First, let's define some terms:

AKI	Anti Knock Index	This is the average of both the Research and Motor Methods. It is also what you usually see at the pumps.
RON	Research Octane Number	Also known as ROM (Research Motor Method). This is a real world number that is derived by running a single cylinder engine on a fuel sample. The timing is advanced until it pings. This advance is then converted to a number.
MM	Motor Method	Using the same engine as above, the same fuel sample is subjected to increased ignition timing. But this time, something resembling reality is used in in determining the number. Load, valve timing, etc. are changed while the sample is being tested. This results in a lower number than RON (the RON test is done at NO LOAD). When you see a single, high number representing octane, it is usually RON (like the old days of 100 octane).
Octane		This is an organic, hydrogen-carbon structure (a ring of 8). It's contribution to all this is that it was designated to have a value of 100 (with Heptane having a value of 0) back in the days when they were trying to rate and standardize fuels. Hence the scale became known as the Ocatane scale, and much confusion on the subject has followed. There is really not much magic to all this. The idea was that a fuel with a rating of 92 could be composed of 92% Octane, and 8% Heptane.

Question: Why does my bike backfire?

Answer: In general, backfiring on deceleration (as opposed to acceleration) is generally caused by a lean condition in the pilot circuit. What happens is that the mixture leans out enough to where it fails to ignite consistently. This, in turn allows some unburned fuel to get into the exhaust pipes. Then when the engine does fire, these unburned gasses are ignited in the exhaust pipe, causing the backfire. Newer carbs have an 'enricher circuit' which cuts in on deceleration to help this problem. Earlier carbs don't have this. Check the pilot circuits and set them a bit richer. It is hard to do this right without some equipment to test where you are. Aftermarket pipes often increase backfiring, probably because they make the bike run leaner. Some backfiring on older models is par for the course, so these owners should learn to enjoy it.

Question: Can mothballs increase octane?

Answer:The legend of mothballs as an octane enhancer arose well before WWII when naphthalene was used as the active ingredient. Today, the majority of mothballs use para-dichlorobenzene in place of naphthalene, so choose carefully if you wish to experiment. There have been some concerns about the toxicity of para-dichlorobenzene, and naphthalene mothballs have again become popular. In the 1920s, typical gasoline octane ratings were 40-60, and during the 1930s and 40s, the ratings increased by approximately 20 units as alkyl leads and improved refining processes became widespread.

Naphthalene has a blending motor octane number of 90, so the addition of a significant amount of mothballs could increase the octane, and they were soluble in gasoline. The amount usually required to appreciably increase the octane also had some adverse effects. The most obvious was due to the high melting point ( 80C ). When the fuel evaporated the naphthalene would precipitate out, blocking jets and filters. With modern gasolines, naphthalene is more likely to reduce the octane rating and the amount required for low octane fuels will also create operational and emissions problems.

Last update: 07:36 PM Saturday, September 25, 2004

• The Mystery of Detonation
• The Meaning of Gas
• Carburetion 101