Normally octane percentage is mentioned on petrol (95% octane). How is this defined? Is it percentage of chemical or some other parameters?

  • $\begingroup$ @Qmechanic Always wanted to learn about Hydrocarbon's in physics class $\endgroup$
    – Hatmix5
    Commented Nov 22, 2014 at 10:42

1 Answer 1


See the Wikipedia article on octane rating. 95% octane is the same as an octane rating of 95, and it means the petrol has the same knocking characteristics as a mixture of 95% iso-octane and 5% heptane.

  • $\begingroup$ Thank you for the answer. So if the higher octane number will it produce more energy? Or is it not even considered on octane indexing? $\endgroup$
    – sugunan
    Commented Nov 22, 2014 at 12:06
  • 2
    $\begingroup$ @sugunan: The amount of energy stored in the fuel is not related to the octane number. However a high octanc number allows the engine to use a higher compression ratio, and this increases the power output of the engine. So powerful engines tend to use high octance fuel, but only for its knock resistance, not because high octane fuels contain more energy. $\endgroup$ Commented Nov 22, 2014 at 12:15
  • $\begingroup$ Wow, only a 50% success rate in spelling octane :-) $\endgroup$ Commented Nov 22, 2014 at 20:04
  • $\begingroup$ I didn't get the point :S $\endgroup$
    – sugunan
    Commented Nov 23, 2014 at 16:09

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