Motorcycle Myth #4 – Unleaded fuels make less power

Discussion in 'Dirt Bike Mods & Maintenance' started by Rich Rohrich, Jun 17, 2014.

  1. Rich Rohrich

    Rich Rohrich BioHazard
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    Myth #4 – Unleaded fuels make less power because they have lower octane values and produce less energy.


    This one seems to be a favorite of the California magazine guys and like so much of what they publish they would better serve their readers by doing some homework on fuel specs, combustion chemistry and how engines produce power before going to print.

    Lead has always been a very inexpensive way to raise the RON and especially the MON octane rating of a fuel. It also works really well at giving the tuner an extra margin of safety when tuning close to the edge due to it's chemical properties and the way it influences active radical formation and the delay period before detonation. If you guys are interested I'll be happy to bore you to death with some of the details[​IMG]at a later date,

    For late model engines (2 & 4 stroke) lead serves no other useful purpose than raising octane and influencing detonation tolerance . Stories of it saving valves, running cooler, or making bottom ends last longer is total bunk when talking about the engines built in the last 10-15 years. It does tend to dirty things up in the engine for both plugs and valves so there are some downsides.

    The only way lead can make an engine run cooler is if it were detonating without lead and the octane increase with lead stopped the detonation. Any fuel with similar distillation characteristics that satisfied the engine’s octane requirement and stopped the detonation would have the same effect on engine temps whether it had lead or not. In other words, adding lead to the fuel of an engine that is not detonating will not change the combustion chamber temps to any measurable degree.


    As for the “energy” argument, that just proves some of these goofs spent too much time smoking weed in high school instead of cracking a Chemistry book.



    Let's take a look at a couple of the high quality Firepower brand race fuels and see if we can't get a better idea about this whole fuel energy thing :

    Firepower Leaded 893 - 118 octane (6.00 grams lead /gallon)
    Firepower Unleaded 324 - 101 octane (oxygenated no lead)


    If you look at the heat energy (heat of combustion) available from a pound of fuel you get :

    Leaded 893 19020 BTU/lb
    Unleaded 324 18747 BTU/lb

    If you only looked this far at the spec sheet it would appear that the leaded race fuel would make more power due to its higher energy value, but that isn't the case. You can't just look at the heat energy of the fuel; you have to factor in how much fuel will react with a pound of air to release that energy. In other words you can't tell how much energy is really released until you divide the true air fuel ratio of a fuel with the heat of combustion value to get what is commonly referred to as the specific heat or combined heat of the fuel.

    So let’s take a look at what these same fuels look with that in mind.


    Leaded 893 19020 BTU/lb of air divided by its air/fuel ratio of 15.29:1 gives us a specific heat value of 19020/15.29 = 1244 BTU/lb of air

    Unleaded 324 18747 BTU/lb of air divided by its air/fuel ratio of 13.84:1 gives us a specific heat value of 18747/13.84 = 1355 BTU/lb of air

    Well look at that, the unleaded fuel will produce more usable energy when all is said and done.

    This is just one example to illustrate a point. If you look at the heat energy value of methanol it's only 8750 BTU, yet due to its very low air/fuel ratio 6.47:1 it has a specific energy of 1352 BTU/lb of air.



    Dr. David Redszus of FirePower Fuels has shown me on numerous occasions that almost all legal fuel blends make about the same power when tuned correctly. The difference between the highest and lowest fuels in terms of BTU/lb air is only 4% unless you start getting into the exotic stuff.Other factors are much more significant so don't worry about the heat energy. While it may be sort of fun to dissect it like this, it is for our purposes pretty unimportant and ultimately so is whether or not there is lead in the fuel.
     
    Ol'89r and Okiewan like this.
  2. 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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