Different Octane Fuels

Friday 5th December 2008-12-08

I rode up to Auckland to stay the night at Pick’s place.
Pick and I were crew on Gary’s Mullet boat (Tere Kane) on Saturday, racing to Waiheke as part of the Peter Blake Memorial.
For the record, in the yacht race we were 2nd on line and 1st on handicap. Stayed the night in Oneroa Bay and motored back into Westhaven marina on Saturday morning. Was perfect weather the whole time.
I then rode back to Tauranga dropping in to see Hil (Farmers Market Greenlane) and Jared/Desma/Paul/Peg at Jared and Paul's Mum’s place near Miranda.

Riding from Tauranga to Auckland and back hardly counts as an adventure ride, but it did allow me to try another run on sealed roads using 98 Octane from BP.

Over the past few months I have been experimenting with the different octane fuels to see if there is any real difference.
Well there is a REAL difference.
The tank on the XT400 holds almost exactly 10 litres until it hits reserve and then the reserve holds 3.5 litres.
On 91 octane petrol the bike will do 180-185 km before hitting reserve.
On 95 octane petrol the bike will do 195-200 km before hitting reserve.
On 98 octane petrol the bike will do 215-220 km before hitting reserve.

I found this quite remarkable. The above data is true for long distance runs and commuting on sealed roads. (NOT adventure riding)

The small distance variation of about 5 km that you see is probably due to a few varying factors:
1. Just how full I fill the tank.
2. The commuting / around-town component versus the open road component on any given tank full.
3. How aggressively I ride (or what mood I am in) on any given tank full.
4. Probably some other factors that I can’t dream up right now. (Head winds, hills, tyre pressures, tongue position, rider/luggage weight, air density and temperature, chain lubriciousness etc, etc)

I do notice that the bike does not run as well on 91 octane fuel. It seems ever so slightly less powerful and will “pink” at low revs with lots of throttle.

So the question is; which fuel is the most cost effective?
Well as of today, the price of petrol is:
- 91 $1.40 per litre
- 95 $1.46 per litre
- 98 $1.54 per litre

So this gives:
- 91 185 km / (10 litres x $1.40 per litre) = 13.21 km per dollar
- 95 200 km / (10 litres x $1.46 per litre) = 13.70 km per dollar
- 98 220 km / (10 litres x $1.54 per litre) = 14.29 km per dollar

Clearly it is best for my wallet and probably my engine to run 98 octane whenever I can.
I am very surprised to see that there is a 91 to 98 difference of 14.29 – 13.21 = 1.08 km per dollar.
Or put another way, a 91 to 98 difference of 3.5 km/litre. This means that on 98 octane, my bike sees about a 20% increase in fuel efficiency (km/l) compared to 91 octane! (22km/litre / 18.5 km/litre => 18.9% more )

To me this seems a little hard to believe. However, I have done at least 2 tanks on each type of fuel. I am very confident in terms of the repeatability and consistency of the data.

Also, here are the maximum distances possible on a complete (13.5 litre) tank full of petrol:
- 91 13.5 litres x 18.5 km/litre = 250 km
- 95 13.5 litres x 20.0 km/litre = 270 km
- 98 13.5 litres x 22.0 km/litre = 297 km

The extra 47 km in fuel range seen between 91 and 98 octane fuel will definitely come in handy from time to time.
It would be interesting to find out the TRUE octane ratings of the fuels and see what the relationship between true octane rating and mileage is for my engine.

I guess I should also try "Force 10" from Gull. It is rated at 98 Octane (Marketing Octane not True Octane) It contains "up to" 10% ethanol. So I guess it is possible that it has no ethanol at all. (Jeez, I am developing an intense disliking of marketing speak).

Had to replace a Ventura pack rack lock-nut that I lost ($7 at Bayride Yamaha).
The silicone rubber repair job on the CV diaphragm seems to be holding in there.
Got a quote back from Bayride Yamaha:
- New gear lever ex-Australia $43
- New CV diaphragm complete with throttle slide ex Japan $137
I ordered the gear lever but NOT the new CV diaphragm complete with throttle slide.