Re: Which fuel is recommended?

Posted by Rusty O\'Toole On 2008/7/24 8:48:13

As I said it is not an exact law. Some engines of the same compression ratio, may be happy with a lower octane fuel, others require a little higher.

It depends on the design of the engine, the combustion chamber, even the amount of carbon buildup in the head which can raise the compression pressure.

In your case, for 8.5:1 I would have said "above 85 octane should be OK". I notice you say it runs fine on 89 octane, and the factory recommended 90 so we are all in the same ballpark.

Have you tried an 85 octane gas or is 89 the lowest available?

Here is another thing most people don't know. There are actually 2 different systems of calculating octane ratings, the Motor and the Research method. They differ slightly from each other.

Since the 70s advertised octane ratings have been the average of the Motor and Research ratings. So if you are looking in an old book today's gas could be called 89 octane, but be the same as the old 87 octane or 90 octane depending which rating they used.