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Octain requirements

Discussion in 'Daihatsu Hi jet' started by dale hynes, Feb 19, 2018.

  1. dale hynes

    dale hynes New Member

    I talked with several specialty shops who service JDM products, and was told that hijet engines run on 100-110 octain in Japan. I switched from regular 87 to 94 octain gasoline, the highest I can get in Canada. What a difference it made! Power everywhere even with 13 inch wheels. Has anyone tried E85 in one?
  2. Cole

    Cole Member

    Disspite what lots of people think running a higher octane doesn't provide more power in itself. Octane in a fuel is the resistance to detenation. That's why in high compression race style engines you need to run a 110+ octane fuel to keep the engine from detenating ( preignition) and killing the motor. Definitely in an engine set up for it, you should run it, for the engine's sake. But it most everyday engines it isn't necessary. Just thought I would throw that out there.
  3. fmartin_gila

    fmartin_gila Active Member

    If a lower octane fuel causes the engine to knock, retard the timing a couple degrees. Optimum timing advance is usually just short of the knock point while accelerating at WOT. Normally the published timing degree is a point where the manufacturers have found that the engine will not knock utilizing most any fuel which one may have accessable and if using high octane, one may usually advance the timing beyond the published value and gain a small amount of power.

  4. These little engines have pretty high compression ratios, to help them make as much power as they can with their limited displacements. They were tuned for a high octane fuel from the factory.

    Yes you can retard the timing to help prevent detonation, but that also increases side thrust on the bore and decreases the engine life, at the same time robbing power.

    We don’t have a lot of power to start with, so losing a little, is really noticeable.

    One thing to be aware of is that Japan uses the research octane number, which is based completely on a lab analysis on how much octane the fuel contains. This is known as the Research Octane Number, or RON.

    Another method of expressing the “octane level” is the motor octane number, or MON. Which is found by running a special engine, at a defined load, and rpm, and increasing the compression ratio with the engine running, until it starts to detonate, and putting that compression ratio into an equation. The motor octane level is always less than the research number

    In the US, and Canada, we use an “octane” rating system which averages the RON, and the MON, to come up with what is known as the Anti Knock Index.

    Since the MON, is typically, about 90% of the RON, the AKI, is roughly 95% of RON. So, a 100 octane fuel in Japan, would be shown on a North American pump as 95, and a 98 in Japan, would show on the pumps here as a 93.
    Last edited: Feb 22, 2018
  5. DWils

    DWils Member

    So, if 91 octane is the highest available locally, that is what we should use?
  6. dale hynes

    dale hynes New Member

    I have found that the difference between 89 and the 94 I use is very noticeable. If you are using 87 or 89 and try a tank of 91 I would think you will feel a difference.
    Right now 94 octain is $1.75 a litre, $6.60 per US gallon, so I’m not driving the truck as often as I would like.
  7. Also forgot: when you are running a normally aspirated engine, at altitude, you need less octane, because the drop of pressure as you go up, results in a lower effective compression in the cylinders.
  8. ttc

    ttc Active Member

    i always run high test fuel

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