Octane Grades

Discussion in 'Engines' started by Sgt. Pappy, Sep 9, 2007.

  1. Sgt. Pappy

    Sgt. Pappy Member

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    After looking some stuff up, I've found that the PN scale states that the 100 grade octane gives 100% power, while the 130 grade would give 130%, and 150, 150% and so on.

    My question is: How would an F4U perform given the same fuel grade as the Spitfire, that is, 100 grade as opposed to the US 130 grade? Is it really as simple as '30% worse than originally'? Or are the grades close enough to disallow a large throttle setting drawback (i.e. late Spitfires had a throttle wall block which could be opened if the Spit was using 150 grade or higher, but the wall had to be closed if under 150 grade)?

    Thanks for the help!
     
  2. P-STICKNEY

    P-STICKNEY New Member

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    There may be a bit of misunderstanding, here.
    While PN does represent "Performance NUmber", the
    performance being referenced isn't overall engine performance, but the
    knock resistance of the fuel relative to a gasoline made up of 100% iso-octane.
    Basically. it's a measure of the resistance of the fuel to detonation (knocking).
    The energy content of various grades of gasoline is for all intents and purposes identical.
    Below the point where detonation occurs, engine performance will be the same for a given RPM and Manifold Absolute Pressure.
    In terms of allowable power settings, effectively there waws no difference between the 100 Octane fuel used by the RAF (And the USAAF in the UK, for a time, and the U.S. Standard 100/130 fuel. The numbers that you see published for the R2800 engine in an F4U-1 or F4U-4 are for 100/130 octane fuel. Using postwar 115.145 Octane, basically the same as teh RAF's 150 PN fuel, The R2800 C-Series engine on an F4U-4 could be run at power levels up to a sea level rating of about 2300 HP. (A gain of 200 HP)

    The Throttle Gate (Stop) on both Brit and American airplanes was set tp provide a MAP (Throttle controls Manifold Pressure, Prop Governor controls RPMs) corresponding to Takeoff Power. (And Military Power in engines rated by the U.S.)
    No matter what fuel was being used, the throttle had to be pushed past the gate (Either by moving the Boost Control Cutout stop on a Brit engine, or pushing the throttle lever through the safety wire stop in the "Gate" of the throttleon a U.S. airplane.

    --
    Pete Stickney
     
  3. Sgt. Pappy

    Sgt. Pappy Member

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    Oh I see. Thanks so much for clearing that up... I can finally write my paper with peace of mind. Woots.

    So essentially, you're saying that the WWII US 130 grade had the same properties and was used in the same way as the British and American 100 grade as well?
     
  4. mad_max

    mad_max Member

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    In the Merlin 100 octane was good for 61" of boost. 100/130 was good for 67" of boost.
    There is no difference if running both fuels at 61" of boost. Exact same power.

    67" of boost provided more power, but you could only make this power as long as
    the supercharger provided 67"; which of course would be at a lower alt. than 61" in both
    gears "stages" of operation.

    I have a British chart that will show you the alt. limits for 72" with 100/150, 67" with 100/130,
    and the alt. where 61" peaks. Over the 61" alt. the supercharger can't provide 61" anymore
    and the boost will decrease as alt. increases.

    If you want them PM me your email. I have not set up a photo bucket type place yet, but can
    file attach them to you.
     
  5. P-STICKNEY

    P-STICKNEY New Member

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    Pappy,
    Yes, that's precisely it - Brit 100 Octane and US 100/130 are completely interchangable.
    A note about the rating numbers. The forst (smaller) number is the anti-knock rating for lean mixture - more efficient burning, used for cruise and such.. The second number is the antiknock rating for Rich Mixture - less efficent, but the charge cooling of the rich mixture allows higher boost.
    The Brit 150 PN fuel was actually, although some references put it as 100/150 Octane, actually about 110/150. The U.S. Equivalent was 115/145.
    mad_max has a good point about the altitudes that those powers were developed -
    above the altitude where the supercharger system (Multiple stages in most cases) can produce a given Manifold Pressure, there's no difference. The performance gains will be at lower altitudes, where hte supercharger can deliver the required output pressure.

    --
    Pete Stickney
     
  6. Sgt. Pappy

    Sgt. Pappy Member

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    Alright thanks very much! I'm getting a much better understanding now.

    So it's all due to the supercharger properties in this case. All I remember is the Merlin 45 blower diameter for F., HF. and LF. Spitfires.. I'll look up the list later on.
     
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