Iso Octane does exist and it's the standard which helps determine a fuels anti-knock or Octane rating. The most accurate octane test is performed with an octane motor which is fed pure Iso Octane at a controlled ambient temp, and humidity, with a correction factor for atmospheric pressure. The Octane motor has a variable compression ratio and is run at a steady load during calibration and testing. During calibration while running on pure Iso Octane (which has an octane rating of 100) the compression ratio is adjusted to set the engine at the knock threshold by using knock detection sensors. Once calibrated the engine is fed the test fuel to be graded. If the engine detonates or knocks the compression ratio is reduced until knock can no longer be detected. Based on the amount of compression ratio change the fuel is given a number for it's anti-knock value or it's octane rating. In this example since the compression ratio had to be reduced to eliminate the knock the fuel has an octane rating of less than 100, if the compression ratio had to be increased to create a knocking condition then the fuel would be rated at greater than 100 octane. This is also known as the Motor Octane Rating. Based on decades of fuel testing chemists can do a good job of determining what a fuels octane rating will be based on the blend of the fuel (Research Octane) so Motor Octane Testing is rarely done anymore. I got a chance to sit through a Motor Octane test at a refinery years ago. Interesting enough they were performing fuel tests for a competitor which did not have an Octane Motor in their own lab. The motor's are large single cylinders with a threaded cylinder which can literally be screwed up and down to change the compression ratio while the engines running, pretty cool stuff. John
