Nov 7, 2005
What is octane

We all know that our engines have a compression stroke before it explodes due to the ignite from the sparkplug. During the compression stroke the engine compresses a cylinder-full of air and gas into a much smaller volume before igniting it. The amount of compression is called the compression ratio of the engine and it is the ratio between the compressed volume and the uncompressed volume. Normal ratios for motorbikes is for ex. 1 to 9.

The octane rating of the gasoline tells you how much the fuel can be compressed before it explodes by itself causing knocking in the engine. The compression ratio of your engine determines the octane rating of the fuel you must use in the engine. One way to increase the power of an engine is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel.

The hydro carbonate Octane (is a gasoline part in the crude oil) handles compression very well and has a high compression value. Eighty-seven-octane gasoline is gasoline that contains 87-percent Octane and 13-percent of another hydrocarbon (gasoline component) heptane with a very poor octane figure. Nowadays the combination of fuels that has the same performance or better of the 87/13 combination of octane/heptane are all compared to the octane of this blend.

The race for more performance in the cars started already in the 1920 and all manufacturers tried to invent better gasoline with higher octane so that the engines could handle higher compression rates. So at this time we started to get antiknock additives in the gasoline. They started with Iodine, Aniline but ended up with Tetra Etyl Lead (TEL) which was a very efficient octane booster. The TEL also required a scavenger to remove the lead from the engine resulting in volatile lead halide salts that escape out the exhaust system.

The octane booster properties of TEL made it possible for the oil companies to manufacture cheaper (low grade) gasoline and compensate it by ever increasing adding’s of TEL. In the 1960 some people started to be worried about the lead effects to nature and when the engineers could not find a solution to the problem with lead gasoline and exhaust catalyst the game was over for the lead petrol.

The lead gasoline was quickly substituted with unleaded gasoline and high octane gasoline was not available anymore. In the unleaded gasoline TEL was substituted with MTEB or more lately ETBE. MTBE is the acronym for methyl tertiary butyl ether that is created from methanol (methyl alcohol). MTBE boosts the octane and it is an oxygenate that adds oxygen to the burning process. MTBE also reduces the unburned hydrocarbons and carbon monoxide in the exhaust. ETBE do the same but is derived from ethyl alcohol.

MTBE production boomed in the 1990s and as much as 15% of the gasoline could contain MTBE. But then started the problems. MTBE is carcinogenic or cancer causing. For that reason oil companies started to look for substitutes for MTBE and the natural choice was Ethanol or pure ethyl alcohol.

What is knocking?


I think that these two pictures which are copied from Mr Reynolds seminars about the influence of Gasohol for Hawaii talks more than hundred words.

The sparkplug ignites the fuel/air mixture and the flame like explosion progress quickly across the combustion chamber. Instead of a smooth burning across the whole combustion chamber the managed burning is confronted by an explosion in the combustion chamber caused by the compression being too high for the fuel.

The result is of course a loss of power and a terrible metallic noise caused by the self “ignited” explosion and the confrontation with the smooth burning flames ignited by the spark plug. Many time people think that “now I have a broken main bearing” because that is how the noise sound. Anyhow driving with too low octane fuel, short time. has no effects on the engine. Long time use may cause “mouse eating” on the top of the piston and valve defects.

Very often you will only hear the knocking when you load the engine, for ex you try to accelerate from a too high gear or you are going up a hill on a too high gear. When driving normal you cannot hear it.

Your bike has a given compression ratio, each gasoline has a limit to how much it can be compressed and still burn evenly, supplying a smooth even thrust to the piston.
When the anti knock index (AKI, octane) of a gasoline is insufficient for your bike’s compression ratio, it burns unevenly and causes the engine to knock.

Octane at the pump.

Here in Thailand we can normally choose between 91 and 95 octane. What does this mean and how are these octane compared to octane’s back home for ex. USA.

Here in Thailand as well as in most other countries we use to display the Octane’s using the RON system. In USA they use a combination of the RON and MON system using the average of these two and it is often called PON.

MON is the best figure for high performance engines. MON means Motor Octane Number.
RON is the best figure for normal engines in daily use. RON mean Research Octane Number. PON is (MON+RON)/2 and is supposed to mean Pump Octane Number
I think that it is no use to look deeper into the differences between MON and RON here. It is easier to post a chart:

90 83 86.6
92 85 88.5
95 87 91
96 88 92
98 90 94
100 91.5 95.8
105 95 100
110 99 104.5

In US it is still more complicated since in some states they sell the fuel with Brand names like Premium, Super, Midgrade, Regular etc without stating the Octane number.

And then just to be “fair and balanced” in Russia they use mainly a modified MON system. So when you buy the most common A76 fuel it will be between 85-88 RON.

So in Thailand we have 91 or 95 RON octane’s and it is the same is it old gasoline or the new and glorious Gasohol.

Then I have to kill some old believes. If your bike work properly on 91 you will not get more “power” by putting 95 into your tank. 91 and 95 fuels will work exactly the same as long as you don’t have any knocking, On BMW:s and Japanese bikes you will hear the knocking clearly. On KTM:s there is so much mechanical noise together with high power pipes giving a high metallic sound so I am not sure if you can hear it then.



Jun 28, 2007
Hiko a very good and useful post !! Is there any rule of the thumb which octane No. you should use for a defined compression ratio, let's say for exapmle </= 1:10 requires 91 octane, 1:10-1:11 should be 95 and so on ?? Cheers, Franz


Nov 7, 2005
Yes Franz there used to be when we still had carburetors and ignition by breaker points. The starting advance (the earlier needed ignition for starting) was taken care by a centrifugal device which set the normal advance at a set level. (advance means how much earlier than the Top Dead Center.

But know we have more modern engines, electronic ignition, electronic fuel system etc. You can see from the brochures that some bikes have very high compression and still they can even get along on 91. Probably it is due to an variable electronic ignition.

That is why I am surprised when "Long and Tall" ?? wrote in the Gasohol thread that he has 16-1 and he still manage to drive the bike with 91...In the good old days he would have needed methanol to run the device!!!!

In the good old days the level when you must use 95 was 9-1.



Dec 6, 2005

I don't think the two are connected.

Isn't pre-ignition, or pinging, when the fuel air mixture ignites without the spark, and therefore, it wouldn't matter if the ignition was at 30 degrees after top dead centre, it would still pre-ignite.

I thought it was two seperate things, one was the advance/retarding of the ignition because the burn was quicker, so with higher compresison/lower octance, the burn needs to be later in the cycle which isn't good for developing power.

The other is pre-ignition which is a seperate animal.

I used to run the Rally Car on 110 Octane mix of Avgas, with 10% tolulene. I was running 13.5:1 (the 1.8L Engine would chew through 1L fuel per Km)


Now Avgas is banned in all motorsport, so the choice is 98octane premium unleaded at $Au1.60 or Elf Fuel at $Au16. Not really fair for us on small budgets.

but I agree with the rest, there is limited advantage in higher octane than your engine needs to function at it's optimum.

Oct 17, 2006
A most concise essay on all the different Octane systems.Thanks.

in the UK it is possible to get 95,97 ,98 and even 99RON fuel I experimented with 98 and 99 RON in my bike but the extra octane and higher cost of those fuel showed no benefit in my M750 ducati in fact it ran best on 95 as recommended by Ducati.

So stick to the best fuel for your motor dont waste money on higher than needed Octane fuels.