Octane requirement and ambient heat???

[kkim]

In another post in an octane thread, someone mentioned they needed to raise the octane level during the hotter months there. (desert type heat temps)

It started me wondering... can this be true? If anything, I would think cold weather would require a higher octane level as the oxygen content is higher per cubic foot of air when it's cold than hot, and having more oxygen per cubic foot would lean out the mixture leading to denotation.

But, does (ambient) heat play a role in denotation as well?

school me with your experiences or your physics class.

[sometimesido]

denotation?

[TnNinjaGirl]

Well, as far as I know one of the reasons we can run low octane in our high compression engines is because of the rapid heat dissipation. Car motors "heat soak" and cannot dissipate enough heat to run low octane on high compression. Maybe that has something to do with it.

[sometimesido]

"Other factors, such as extremely hot weather, changes in altitude or hard driving conditions (like towing a heavy load) may also cause knocking."

http://www.api.org/aboutoilgas/gasol...ine-octane.cfm

[sometimesido]

"Hot air expands and is less dense. Air that is thin can only be mixed
with a smaller amount of fuel."

http://64.139.33.19/tsn/bb/9-5/index.html?bID=116188

[sometimesido]

"The desired volatility depends on the ambient temperature: in hotter climates, gasoline components of higher molecular weight and thus lower volatility are used. In cold climates, too little volatility results in cars failing to start. In hot climates, excessive volatility results in what is known as "vapor lock" where combustion fails to occur, because the liquid fuel has changed to a gaseous fuel in the fuel lines, rendering the fuel pump ineffective and starving the engine of fuel. (This effect mainly applies to engine-mounted fuel pumps; a fuel pump located in the fuel tank, as in most modern automobiles, is much more resistant to vapor lock.)"

http://en.wikipedia.org/wiki/Gasoline

[voodoomaster]

So, what is the easy fix for a vapor lock situation? I appear to be having the symptons of vapor lock, the machine takes forever to finally turn over after sitting out in the sun (100 + degrees outside)

[ASecretNinja]

Originally Posted by voodoomaster

So, what is the easy fix for a vapor lock situation? I appear to be having the symptons of vapor lock, the machine takes forever to finally turn over after sitting out in the sun (100 + degrees outside)

Can you throw a cover (or large towel) over it to keep the sun from beating down on it? 100 degree air temperature isn't the problem. I'd wager it's the ambient heat accumulating in the machine from sitting out in the sun.

[voodoomaster]

Originally Posted by ASecretNinja

Can you throw a cover (or large towel) over it to keep the sun from beating down on it? 100 degree air temperature isn't the problem. I'd wager it's the ambient heat accumulating in the machine from sitting out in the sun.

Got a cover...will start using it now that we're at 105 F plus, Just keeps getting better here in AZ.

[sharky nrk]

its two seperate items really - in theory the increase the density of the air charge in colder weather does warrant an increase in fuel to keep a fixed ratio. Now the temp of the air charge and the ambient temp are not the same item. In an auto you have a relatively enclosed space with alot of radiant heat coming from various areas, this will heat the incoming air charge significantly (and many have hot coolant running through the intake allowing for thermal balancing as well). On a bike with a much more open air space to "loose" heat to, I imagine the variance is greater. Now the total thermal load placed on the motor as a whole is different than the air charge itself. Increased thermal load on the oiling, cooling, and mechanical systems of a motor can lead to increased chance of detonation due to hot spotting and ect. Now with the carbs and the ninjette I am unfamiliar but I have had the pleasure of tuning many Subarus and Hondas. Most were turbocharged temperature fluctuations can lead to significant differences in your tune.

[voodoomaster]

So....what are you attempting to say then Partner? Yes, it is hot air the engine is sitting in, say 100-105 deg F, for several hours. I've tried the cover, she still wants to crank and crank before firing over. I've opened the tank cap,no difference. This is a California model, I can see the evap can. At this point, I have run out of things to try.