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Fuel Testing

87 Octane or 93 Octane?

Some time ago, I thought I started to see a pattern developing while dyno testing my V-Max. It seemed that a mix of 87 and 93 octane fuel was developing more power than just 93 octane fuel alone. I was intrigued enough to test the two fuels and it rendered this result:

The above chart is the result of back to back test performed within 13 minutes of each other. Also, the engine was at the same operating temp for both run # 2's as verified by infrared temp gun on the cylinder head. 

After seeing this conclusive answer, I am left to believe other factors had caused the slight increase I had noticed in previous days of dyno testing with 87/93 Octane fuel mix. I already knew a few things about fuel and timing from the Fuel Injection project I was involved in some time ago. Before that project, I had a few misconceptions about the Octane rating of fuel. My first was higher Octane fuel was more explosive and burned faster, therefore it made more power. Completely wrong. Higher Octane fuel burns slower and is more reluctant to pre-ignite or predetonate. Here is a good web site to read about Motorcycles and Fuel Octane.

I think what Yamaha did was match the ignition timing to 93 Octane fuel because of the cylinder pressure created by this high compression engine. The engine may not knock from a lower Octane fuel, but that does not mean it has created it's peak cylinder pressure at the correct time. Also, since the air is thinner at higher altitudes, this means the cylinder pressure will be lower and a lower octane fuel may produce more horsepower. Since I am 45ft above sea level, I will be running 93 Octane fuel, as Yamaha recommends.