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The test involves a standard test motor fitted with a detonation sensor. The blend being tested is compared to standard blends containing only normal heptane (defined as having an octane number of 0) and iso-octane (defined as having an octane number of 100). The octane number of any material is equal to the volume percent of iso-octane in a normal heptane/iso-octane mixture having the same detonation charateristics as the material being tested. That is, a 92 RON material will have the same detonation charateristics in the Research Octane test as a mixture containing 92% iso-octane and 8% normal heptane. As far as combustion temperatures are concerned, normal heptane (0 octane) releases 118,668 BTU/gal of heat when burned and iso-octane (100) releases 119,388 BTU/gal so iso-octane actually contains slightly more energy/gal than n-heptane. The difference is small, however, less than 1% for a 100 octane difference. This slight amount of extra energy is, however spread across more mass. As a result, the adiabatic temperature for 100 octane iso-octane is 99.45% of the adiabatic temperature for 0 octane normal heptane (slightly colder, but not by much). For real world gasolines, the octane differences are much smaller - in mos no more than 5 or six numbers between regular an premium grades. The differences will be much less - essentially zero for all intents and purposes.
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