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Which Octane runs cooler??

Discussion in 'Technical and Troubleshooting Torque' started by vossy53, Oct 19, 2010.

  1. Good day... This is one for the engineers. I thought that I read somewhere that the higher the Octane content in petrol the cooler the bikes engine runs! ie an engine (high compression/high revving type) runs cooler using the 95 PULP and the 98 runs cooler than the 95...
    Any comments?

  2. lol, i remember learning about octane ratings in 2nd year, but ive forgotten half of it.

    From my understanding, that is not necessarily true.

    The octane rating is the fuels ability to resist premature ignition and hence engine knocking. I will point out that this premature ignition is brought about by pressure. When the critical pressure of the fuel is exceeded, this results in an explosion. This explosion is out of phase and this causes loss in power. Pinging (or knocking) is the result of a localised explosion against the chamber wall. They can also cause damage due to the high impact.

    This is in contrast to normal function, where the spark brings about a controlled burn (not explosion)

    Increasing the octane rating is usually achieved by the addition of compounds which increase the critical pressure.

    Back in the good ol' days, lead was added, as this was excellent at increasing octane rating. However, as we all know, lead is quite toxic, and so lead was outlawed, hence we now have unleaded fuel.

    So temperature has not a lot to do with it, its mainly pressure.
    The only reason i can think that temperature may change, is that additives burn with different temperatures, however, i dont think they make up a large enough portion of the fuel to have any significant effect.

    It is possible that the different temperatures you hear about are in reference to the new E10 fuel which contains up to 10% ethanol. Ethanol burns much hotter than iso-octane and this difference IS significant. This is why some cars/bikes cannot run E10 as the higher temperatures will screw up some part of the engine (however i dont remember which part).
    However although there is a temperature difference, im not 100% sure its the problem. Like I said, i've forgotten half of it :p

    Hopefully someone can confirm/correct this

    Aside: while we're at it, what about diesel? Diesel engines have no spark plugs. Instead, the pistons compress the air, to a pressure above the critical pressure of diesel. Once at the top of the stroke, the fuel is squirted into the chamber where it immediately combusts due to the pressure. So diesel engines operate entirely on the phenomenon we are trying to avoid in cars. Ha
  3. Octane is the fuel's resistance to detonation.
    Higher octane allows the use of higher compression ratios or forced induction, using higher octane than the engine is designed for is overall safer due to the variations in fuel from servo to servo but otherwise will make no difference.

    It won't affect temp.

    American/Australian vehicles are mostly designed for 91 octane..
    Japanese are designed mostly on 96, so using standard crappy unleaded can be dangerous for someone who likes to rev.

    If you see the word ethanol at all.. avoid it
    Unless your car/bike is SPECIFICALLY designed to run on ethanol

    Hope that helps

    [edit]: a little more on the ethanol thing...
    What I'm lead to believe is that if you use ethanol in a car not designed to handle it, and then park somewhere for an extended period of time the fuel starts to corrode the weaker parts of the fuel system. I have also personally seen a case where the fuel started to form precipitation after a week of sitting around and the chunkiness then proceeded to block fuel flow almost entirely stopping the engine.

    Using it as a last resort is ok as long as its all used up and the tank is then refilled with regular fuel and run through the engine a few mins to wash out the ethanol before parking overnight.

    These issues are mostly seen on things built pre-2000, most newer things can handle it better but I'd still rather be safe than sorry.
  4. It's a bit of a loaded question, but here's how it works.
    A higher octane fuel burns slower, and needs a higher energy to ignite. This can be achieved through a hotter spark, but usually higher compression. These two characteristics combine, to give higher octane level fuels less tendency to pre-ignite.

    Now depending on what sort of engine you are running them in, pre-ignition of fuel (knocking) causes an engine to run hotter, and have decreased efficiency. More modern efi engines, have knock sensors and the ability to automatically retard and advance the igniton timing, so it is less of an issue, but certainly in an older engine, if the timing is out, and the engine is knocking, running a higher octane level can help reduce this effect, and the resulting heat. A better solution would be to just fix the timing.

    So in circumstances, yes, higher octane can reduce temperature, but it is neither a direct effect of octane rating, nor the intended operation of such a fuel.

    So in the example of your high revving, high compression example, this would depend on the condition of the engine and it's state of tune as to whether it is suffering from ignition problems or not, for fuel to effect its operating temperature.

