fuel /octane question

[hipshot]

i had a race car driver tell me, that the higher the octane in fuel, the more compression it takes to vaporize. and vise versa, the lower the octane level,the sooner it will vaporize and ignite.

in a nutshell, what he said is "lower octane fuel fires before it is supposed to,giving you less than optimum power.".

anyone have any knowlege about this subject?

 

oh . this guy also

said that avgas,works better in "older" motorcycles, than super unleaded fuel, because it is a higher octane,(110 to 120 octane)

just jt

[Stoutman]

i had a race car driver tell me, that the higher the octane in fuel, the more compression it takes to vaporize. and vise versa, the lower the octane level,the sooner it will vaporize and ignite.

in a nutshell, what he said is "lower octane fuel fires before it is supposed to,giving you less than optimum power.".

anyone have any knowlege about this subject?

 

oh . this guy also

said that avgas,works better in "older" motorcycles, than super unleaded fuel, because it is a higher octane,(110 to 120 octane)

just jt

 

The first statement is not quite accurate. The vapor pressure is not really related to octane. Octane ratings are about how fast a fuel will burn. The lower octane fuel burns faster than the higher octane fuel. That means in a high compression engine the fuel can burn too fast and is more of an explosion than a controlled burn. That is the knock you hear in the engine when you don't have high enough octane fuel.

 

The part about it firing before it is supposed to is not accurate. The fuel fires when the spark plugs ignite it, unless you are talking very high compression ratios (which the racing guys might be talking about). The lower octane may seem like it fires before it is supposed to because it burns so fast at high compression. Higher octane fuel burns slower giving a more controlled flame front for a smoother power cycle at higher compression.

 

As far as av gas goes most av gas today is 100 octane Low Lead. The low lead av gas has much more lead that automotive gas used to have. There also used to be a 130 octane av gas that had much more lead. I think the 130 octane has been hard to find for about 20 years. The thing that makes av gas good for motorcycles is that is stores much better than auto gas. It has very low vapor pressure because it has to work at higher altitude. That means it doesn't evaporate as quickly as automotive gas. It is also a much cleaner fuel in that it doesn't have as much of the high vapor pressure impruities like benzene and other volotile organic compounds (VOCs). It is the VOCs that evaporate and leave behind the nasty stuff that fouls up your carberators. Todays price for 100 Low lead is about $3.80 per gallon

 

Most engines (except racing engines) don't need the high octane gas. Any engine with compression below about 10.5/1 won't really get much benefit from the high octane gas unless you can advance the timing a lot. The newer computerized, fuel injected engines will run fine on regular because the computer will adjust the timing and air/fuel ratio. They won't have as much power, but it won't be very noticable either. Most car manufacturers that state you must use high octane gas in the newer cars do so because of their horsepower claims. The car will run fine on the lower octane gas, but won't generate the claimed horsepower.

 

Here is a good discussion on octane and what it means.

[hipshot]

thanks ,jon!

i should-a known that a fellow texan, would have the answers!

lol

just jt

[RPG]

Short and simple is, Octane raises the flash point of gasoline. (causes the gas to ignite at a higher temp. than without octane) Pre-Ignition is caused by gasoline igniting due to high pressure and high temperature before the sparkplug fires, this can cause your motor to make a knocking sound and can seriously damage the engine. Octane in the proper ratio prevents pre-ignition.

[V7Goose]

Without quibbling about the specific wording, this is basically correct. RPG's response more accurate than Stoutman's (but much of wha Stoutman says is absolutely correct). The ping or knock that you hear in an engine is also called pre-ignition, and that is an accurate name. The ping is heard when two flame fronts (one from the spark, the other from the pre-ignition) meet. If I go into any more detail, this thread might put my tire discussions to shame for length! Suffice it to say that pre-ignition can be caused by numerous things, and low octane fuel in a high compression engine is one of them.

Goose