higher compression engines don't allow the lower octane ratings. They're made to compress the fuel more thna 87 octane fuel can withstand. Of course, by changing the timing and the amount of fuel and air that enters the cylinder, they will work with lower fuel, but less powerfully.

  • Re:10% Ethanol (Score:4, Informative)

    by dgatwood ( 11270 ) on Monday January 16, 2012 @05:15PM (#38717874) Homepage Journal

    What you're missing is that the people saying that Ethanol takes more energy to produce are not including the sun's energy that causes the corn to grow, nor are they including the sun's energy that caused the plants and animals to grow that eventually turned into oil. They're talking about the production process itself. If the production process itself takes more energy than it produces, then the system as a whole isn't just a net loss; it's a *huge* net loss.

    It would be as though the amount of gasoline your chainsaw took to chop down the tree could produce more heat than burning the tree. That's what happens with ethanol. That just isn't true for gasoline, coal, or wood. I'm not certain about the lab assistants. They generally don't like it if you try to burn them for warmth.

  • Re:10% Ethanol (Score:5, Informative)

    by bkaul01 ( 619795 ) on Monday January 16, 2012 @05:17PM (#38717902)

    Octane rating is not a measure of octane content. It's a comparison of the autoignition (e.g. knock) resistance of a given fuel blend to a scale defined by the properties of pure iso-octane (100) and heptane (0). A gasoline with an octane rating of 97 has the same autoignition properties as a mix of 97% octane and 3% heptane. Ethanol's octane rating is at or over 100, and E85 typically has a R+M/2 octane rating of around 95, which is a little higher than retail premium gasolines, and much higher than standard 87-octane gasoline.

    The high knock resistance of E85 actually enables engine designs that have higher compression ratios, more boost, etc. to improve efficiency (and power output) which can actually make up some of the reduced range in a vehicle if the engine is designed primarily with E85 in mind rather than standard gasoline. Still, with the fuel only having about 2/3 the energy content of gasoline per unit volume, it's a big gap to close.

    Ethanol isn't primarily added because of its octane-boosting properties, however (though those are taken advantage of in formulating the base stock to blend with). It's added because the EPA mandates oxygenated fuels to reduce emissions of CO and other pollutants. Fuels can be oxygenated through the addition of either ethers or alcohols, MTBE being an example of the former, and ethanol an example of the latter. Most states have mandated that oxygenates be specifically made up of ethanol, due to the harmful health effects of MTBE, methanol, and other potential chemicals if they leach into ground water, as well as support from the powerful corn-farming lobby.

  • Re:Kinda sucks (Score:2, Informative)

    by Anonymous Coward on Monday January 16, 2012 @05:26PM (#38718038)

    Yeah, modern FFVs are nearly all good enough wrt timing and mixture. But that's barely worth talking about, because they're a MASSIVE compromise in a far more significant area. Look up Otto cycle [] and note the dependence of ideal efficiency on compression ratio.

    That's the magic right there -- but of course, in America, land of the naturally-aspirated big block, your compression ratio is fixed at a valuable suited for gasoline, and usually for the lowest (85) ethanol rating. You can play with the timing and mixture, and that's good, as far as it goes -- but that just improves the burn -- the heat-to-work conversion is still happening with low efficiency due to your piss-poor compression ratio.

    Turbocharged cars are much better off, because compression ratio is variable on the fly -- within limits usually designed around gasoline. So they'll go to the maximum pressure they can reach, improving efficiency, but there's usually some more to be gained. Any alcohol-burning race setup will be designed to utilize the fuel's good points, so it will have more compression available than road cars, and will be even more efficient -- enough to make up for the decreased energy content and still get more work per gallon. As a nice bonus, it'll be producing comparable power out of a smaller block, saving weight...

  • Re:10% Ethanol (Score:5, Informative)

    by j-turkey ( 187775 ) on Monday January 16, 2012 @05:33PM (#38718134) Homepage

    That's just semantics.

    It takes less energy to drill a gallon of gasoline out of the ground and deliver it to your fuel tank than you gain by burning that fuel in your engine. It takes more energy to grow corn, turn it into ethanol and deliver it to your fuel tank than you gain from burning that ethanol.

    If you were using solar powered tractors to grow the corn, and solar powered trucks to move it around it might make sense (just might, it wouldn't necessarily.) Given that most of the energy to produce the ethanol comes from gasoline or diesel, it makes no sense to use ethanol.

    I believe that in most cases, it's more than just semantics. Most (not all) corn is grown using conventional (petroleum-based) fertilizer. According to Michael Pollan [], producing one calorie of corn uses two calories of petro-fertilizer. This is only counting fertilizer use, not the additional energy used for farm equipment, moving product/raw materials, the distillation process or loss of energy during distillation.

    I'm shocked that this is not cited elsewhere when discussing Ethanol as an energy source, especially when used to reduce our dependency on petroleum (foreign or otherwise). Given that we're using more petroleum to make it than it would save, it appears to be a bit of a boondoggle.

