Advice on Octane Boost for 2002 Yamaha WR250F

Rich Rohrich

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Originally posted by Lobster
If I have a grasp on that, and the octane level makes the chain more stout, then what happens to the attached(?) octane molecules during the combustion?

The more stable hydrocarbons are reacted in a normal controlled way, they are just less likely to be be effected negatively by the temps and pressures that come into play during combusiton. So there is less of a chance of a stray hydrogen molecule teaming up with an oxygen molecule and forming radicals.
 

Lobster

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Jan 7, 2002
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And here I thought you were just ignoring my ignorance! I was questioning due to some dyno time(22 hp twin air cooled Briggs on an eddy current crankshaft dyno) and different fuels we tested waaaaayyyy back when and didn't see a benefit from additional octane when we mixed our own. However, I notice you always add "race" in your replies, and a 700cc twin making 22 hp isn't a race engine!
 

Rich Rohrich

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Originally posted by Lobster
And here I thought you were just ignoring my ignorance! I was questioning due to some dyno time(22 hp twin air cooled Briggs on an eddy current crankshaft dyno) and different fuels we tested waaaaayyyy back when and didn't see a benefit from additional octane when we mixed our own. However, I notice you always add "race" in your replies, and a 700cc twin making 22 hp isn't a race engine!


Sorry, I wasn't ignoring it, I just MISSED the question the first time. :ugg:

For what it's worth I've seen instances where higher MON octane fuels will knock far sooner than some lower MON octane fuels, especially on air cooled engines like RD350s and modified XR600 engines. I was never really sure WHY that was the case until Dr.Dave Redszus from Phillips explained it to me. He clued me in to the signifigance of piston crown and combustion chamber temperature on radical formation. The lower octane fuels I used had higher END POINT evaporation temperatures than the flat curve 118 octane stuff. The higher temps of these heavy ends would allow some liquid droplets to reach the combustion chamber and leech some heat. Reaction rates are sensitive to temperature, so even though the lower octane fuel had less stable hydrocarbons in it's mix, the lower temps helped to limit the formation of active radicals. I believe this is why Honda specs AvGas as the fuel for customer RS125s. AvGas has a fairly high end point and provides an excellent piston crown cooling effect when tuning at the ragged edge, as road racers are prone to do :)
Those conversatoins with Dr. Dave certainly made me appreciate the value of trying to understand as much of the chemistry of combustion as possible ( clearly I have a LONG way to go ;) ).

The moral of the story, octane doesn't MAKE power, it just can help establish a less hostile environment for power to be made in, and it's just ONE SMALL PART of the whole fuel equation. Controlling temperatures plays a key role in controlling knock.
 

Rich Rohrich

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Originally posted by jodog
Be careful in the altitude because high octane in high altitude can make your bike run like ****.

If your bike runs poor at altitude with race fuel it has NOTHING to do with the octane of the fuel. It's purely a function of atmospheric pressure changes that effect jetting and the vaporization of fuel.
 

techman

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Feb 18, 2000
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I'll step into the fire. If your hp produced per mass-based (stoichiometric?) carb generated mixture is down because the effective mixture chemically speaking between fuel and oxygen is different, due to the different gas, and your usage situation needs the same hp - you suck more mass through the carb and down goes your mileage.

This is s SWAG.
 

techman

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Feb 18, 2000
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Ok, I'll leave my foot firmly planted in my mouth, stoichiometric is the chemical mix ratio (mass flow ratios caused by Bernoulli equations at a carb venturi may or may not result in stoichiometric ratios), and I have now had my dose of education for today by reading page 2. I'll leave proper answers to those who know.
 

APBT

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Mar 13, 2005
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Rich i . It's purely a function of atmospheric pressure changes that effect jetting and the vaporization of fuel.[/QUOTE said:
The atmospheric pressure changes actually effect the air density (slugs/cubic foot of air) which then effect jetting and fuel vaporization. Temperature and humidity also effect air density, humidity less so than the other factors.

Excellent posts, Rich. Just attempting to be technically correct.
 

Rich Rohrich

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Cr85RaCeR607 said:
How about that "Cals Products Rocket Fuel" stuff? It comes in 1 oz. packets that you mix with 2-6 gallons of gas. Do you think that would help at all?

No
 
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