Thread: gas octane

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a thread on compression ratios has just got me thinking....

with u guys giving great advice on my motor, the CR question would crop up sumwhere. around 9:1 is the norm for a street motor isnt it?

what octane do you have in america cos over here the standard is 91-3 with most places selling upto 98 octane. is this higher than what you have, and so could i run higher compression?

Before you can compare octane ratings you need to know which testing method the posted number represents. In the States there are three methods, two of which your folks probably use, and maybe more depending on who controls what's published.

The two standards for the fuel industry are Research Octane Number (RON), and Motor Octane Number (MON). The same fuel tested against these two methods will produce different numbers. A higher value will result from the RON method than from the MON method. The MON is a more severe test. These were developed eons ago for different classes of trade useage.

The number we post on the pumps here is an average of the two methods mentioned above, it's known as Anti Knock Index (AKI). It was "invented" by our government about 30 years ago "to make it easier for the consumer". Unfortunately most manufacturers were using the RON standard values in their manuals and literature, so it really caused more confusion for the general public who usually had no awareness of all the different methods. Some would say, "typical government deal". In years past my experience with European vehicles had them using the RON method in their listings. Today I'm not in that loop so I don't know, you'll need to figure out what your people are doing.

Just to add more clarification by example. Depending on geographical considerations, the typical highest octane fuel at our stations is 91 or 92 AKI. Given common fuel variances this same fuel would yield a RON rating of 95/96 and a MON rating of 86/87. These numbers could be different in reality depending on the stock the fuel is made from.