    ...either that or I'm horribly misinformed. (Note: Pollan's book cites a peer reviewed study for this claim - I'm just citing what I read from memory)

  • by theNAM666 ( 179776 ) on Monday January 16, 2012 @05:33PM (#38718138)

    >My car is relatively newer and I *hate* when gas stations are forced to use E10 (10% ethanol, ie. Winter fuel). My mpg drops by 10% - 15%.

    That's bullshit. You're only adding 10% of a *FUEL*. If you added 10% water, and it still ran, you'd expect an approx. 10% loss in efficiency. You could mix in kitchen oil (which will burn) and if you could get it past the injectors, you wouldn't expect a loss anywhere near that.

    Even if E-10 were 25% less efficient than gas (it's not), at 10% blended in you'd see an approximate 2.5% loss.

    Damn, people are stooopid. It's math and science, people, not whatever prejudice you've majnaged to convince yourself of. Just measure the amount you drive and the gallons of fuel consumed, and divide-- and be suspicious of extraneous factors, such as warming up or using more gas due to bad weather in winter.. It's *so* simple.

  • by jpstanle ( 1604059 ) on Monday January 16, 2012 @05:38PM (#38718186)

    This is the case for ANY motor not specifically designed to run on high-ethanol-content fuels. Ethanol is a strong solvent and strips oil films, breaks down hoses and seals, oxidizes ferrous metals, and generally tears apart gasoline motors. E85 "flex-fuel" motors are designed with ethanol's nastiness in mind, using different materials and lubricants, but even then, running E85 is harder on the engine and usually calls for more frequent service intervals.

    Running E85 in ANY engine that does not explicitly state that it is designed to run on E85 will cause permanent and rapid damage. It'll probably completely destroy the engine before your next oil change.

    Ethanol is complete crap as an engine fuel, with the lone exception being purpose built race engines that can utilize the higher detonation resistance for more horsepower per unit displacement. And those race motors tend to get rebuilt at least once a year, mitigating the wear factors.

  • Re:10% Ethanol (Score:5, Informative)

    by OnlineAlias ( 828288 ) on Monday January 16, 2012 @05:39PM (#38718192)
    Well, to be a pedant, backfiring has nothing to do with pinging or detonation. It is an entirely different phenomenon.
  • Re:10% Ethanol (Score:4, Informative)

    by dfm3 ( 830843 ) on Monday January 16, 2012 @05:40PM (#38718208) Journal
    Not sure if your good ol' boy was talking about E85 or that "up to 10% ethanol" blend that most stations sell, but my personal experience with E85 is that you either end up paying slightly more per mile versus regular gasoline, or it's a wash (depending on the current gasoline price).

    Several years ago I took a few cross-country business trips in a rented "FlexFuel" Chevy HHR- definitely not my vehicle of choice, but it's what they paid for. I obtained a list of E85 stations along my route (turns out they are exceptionally rare in some regions) and did a little cost analysis with the E85 versus the usual 87 octane (10% ethanol) gasoline. Looking back at my mileage logs, I estimated about 34 MPG with regular gas and 25 MPG with E85. However, the price difference between the two fuels wasn't great enough to make up for the reduced fuel economy, and E85 actually ended up being about 5% MORE expensive per mile at the time.

    My most interesting E85 experience was back in the summer of 2008, when Georgia and the Carolinas were faced with fuel shortages and price hikes. Regular gasoline- when you could find it- was about $4.60 per gallon and most stations were sold out. I happened to be attending a conference in the region and had ended up with an E85 rental car. I printed out a list of stations and had no trouble finding fuel wherever I went... and it averaged about $2.80-3.00. A number of people actually got stranded at the conference when every station in the county, and every station in the next county, ran out of gas. Some folks resorted to waiting for hours in lines dozens of vehicles deep when delivery trucks finally came through with fuel; however, I found that there was always plenty of E85 to spare even after the regular gasoline sold out.
  • Re:10% Ethanol (Score:5, Informative)

    by Rich0 ( 548339 ) on Monday January 16, 2012 @05:40PM (#38718216) Homepage

    I think the real problem is that the standards aren't performance-based. Fuel is required to be oxygenated, rather than requiring that it has some level of emissions in some reference test. If the standard were performance-based then the refiner could use a number of different means to accomplish the standard rather than just adding one or two particular substances - both of which are expensive and have certain drawbacks (environmental and otherwise).

    However, ethanol in gas is more about agribusiness subsidies and the environment is just a red herring. At work they go on about the environment as well, but I've noticed this tends to be only in situations where environmental interests are strongly correlated with corporate financial interest. Saving on pounds of CO2 on an airline ticket tends to mean lower airline costs which means lower ticket prices. Saving on pounds of CO2 from power use means less power use which means a lower electric bill. No harm in it, but the appeal to the environment sounds disingenuous. If I found some more renewable supplier of paper for an extra $3 per ream it isn't like they'd be tripping over themselves to buy it.