    Hope this helps you out.
  5. Conversely, many older air-cooled engines tend to pink a bit when very hot, such as after sitting in traffic for a while. Using a higher octane fuel can reduce or eliminate this tendency, not because the engine runs cooler but because the fuel is better able to resist the effects of the high temperature.
  6. Da one with nitrous mixed with it
  7. hmmm, now that i think about it, i would really like to get into reading some serious research regarding octane ratings, would be an interesting holiday task. Researching something i might actually find interesting lol

    Just along the lines of this thread, i remember reading a thread (not netrider) about 6 months ago, and there was one really good post in there. But i can for the life of me remember where it was :(
  8. how about poking your eye out with a blunt spoon :)
  9. lol, now i read my post, its kind of depressing
  10. In that case, as a start, I recommend reading anything you can find by or about Harry Ricardo, the man who, quite literally, wrote the book on how the petrol engine actually works. I've only read his autobiography but that, in itself, was pretty good.
  11. E85 is where it's at, especially if you have a turbo hanging off the side of the block, shame about the consumption though...
  12. Step one: Run several tanks of different fuel types (91, 95 & 98).
    Step two: Pick the one that runs best for your bike.


    Alternatively, you can run 98 octane goodness (assuming your bike doesn't run best on this) and then tune to suit. This involves carburettor work or fuel injection tuning to suit, and also advancing the timing events to take full advantage of the fuels characteristics. This can be done by yourself with the proper gear, or a dynotuning centre.

    Cheers - boingk.
  13. With spare time, spare cash and spare bike Id like to mess around with water injection and advance that sucker +50 degrees BTDC :grin:

    Something like a bandit or gsx1400 would be a fun candidate.
  14. Just run methanol and give it a zillion to one compression ratio. Oodles of power and it'll smell nice :D.

    (you may have to leave the jets out :LOL:).
  15. +1 methanol FTW
  16. I suspect using a higher octane than the timing+compression is tuned for will result in cooler temperatures. As people have mentioned, the higher octane is harder to ignite at a given temperature/pressure, which means it will burn slower which equates to slight retardation. I suspect this reduces compression pressure? It also causes incomplete combustion which leaves carbon deposits* on valves and pistons... If less burning is going on, that would imply cooler temperatures...

    BUT... if, because of the retardation, burning continues into the exhaust headers when the valves are open, that could cause them to heat up more than usual, increasing "perceived temperature". Don't know if this is the case, just speculating.

    When you add in automatic advance/retardation (do modern bikes do this? mine doesn't), then you can't really tell what the story is...

    * If it gets bad enough, it will increase compression and start glowing red, both increasing temperature.

    I'm not a mechanical engineer though, would love it if someone set the record straight.
  17. Spanner in the works.

    Higher octane fuels burn slower, so there is more time for the energy to be absorbed by the block.

    Also higher octane fuels tend to have more complex bonds in the carbon chain, therefore they have more energy.

    So an engine running a higher compression ratio to cope with the higher octane fuel would definitely most likely run hotter. Sort of.

    Running higher octane fuel in a 91 block? The fuel has more energy, but it doesn't necessarily translate into more power at the wheel. Where does it go? Exhaust and cooling is the only place it can be. This is provided combustion is complete of course.

    Just spurring the debate along.
  18. I doubt if any modern bike engine lacks auto-advance. AFAIK, this is now handled by the electronics of the ignition unit/ECU, so you won't find little springs and bobweights anywhere. Fixed timing is for lawnmowers.
  19. I'm staying well away from this one because the chemical side of engineering ain't my strong point at all. :D

    Pat, I think blackjacket meant in the sense of automagical knock/pinging detection, where the engine detects that it's suffering from predetonation and retards the ignition and takes other precautions.

    I don't know of any engine management system which automatically *advances* timing, though. Like, say, if you fed the bike 110 octane race fuel, it wouldn't automatically transform itself into some fire-breathing race-tuned engine with ludicrous amount of ignition advance.

    To the best of my knowledge most modern vehicles simply have the pre-programmed map for operation with the octane rating the vehicle is designed to use, and then a "fallback position" for when predetonation/pinging is detected.

    I, too, also do not know if my motorcycle can identify pinging and take measures. Haven't been game to try it.
  20. I've seen little doohickeys on the heads of car engines referred to as knock sensors. Presumably they're some sort of solid state accelerometer that detects the hammerblow that pinking delivers to the engine internals. IIRC, even my primitive and horrible XF Fairmont EFI had one, mounted to one of the front head studs.

    I haven't seen anything similar on a bike engine, but then, I haven't been looking.

    On a related note, I used to set the timing on my wet weather hack VW Beetle by ear. The dizzy was so worn that, checked with a strobe, the timing was all over the place. I'd get it close to right and then give it a bootful in top on a local hill. If it pinked under such provocation, I'd whip out the 10mm spanner and twiddle the distributor round a bit and have another go until it wouldn't quite. By doing so, I discovered I could use several degrees more advance on Premium than with regular unleaded, with a corresponding improvement in mpg. Almost enough to cover the extra cost of the PULP :D.

    Not sure if I'd risk it with a modern bike engine, but then, I haven't had to adjust ignition timing for 15 years or more, since I got rid of my last MZ.