If octane is good, is more better?

Text: Robert Smith • Photography: RoadRUNNER Staff

Have you ever taken your baby to the gas station and decided to treat her? (I'm talking about the bike, of course.) So you pull up to the pumps and go for the highest octane you can get. After all, nothing's too good for your bike, or is it?

What's special about higher octane fuel? More power? Better gas mileage? Longer engine life? Most of us know that you need a higher octane fuel if you get "knocking" (detonation) or "pinking" (pre-ignition) in your engine; but is there any benefit in using a higher octane fuel than your engine needs? Conventional wisdom held that the higher the compression ratio, the higher the octane you needed to avoid detonation. So how do modern fours with 12:1 compression get away with regular gas while my '70 Bonneville at 9:1 needs 94 octane?

Early motorcycles weren't designed with efficient combustion in mind: fuel quality was poor, and compression ratios were as low as 4:1 to avoid detonation. In England, Harry Ricardo carried out the first scientific examination of combustion efficiency. By incorporating an inspection "porthole," Ricardo was able to observe the flame path inside the combustion chamber. His experiments led him to recognize the importance of combustion chamber turbulence in promoting efficient combustion and preventing detonation. An engine so designed could run with higher compression (and therefore more efficiently) on the same fuel. The earliest motorcycle incorporating this concept is the 4-valve Triumph Ricardo of 1921.

Along the way, Ricardo also discovered that the more iso-octane in the fuel, the more resistant it was to detonation, and developed a scale to measure detonation resistance: the octane rating. Gasoline made up of 100 percent iso-octane was given an octane rating of 100.

But producing a fuel with 100 percent iso-octane was impractical and expensive. Thomas Midgley Jr, working for DuPont in the US, discovered that different chemicals added to gasoline would prevent detonation, therefore increasing the "octane" rating. Of the compounds he tested, tetra-ethyl lead, TEL, was the most cost effective to produce. Alkyl halides also added to the fuel avoided lead buildup in the engine by combining to form volatile lead halides, which went out the exhaust.

TEL had other advantages. The lead acted as a metal lubricant, preventing exhaust valves from "welding" themselves to the soft iron valve seats then in use, and so preventing valve seat erosion. But it had one big disadvantage: Lead is extremely toxic, especially to children, and the high levels of lead in the air around major highways became associated with a number of illnesses. As a result, lead has been banned as a fuel additive in most jurisdictions. It's been replaced with a swath of other octane boosters, like toluene, xylene, ethanol, methylcyclo- pentadienyl manganese tricarbonyl (MMT), methyltertiarybutyl ether (MTBE) and more.

Meanwhile, developments in cylinder head design were improving combustion efficiency also, allowing higher compression ratios and therefore more horsepower. In the UK, Harry Weslake discovered that creating "swirl" in the combustion chamber improved mixing and combustion efficiency increased, while also reducing detonation. The best-known example of Weslake's principle is the "bathtub" combustion chamber used in the first-generation Minis.

But there was another important feature of Weslake's design. Although the combustion chamber was heart-shaped, the piston crown was, of course, round. This meant gases were "squished" back into the combustion chamber. This forced unburned mixture back toward the flame front, improving combustion efficiency further by "scavenging" these gases. Squish became the holy grail of engine builders and led to some innovative designs, such as the wedge combustion chamber (Erling Poppe's Sunbeam S7 for BSA), and the in-piston combustion chamber (the Moto Morini "Heron" cylinder head).

Until the 1970s, most motorcycle engine designers stuck with the hemispherical combustion chamber, which offered high volumetric efficiency (good gas flow) and therefore, high performance. But it had two major drawbacks. First, for increased performance, larger valve sizes, valve lift, overlap and higher compression were all needed - but the geometry of the design limited these. Second, combustion efficiency was low, also limiting compression ratio and generating higher emission levels.

Formula 1 has often been the proving ground for new ideas, and so with combustion chamber design. Around 1966, Frank Duckworth (the "worth" in Cosworth) designed a new four-valve cylinder head with a shallow combustion chamber and narrow included valve angles for the Ford-Cosworth F1 car. The Cosworth DFV V-8 became the "winningest" F1 engine ever with 167 Grand Prix victories. It's still the basic blueprint for all modern high-performance four-stroke engine design.

All of these developments allowed the use of higher compression ratios without detonation by improving combustion efficiency, so that now a modern high-performance sportbike with 12:1 compression can run comfortably on 87-octane gas.

So back to the original question. Given the advances in engine design, is there any benefit to using higher-octane fuel? First, always use as a minimum the octane rating recommended by the motorcycle manufacturer. And also bear in mind that octane is measured differently in different countries. Triumph, for example, recommends 95 RON octane for its bikes, but in most of the US, the highest octane you'll find is 92. Turns out that's quite OK. In the UK, octane is measured by the "Research" method (RON); while in the US, we measure the average of "Research" and "Motor" octane ratings (R/2 + M/2). US ratings are typically five points lower on the scale, so UK 95 RON is roughly equivalent to 90 octane US.

The higher the octane rating of gasoline, the more detonation inhibitor compounds it contains. Most of these compounds have lower heat content - combustion energy - than the alkanes in the fuel they displace. So, in most cases, the higher the octane, the lower the combustion energy. Not only are you wasting money buying higher octane fuel, in most cases you'll be getting marginally inferior performance too.

Bottom line - stick to the manufacturer's recommendation!