Octane Ratings

[JohnMyer]

I recently purchased a 1963 Buick Skylark convertible. In reading the Owners Guide, it states that the Skylark requires fuel having an octane rating of at least "93 Research Method" or "84 Motor Method". How are these ratings determined and how do they relate to the typical octane ratings of 87, 89 or 93 that is typically shown on the various grades of fuel pumps?<P>I will use premium fuel in the Skylark, but I want to understand these octane ratings described in the Owners Guide.

[Guest]

John-I'm not an engineer and I am relying on memory. But here goes-The difference between research and motor is similar to the difference between real and expected. Research is a math determined figure and motor is experimentally derived. BTW the 93 figure your book is requiring is going to be about the same as the current 93. I hope my memory is serving me well on this issue; if not maybe someone will correct me.

[ply33]

All octane ratings are made by comparing the fuel to be checked against a test mixture of iso-octane and another chemical (memory fails on what that other chemical is). The tests are all performed in a special one cylinder test engine designed specifically for this purpose. The test mixture is varied until it provides the same amount of knock in the test cycle as the fuel to be checked. The percentage of iso-octane in the test mixture that matches the fuel to be checked is the octane rating of the fuel.<P>There are lots of different test cycles the one cylinder engine can be put through. The two used most often are the "research cycle" and the "motor cycle". Thus the two octane ratings you hear about are Research Octane and Motor Octane.<P>For purposes of selling gas to the public, the octane rating on the pump is, by law, the average of the research and motor octane. Note that really good gas has a research octane very close to the motor octane. You can create the same "pump" octane with gas that has the same average but a wider spread between the research and motor octane ratings. The two gasolines will have the same pump octane rating but not necessarily the same performance in your car. For example if your car needs a motor octane of 84 and the 87 octane gas has a motor octane of 86 and a research octane of 88, you can use the 87 octane. But if the 87 octane gas has a motor octane of 82 and a research octane of 92 your car may not be happy. Two different 87 octane gasolines will seem to perform differently in your car.<P>So for a car that required a Research Octane of 93 and/or Motor Octane of 84, your average is 88.5. So I'd start with the 89 octane that in this area is the intermediate grade. Of course, if your engine is knocking, go to a higher octane.

[JohnMyer]

Thanks guys for your responses on how octane is determined.

[Guest]

Ply33 had a good explanation. The other chemical is heptane. Al