This propensity to premature ignition is measured by the octane rating of the fuel, where 2,2,4-trimethylpentane (isooctane) has an arbitrary value of 100, and heptane has a value of zero.
ðɪs pɹəpˈɛnsᵻɾi tə pɹɪmətʃˈʊɹ ɪɡnˈɪʃən ɪz mˈɛʒɚd baɪ ðɪ ˈɑːkteɪn ɹˈeɪɾɪŋ ʌvðə fjˈuːəl, wˌɛɹ tˈuː,tˈuː,fˈoːɹtɹˈaɪmɪθəlpˌɛnteɪn ˈaɪsuːktˌeɪn hɐz ɐn ˈɑːɹbɪtɹˌɛɹi vˈæljuː ʌv wˈʌnhˈʌndɹɪd, ænd hˈɛpteɪn hɐz ɐ vˈæljuː ʌv zˈiəɹoʊ.