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question for rich about octane

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Ive read your prvious posts and want to know a few speciic answers. In Australia the two main pump fuels avail are 98 Shell Optimax or standard unleaded ranging from 92-94.

Youve mentionined in other threads the limitations of high octane in these engines, is there any benefit in running the 98 over the standard 92-94 pump fuel. There is about 8cents per litre diff in price, but price is not a concern for me. Also it says in the manual to run standard unleaded in Aus, as the 98 runs to rich. If I changed the jetting to accommodate the 98 would it lead to significantly increased performance.. Thanks in advance.

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is there any benefit in running the 98 over the standard 92-94 pump fuel.

I'm not really familiar with the make up of your fuel so it's tough to say for sure. Chances are good that the 92-94 rating is an RON number rather than the (RON + MON) / 2 number we use at US pumps. If that's the case then the cheaper fuel would be below Yamaha's minimum 95 RON recommendation. With the 92-94 you might be running a risk of spark knock under high load situations like sand or mud, or when the outside air temps are very high . It would depend a lot on how the octane numbers are distributed within the fuel. Without that info it seems pretty risky to me run the cheaper fuel. I'd jet it for the better quality fuel and take advantage of that margin for error.

In terms of absolute performance I doubt you would see any real difference between the two fuels unless they used some very heavy hydrocarbons to get the additional octane for the 98 fuel.

I hope this helps some. :thumbsup:

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Sure does. Heres some specifications I found that may help.

It is 98.4 Octane I think thats Just RON

Density 760kg/m cubed

Distillation

10% evaporation @ 45 degree celsius

50% evaporation @ 105 degree celsius

90% evaporation @ 155 degree celsius

Final Boiling Point 196 degree celsius

Copper corrosion 3hrs at 100 degree celsius = 1a

Benzine 3.3% Vol

Sulphur .015 % mass

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Do you have the data on the lower octane fuel?

If you look at the distillation curve of your 98 pump fuel and compare it to a proper race fuel for high speed engines you'll see that even though the pump fuel has a reasonable 10% point the 50%, 90% and end point temperatures are much higher on the pump fuel than they would be on a good race fuel. This contributes to the somewhat sluggish response of pump fuel compared to a good race fuel. The big jump between the 10% and 50% point doesn't do anything good for throttle response either. Fuels like your 98 pump will tend to make less power above about 7000 rpm than a proper race fuel. At lower revs the main difference will be in the throttle response and how it plays with the fuel curve.

Here's the numbers for FirePower 324 (formerly Phillips 66 B35) for comparison :

10% evaporation @ 69 degree celsius

50% evaporation @ 96.1 degree celsius

90% evaporation @ 102.7 degree celsius

Final Boiling Point 120.1 degree celsius

This is pretty close to an optimum fuel curve for these motors while still keeping the fuel affordable. You can do more with a fuel from costing $30.00/gallon but most of us will never get to reap those benefits. :devil:

If you look at other good race fuels (excluding those specifically designed for two-strokes) you'll see a similar pattern in the distillation curve.

If you can get the curve for the 92 octane fuel I'd love to see it so we can compare them. :thumbsup:

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Thanks still disaminating info.

Standard Unleaded

92 octane

10% at 44 deg c

50% at 88 deg c

90% at 150 deg c

end point 200 deg c

Copper corosion same

Benzine 2.7% vol

Sulphur 0.01% mass

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my 2 cents

i race karts in australia as well as the bikes

the use the 2stroke yammy engines they rev ultra hard,

i have done extensixe talking testing and research on pump fuels and the engines love the octance but heres what is best from my experience

stay away from low octance fuel ie regular it screws the engine and performance

shell is bad!!!!it fries alot of pistons

caltex vortex is bad it runs to hot

mobil synergy 8000 (not 4000) tip top good for the parts but goes off faster than vortex shel etc ie 2weeks max after purchase

my pick of pump feul BP ultimate most consitant and best performer go for it

if you like track down to elf 98oct race feul runs like a dream but it would want to for $4.00 au per litre so $40 each time u fill up

hope this helps :thumbsup:

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Thanks still disaminating info.

Standard Unleaded

92 octane

10% at 44 deg c

50% at 88 deg c

90% at 150 deg c

end point 200 deg c

Copper corosion same

Benzine 2.7% vol

Sulphur 0.01% mass

That looks a bit better but still nowhere close to what we'd like to see for the 250F. If you decide to try the lower octane fuel (which I don't think is a good idea) I would suggest taking great care in looking for the signs of knock or detonation on the plug ceramic and just listening for the pinging sounds. It's sort of tough to hear it on these engines so make sure and keep close track of the plug.

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Rich the cheap 92 pump fuel has a better 10% to 50% curve but the two final ponits are similar. The 92 octane standard jumps 33 between 10% & 50% while the 98.4 optimax jumps 60 between the 10% and 50%. So would the 92 fuel be a better fuel than the 98 with an octane booster or will that change the curve dramatically. There end points are preety similar.

I guess the bottom line is with your knowledge which fuel would you run out of the two above and would you make any changes eg octane booster, jetting etc. PS at sea level 20km from coast. Ambient temp 22-30C but soon will be 28C to 40C generally 700-950hPa if its above 950 its generally raining and not on.

To ilike mud, thanx

I dont have a BP dealer within 100km so BP ultimate is out of the question. And I cant get the data sheets on them. My brother has raced clubman and rotax very competitively for the last ten years so im aware of what your saying. Those engines are operating at significantly higher temps and rpm due to no gearing. The karts seemed to fix the detonation problem once they went to the water cooled rotax engines.

My local fuel stations are shell or caltex. Caltex has no data sheets and there RON is lower in both standard and vortex so thats why i have given Rich the Shell data.

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Rich I got the stats for BP ultimate and found where i can get it. Its stats are

Octane 98 RON

Temp for 10% evap. 65°C max 49°C typical

Temp for 50% evap. 115°C max 110°C typical

Temp for 90% evap. 183°C max 150°C typical

FBP 225°C max 195°C typical

so which one is best.

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Rich I got the stats for BP ultimate and found where i can get it. Its stats are

Octane 98 RON

Temp for 10% evap. 65°C max 49°C typical

Temp for 50% evap. 115°C max 110°C typical

Temp for 90% evap. 183°C max 150°C typical

FBP 225°C max 195°C typical

so which one is best.

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Well the 92 octane fuel has the better curve from the standpoint of throttle response but I would be concerned that it won't provide enough protection from knock. If you add octane boosters to the 92 fuel you'll just screw up the distillation curve and put you in the same situation as the 98 fuel. If you started with the 92 fuel and added a small amount of a proper race fuel or you use something like MTBE to bring the octane up you could get good results and enough octane to protect the engine. If these are your choices it's probably simpler to play safe, use the 98 octane fuel and live with the reduced throttle response.

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