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Motorcycle Myths : #1

Discussion in 'Dirt Bike Mods & Maintenance' started by Rich Rohrich, Feb 20, 2014.

  1. Rich Rohrich

    Rich Rohrich BioHazard
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    Some of you older guys might remember an article Gordon Jennings wrote many years ago debunking various motorcycle myths. It was a fantastic article full of interesting facts and lots of good humor. I'm certainly no Gordon Jennings but I'd like to revive that idea with an article of my own but I'd like some thoughts from you guys on myths that you would like to see addressed.



    Here's my starting point for the article with a couple of pet peeve myths of mine :


    Myth # 1 – Toluene is a magic fuel additive that adds power by just dumping it in gas tank

    Reference grade pure toluene vaporizes in the 240-250F range with a basically flat curve, but only lab guys with big checkbooks get that stuff. The commercially available stuff (Home Depot, paint store etc. ) has lots of distillation byproducts in it so just like pump fuel it has a high temperature distillation curve that varies from load to load, so you'll see a curve with anything in the 280- +400F range in commercial grade Toluene.

    Anything in this temperature range isn't likely to vaporize above 7000rpm in current MX engines so all the additive accomplishes is tweaking the distillation curve and the overall fuel curve i.e the jetting.

    There is no extra energy release value to Toluene either as the gross combustion value for it is on par with everything else normally used in fuels.

    Toluene is a decent additive if all you want to do is protect the engine from fragging itself due to detonation. It has reasonably high octane and it is relatively cheap. The downside is because additives like these are fairly reluctant to vaporize they have proven to make for sloppy throttle response, poor carb circuit transitions and poor high rpm running in engines like ours that have short intake tracts and run for sustained periods above 7000 rpm. The greater the percentage of Toluene the more these issues are likely to surface. This will tend to be true for Toluene based octane boosters as well.

    I’ve seen a number of instances of Internet automotive sites taking little chunks of info and using them out of context in an attempt to prove the efficacy of Toluene in producing additional power by virtue of “heat of combustion” numbers. In doing this they are are ignoring the fact that the maximum combustion heat produced by any fuel can be expressed as a function of its stoichiometric ratio and net heat of combustion. While fuels may have varying heats of combustion and stoichiometric ratios, when compared by their heat of combustion per pound of air, most aromatic hydrocarbons toluene included, and standard pump fuels are all pretty similar.

    For those who haven't dozed off, here's how it works:

    Reference grade toluene has a net heat of combustion = 17424 (btu/lb), and it's chemically correct (stoichiometric) A/F ratio is about 13.8:1 . This comes out to a heat of combustion per pound of air value of 1262. Average pump premium has net heat of combustion in the neighborhood of 19,000 (btu/lb) andthe A/F ratio is about 15:1 which comes out to a heat of combustion per pound of air value of 1266 just a touch better but nothing we would notice. A good oxygenated race fuel like Firepower 324)has net heat of combustion of 18,747 (btu/lb) and the A/F ratio is 13.84 which comes out to a heat of combustion per pound of air value of 1355, an increase of about 9% over standard pump fuel.

    The point of all this you ask? Usable energy values only really change when you go to specialty race fuels that contain oxygen bearing components or other voodoo components like isoprene. If you have an engine that is detonating and dump a load of any aromatic hydrocarbon in your fuel and you feel a difference it's likely you just modified the fuel curve in a positive way, which is something you could have done just as easily with brass aka JETTING.

    A little more about Toluene and combustion chemistry.

    The willingness of a hydrocarbon based fuel to ignite is essentially a function of it's ability to vaporize. Once it's vaporized they ALL light off pretty much the same. Pump fuel is adjusted throughout the year in an attempt to ensure that the fuel will vaporize quick enough for the lowest temperatures encountered but not so easily that you run into a vapor lock condition in warm temps.

    If you pick a high octane race that has a vaporization curve that is poorly suited to your riding conditions you can effect it's starting behavior. Fuels that vaporize easily will tend to richen the jetting while fuels that take large amounts of heat to vaporize will in effect lean the mixture and force you to compensate with richer jetting.

    ... to be continued
     
    #1 Rich Rohrich, Feb 20, 2014
    Last edited: Feb 25, 2014
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  2. Mully

    Mully AssClown SuperPowers
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    Rich, anytime I can learn from someone who knows is always good. DO IT !!
     
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  3. RM_guy

    RM_guy Scared of DirtWeek<BR>Club *********
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    Very much interested! There are so many myths and just down right bad and sometimes dangerous information out there that we need something like this. :)
     
  4. Okiewan

    Okiewan DRN is my fault.
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    Yes, more please.
     
  5. Papakeith

    Papakeith COTT Champ Emeritus
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    I can always learn more. Thanks for posting this Rich
     
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  6. RM_guy

    RM_guy Scared of DirtWeek<BR>Club *********
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    Nice! Keep 'em coming!
     
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  7. Okiewan

    Okiewan DRN is my fault.
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    Motorcycle myths now how their own threads in this fourm.
     
  8. Rich Rohrich

    Rich Rohrich BioHazard
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    Now that the season is in full swing lets push a few threads back to the top and see if people want to contribute to the discussion.
     

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