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Thread: Premium Fuel

  1. #1
    Junior Member Tferre's Avatar
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    I have a 87 t dub. On my bike the gas cap says "Regular Fuel" so I've always ran regular. I read a thread were some guys said they only run premium fuel in theres. So I went out riding this last weekend and filled up with premium. Wow big mistake. It was to rich for my engine and ran rough all day. I ended up draining my tank and filling with regular, and it returned to running great. Does anyone know why this is?
    Welcome to Utah, the State of Adventure!

  2. #2
    Senior Member Xracer's Avatar
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    Was it E- whatever and did you check for water? Or maybe it's just your Dubs way of telling you it's ok to be cheap!!

  3. #3
    Member truepath's Avatar
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    Take a look at the "Clark Tank" discussion a few topics down and there is a big discussion going on now about all things having to do with gasoline. There have been some very good comments there and they should answer your question.

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  5. #4
    Junior Member Tferre's Avatar
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    No there wasnt water, and I added a touch of seafoam (my favorite additive for everything).
    Welcome to Utah, the State of Adventure!

  6. #5
    Senior Member bikerjosh's Avatar
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    I remember some thread on the old forum, explaining that premium fuel wasn't great for small displacement aircooled motors. Something about making them run hotter. Never really gave it any thought since I've always run 87 octane unleaded and a shot glass of sea foam in each tank, since it sits a lot between rides or mods.

    thanks Josh
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  7. #6
    Senior Member fishhuntr1's Avatar
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    Higher octane fuel resists detonation. Another way of looking at it, is it's harder to light and actually has less energy. With a non-sophisticated engine (no variable timing, knock sensors, etc.) there's no advantage to using higher octane than required to avert detonation.

  8. #7
    Banned qwerty's Avatar
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    Quote Originally Posted by fishhuntr1 View Post
    Higher octane fuel resists detonation. Another way of looking at it, is it's harder to light and actually has less energy. With a non-sophisticated engine (no variable timing, knock sensors, etc.) there's no advantage to using higher octane than required to avert detonation.
    Octane does not affect potential energy level of gasoline. Octane affects how quickly the flame front propagates through the combustion chamber. Burning fuel raises the temperature of the gasses in the combustion chamber, increasing the temperature of the gasses in the combustion chamber increases the pressure those gasses exert on the walls of the combustion chamber (which includes the top of the piston), and it is the pressure on the top of the piston that pushes the piston down, which pushes on the rod, which pushes on the crank, etc. Since the flame front propagates slower, the maximum cylinder pressure may not be generated until well after the piston passes top dead center on the power stroke, which means the push on the top of the piston does not begin until after top dead center. Best power is delivered when the push begins just after the piston passes top dead center. If the push doesn't begin until 20* after top dead center, no power is generated for that 20* arc of crankshaft revolution. That is why higher octane fuels require advanced ignition timing to make power--the flame front needs more time to generate heat and pressure. In some engines, higher octane fuels simply don't burn as efficiently as lower octane fuels. It is the lower efficiency that results in lower engine output, not lower energy content of the fuel.




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