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305 Small-Block Build - Questions and Answers - What's Your Problem?

Have questions or concerns? Jeff Smith has answers.

The Octane Game
Ronald Corte; Draper, UT: Is there any specific reason why the eastern half of the United States gets 93-octane premium unleaded and the western half only gets 91-octane gasoline? I made a trip last year to Virginia and was able to fill up using 93-octane fuel, and my Jeep SRT8 ran great. When I crossed the Missouri River and started using 91-octane fuel, I noticed a difference. The prices seem to be the same for either grade, so in a sense, I feel I am paying more for less with the 91-octane fuel. Why not 93-octane for the whole country?

Jeff Smith: We presented your question to our friend Tim Wusz of Rockett Brand Racing Fuels, Ronald. According to Wusz, the pipeline that transports fuel all along the western coastline is a privately owned entity, and all the fuel companies that use the pipeline, such as BP, Chevron, Conoco Phillips, and Shell, have to agree on the octane of the fuel that will be transported. Several years ago, when reformulated gasoline was first introduced in California, 92 was the standard. But sometime later, they all agreed to lower it to 91. Also according to Wusz, 93-octane is not necessarily universally available outside the West. There are several regions that also use 91. You may have also noticed that when you drive into any mountainous or high-altitude area, octane ratings drop because the density of the air is lower. This means cylinder pressure is lower, which results in a lower octane requirement. In these areas, 89-octane may be the premium fuel. It's also worth noting that California has its own requirements for fuel, as dictated by the California Air Resources Board (CARB) with a reformulated fuel that in 1999 eliminated MTBE (methyl tertiary butyl ether) as an aromatic and replaced it with ethanol, which is classified as an oxygenate because it incorporates an oxygen molecule. This reformulation is also only used in California, which is another reason California gas prices are among the highest in the continental United States.

All fuels are rated on domestic gasoline pumps as R+M/2 = AKI, which means the research (R) octane number plus the motor (M) octane number divided by 2 calculates the antiknock index (AKI). The research number test is the milder of the two and generally produces a higher octane number. What most car crafters should be more concerned with is the motor octane number. This is a higher temperature test that produces a much lower octane number. Averaging the R and M numbers produces a more accurate AKI. According to Wusz, high-performance engines under wide-open-throttle load generally respond better to a high motor octane number while part-throttle issues are generally more related to research octane numbers.

Another high-octane fuel that is finding increasing popularity with horsepower-addicted car crafters is E85. At 85 percent ethanol and 15 percent gasoline, as you can see from the following AKI chart, ethanol features relatively high research and motor octane numbers. Part of the difficulty in using E85 is it is not a consistent fuel at the pump. Blending ratios change according to the season, which means that even though it's supposed to be 15 percent gasoline and 85 percent ethanol, in the winter that could change to more like 30 percent gasoline. This obviously will also dramatically change the fuel's octane rating (despite the number posted on the pump). Rockett has just released an E85 race gas that looks promising with an AKI of 112. It's worth noting Rockett's much higher motor octane number of 108 versus straight ethanol's 103. Wusz told me that straight ethanol easily raises the research octane number, but the spread between research and motor octane is greater, as indicated in the chart. So this could mean that Rockett is using a high-quality race gasoline as its blending fuel, which would raise the motor octane numbers. Combine this with ethanol's excellent ability to pull heat out of the air, and supercharged engines really respond to an E85 fuel. While for everyday street driving, Rockett's racing E85 is overkill, this would be the fuel to use when pushing one of those E85-fed, supercharged bad-boy big-blocks.

Wusz also mentioned that using octane numbers and octane tests initially designed for use with gasoline are not necessarily as simple when testing methanol or ethanol. One post I found on the Internet quoted a highly respected engineering book as listing research and motor numbers (R/M) for ethanol as 107/89. These are drastically different from the numbers Wusz supplied and different yet from other numbers on the Internet. My money is with Rockett.

An interesting thing happened just as we were about to go to press. The EPA has been studying the effects of allowing pump gasoline that is currently limited to 10 percent ethanol to go to 15 percent. I've received these alarming press releases from a coalition of very strange bedfellows that oppose this move, calling it a "50 percent increase in ethanol." While that number is true, it's also intended to alarm potential customers. The reality is that the proposed move is to increase the percentage from 10 percent to 15 percent total ethanol blending. This probably won't change the octane rating because the fuel companies will just change the total makeup of the fuel. A study we saw from the noted engineering firm Ricardo Inc. in which the company tested cars and light trucks from six different manufacturers from 1994 to current concluded that there would be a minimal impact on driveability and performance.

They only tested cars as old as 1994, and all these were obviously electronically fuel injected. This change of 5 percent will certainly affect nonfeedback carbureted engines, but the overall impact will tend to lean out the air/fuel ratio slightly. The only carbureted cars this might dramatically affect would be those that are right on the ragged air/fuel ratio edge that might need a main jet or idle feed restrictor increase. But for the most part, despite the scare tactics you may hear, the long-term effects of this decision (which at this time is not official) on engines and fuel system durability will probably be minimal. There could be some minor fuel system durability issues with water in the fuel system with this increase, but again, we are not aware of any testing that has been done to address the increase from 10 to 15 percent ethanol.

More Info

Rockett Brand Racing Fuel; Mount Prospect, IL; 847/795-8400;

91 pump gas 96 86 91
Rockett 100-octane 106 96 100-101
Rockett E85 Race 116 108 112
Ethanol 120 103 111.5

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