  • by Solandri ( 704621 ) on Monday January 16, 2012 @06:12PM (#38718634)

    Ethanol is an extremely inefficient way to "store" solar energy.

    Corn ethanol is an extremely inefficient way to "store" solar energy.

    This whole boondoggle started because the U.S. always runs a corn surplus. The U.S. doesn't want a repeat of the 1930s, where crop failures led to hunger and near starvation, so the government deliberately subsidizes food production (mostly corn) to insure there's an oversupply. The question then becomes, what to do with all this extra corn? A lot of it is sent overseas as foreign aid. A bunch of it is converted to high fructose corn syrup, as a substitute for cane sugar. More still becomes grain feed for livestock, to satisfy our appetite for beef, milk, and cheese. And a few decades ago someone got the bright idea of converting it into ethanol to help ease the country's dependence on foreign oil.

    That's the reason the country started making corn ethanol instead of using a more energy-efficient crop like sugar beets. Unfortunately, somewhere along the line, it took on a life of its own, and under the influence of heavy lobbying we started growing corn for the sake of converting it into ethanol, rather than converting only excess corn into ethanol.

    Ethanol, provided you make it from a sugar-rich crop, is actually a pretty good way to gather and store solar energy for transportation applications. The alternative (PV solar to electricity to batteries to electric vehicle) is horribly expensive. Wholesale cost of PV solar electricity is about $0.20-$0.25 per kWh, vs. about $0.055 (wholesale) for coal. The Leaf is rated at 34 kWh per 100 miles, or $6.80-$8.50 per 100 miles at wholesale PV solar electricity prices. To travel 100 miles requires 411 kg of batteries (EPA rage of 73 miles on 300 kg).

    Brazil estimates its sugar cane ethanol costs $0.83/gal to produce []. If you figure a Leaf-like car would get 35 mpg, modify that for ethanol's 70% energy density vs. gasoline, that would mean 4.08 gal per 100 miles, or a cost of $3.36 per 100 miles at wholesale cane sugar ethanol prices. The 4.08 gallons needed to move the vehicle 100 miles would only weigh 12.1 kg. So sugar cane ethanol is 2x - 2.5x cheaper and 34x lighter than PV solar (this ignores the engine weight, but I'm just following the criteria of this argument - "storing" solar energy).

  • Re:10% Ethanol (Score:5, Informative)

    by Jane Q. Public ( 1010737 ) on Monday January 16, 2012 @06:30PM (#38718890)

    "Ethanol in fuel is what CAUSES the condensation..."

    Nonsense. Ethanol has been used as an additive to dry gasoline tanks since LONG before it was ever forced upon us for other reasons.

    Ethanol forms a permanent bond with water... up until the point where it is actually combusted. That is precisely why you can't distill 100% alcohol... you simply can't separate it from the water that way. It is possible to separate it chemically, but you really don't want to do that to your booze.

    You are just plain incorrect. Alcohol works fine in the winter to dry your gas tank. As others have pointed out, though, too much of it is not good for your gas mileage.

  • Re:10% Ethanol (Score:5, Informative)

    by toadlife ( 301863 ) on Monday January 16, 2012 @08:01PM (#38719670) Journal

    It's not the ethanol that's sapping your fuel economy, it's the climate. Cold air is denser, and causes the engine to run richer, i.e., inject more fuel into the engine. This gives you a bit more power, but at the expense of fuel efficiency.

    The denser air also provides more wind resistance.

    Also, your car takes much longer to warm up in the winter, and until the car reaches normal operating temperature, it runs extremely rich.

    Wet and/or icy road conditions probably also sap efficiency too.

  • BigCorn (Score:5, Informative)

    by mbkennel ( 97636 ) on Monday January 16, 2012 @08:14PM (#38719762)

    ArcherDanielsMidland, and Senators from Kansas, Nebraska, Illinois, Dakotas and Minnesota

  • Re:10% Ethanol (Score:5, Informative)

    by dryeo ( 100693 ) on Tuesday January 17, 2012 @12:15AM (#38721312)

    Backfiring is often detonation in the exhaust. I've had mild backfiring from having an exhaust leak close to the engine, during part of the cycle there is negative pressure in the exhaust and it sucks in air which mixes with unburnt fuel in the exhaust and detonates with a small bang.
    I've also had the engine die while in gear, gas goes into exhaust, engine comes back to life and you get a huge bang when the air + gas mixture in the muffler detonates. The odd time this has happened to me I thought the exhaust was blown totally off my truck.
    You can also get backfiring from broken non-sealing valves, same thing, gas and air gets in the exhaust then detonates loudly.
    The detonation you get from too low of octane (also too advanced ignition) might be better described as premature detonation or uneven detonation. Instead of a nice explosion starting at the spark plug and expanding evenly through the combustion chamber you get spontaneous detonations in different parts of the combustion chamber, often early detonations as well. This causes stress in the engine especially when the detonation happens too early while the piston is on the upstroke as well as excessive heat buildup.

How many NASA managers does it take to screw in a lightbulb? "That's a known problem... don't worry about it."