505 fuel octane
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Thread: 505 fuel octane

  1. #1
    Fellow Frogger!
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    Default 505 fuel octane

    hello
    my 505 has a 7.5 compression ratio engine, and as such it needs 88 RON octane. obviously this is not available these days, so i'm using 95 unleaded. the ignition is set, using a vacuum gauge, at about 14 degrees btdc instead of 10, in order to keep a steady reading of 18, and it drives ok now, but am i missing something?
    maybe it needs bigger/smaller jets? the book says 140+-2.5 for main, 220 for air corrector.
    thx

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  2. #2
    Fellow Frogger!
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    Assuming that the distributor and carb. were designed for this low compression engine then I think it's a mistake to play with timing or jetting. There is no problem running a fuel that is more knock resistant than necessary, and there is nothing but harm in sparking too early just because the high octane fuel will tolerate it.

    If you're keen to optimise and don't want to pay for a dyno, you could time some full throttle accelerations point-to-point on a quiet stretch of road, trying various advance settings. Shortest time is where the engine's happiest. At least you're unlikely to have problems with pinging. It will be a compromise since you are only able to advance/retard the entire advance curve.

    One thing that can be well worth checking is that the distributor isn't worn; that it's still giving the advance curve M.Peugeot intended.

    Have fun,

    Rob.

  3. #3
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    higher octane fuel also burns more slowly, doesn't it?

    i have a 123ignition dizzy, and i set the timing with vacuum gauge. at 10 degrees as per the book the reading is low and unstable.

  4. #4
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    Higher octane fuel doesn't burn more slowly, it is just more resistant to detonation (i.e. exploding all at once under pressure).

    The 123 dizzy pretty much eliminates the distributor being out of calibration -- though I suppose it might not be set with the right advance curve for this low compression engine. Will assume it's right.

    You're having trouble with a rough idle, but timing at idle is not very critical. Engines typically use ported vacuum to give a very retarded idle; this is to increase heat in the exhaust and reduce emissions. Adding a bit of advance can certainly smooth up the idle, but doing it by adjusting the whole distributor also advances everything else meaning you quite likely end up with too much advance at revs and under load when timing is much more critical.

    When I was converting my 604 to EFI, I ran it for several months with straight manifold vacuum to the distributor. This gave it something like 23 advance at idle and it idled beautifully. If you're feeling like experimenting, you might try running full manifold vacuum to your dizzy. Adjust the timing with the vacuum advance disconnected, a few tweaks to idle speed and mixture should have it idling smoothly without compromising the rest of the rev range.

    About the only danger I see in this (besides emissions) is that there might be a lag in getting the ignition retarded when you go from idle to planting your foot. This didn't trouble me with the 604. Quite likely the 123 dizzy's vacuum advance would respond even quicker.

    Have fun,

    Rob.

  5. #5
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    Eventually i fitted a bigger idle jet- 50 instead of 45 (the data for this engine states 47 +/- 5), and now it idles fine and shows 18 on the vacuum gauge at the official 10 degrees.

  6. #6
    1000+ Posts Wildebeest's Avatar
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    2pac,
    I'm thinking that the idle jet size difference is minimal and that you may have dislodged some dirt in the change over. Could have been your problem all along ?
    Don't bother with changing back, the 50 should be fine.
    2pac likes this.

  7. #7
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    Good result. Glad you got it sorted.

    Have fun,

    Rob.
    2pac likes this.

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