The other Weekend my KX500 came to a grinding halt whilst racing at Manfield. After a top end tear down, the electrosil was found to have flaked off the barrel. Not good as I had not long just got it back from the platers. Turns out it was due to detonation. I was running BP Ultimate 98, so I was surprised to find that it had been detonating that badly. A respected motorcycle figure did some research on BP98 by contacting BP themselves. Guess what, you can forget NZ petrol octane ratings as they are quite misleading. Read on....

Here are some of the details supplied to me on the matter. (In italics)
The specs for BP98 are alarming. I don't know whether you are aware of same but all "pump" petrols (98 is a 'pump' petrol as such) have their octane ratings listed as the RON or research octane method. This is a little misleading and is not the information that racers really require.
Looking at the attached spec sheet the MON or motor octane rating of this fuel is in fact only 84. The MON number is always lower but not by such a huge amount.

Note that as a comparison the MON number of 91' pump petrol is 82, not a lot different.
Would you run your motor on straight 91'?
The respective anti knock index ratings are also not that encouraging.
Note that although the spec sheet is written / itemized a little differently for No. 1 / Avgas, but in fact the RON number is typically within 105 - 110
even though they don't rate Avgas in this way. The MON number is 100 or more, which is exactly the "territory" you need for your motor.
Note that BP "Ultimate 98" is an "Environmental" fuel, it is not marketed as a racing fuel as such and has low benzene count.

So there you go
folks, always be very aware of the octane sensitivity of your racing 2 stroke. Some motors may run happily on pump fuels but many are very sensitive and can detonate quite readily due to a whole host of variables.
Many of the late model highly tuned 2 stroke MX based bikes require at least
a 50% concentration of Avgas and 50% 96. Those most sensitive to detonation will require an even higher concentration and so on. As Avgas has a slower burn speed it is popular to use "cocktailed" with pump fuel to raise the burn speed for decent
engine response.

The flip side is that with very high concentrations of Avgas, reed petal life becomes shortened."

The other factor to be aware of is that in the autumn months fuel companies change the volatility of their fuel for easier starting in cold weather. "The fire lights easier" and burns more rapidly. This is no problem for your average passenger car, which accounts for 99% of the fuel company's market.
However, this can very often can be detrimental to a highly tuned racing motor, engine problems can suddenly appear in the
autumn months that weren't there during the warmer months.

By law of averages we are just a small minority and a relatively small annoyance to fuel companies who are only concerned about a bigger picture.
Nothing is ever as simple as it seems......................

I find it interesting that when approached at a technical level, BP say they market 98 as an environmental fuel, but in actuality, looking at the adverts for it at their stations, they seem to market it as a "fuel for performance vehicles"

Oh well, time for time to go and have a good cry and work my ass off to get some $$$ to do it all over again.

Not to much gloating now you 4 stoke riders, through all of this, I've found all sorts of ways to improve reliability, ridability and get more power!

I'll be back!

Anthony