Octane

[sydeem]

Might have an answer here - This is from http://www.faqs.org/faqs/autos/gasoline-faq/part1/ a pretty comprehensive discussion of octane (sorry about the otain spelling)

Combustion Characteristics
As gasolines contain mainly hydrocarbons, the only significant variable
between different grades is the octane rating of the fuel, as most other
properties are similar. Octane is discussed in detail in Section 6. There
are only slight differences in combustion temperatures ( most are around
2000C in isobaric adiabatic combustion [45]). Note that the actual
temperature in the combustion chamber is also determined by other factors,
such as load and engine design. The addition of oxygenates changes the
pre-flame reaction pathways, and also reduces the energy content of the fuel.
The levels of oxygen in the fuel is regulated according to regional air
quality standards.

Comment - don't know why one would have more carbon build up with regular but I did not read the whole article in detail.

 

[discomike]

Run your truck around town for a couple of weeks on regular and then go put your foot in it on the highway, look in your rear view mirror as your stytter up to 5500 RPM's with little puffs of smake blowing out your tail pipe. Then try it a second and maybe even a third time at which point your truck will hit 5500 RPM's smoothly and with out any more smake, carbon blowing onto the car behind you.
This won't happen on supreme, I can't give you a technical reason for it.
Mike J.

 

[sydeem]

Ok - that sounds pretty convincing

 

[beemer]

High or low octane, C8 H18 is C8 H18.

Disco Mike, gas is gas, high or low octane, it's C8 H18. You will not get more carbon buildup between the two. More additives are added to higher octane fuels, if any the added additives in higher octanes will add more deposits to the combustion chamber, piston top and valves. On the newer, 96 up Rovers with anti-knock sensors they will correct timing to prevent detonation due to fuel grade, load, air temp, altitude and humidity. Timing set back will cause a slight loss of power. This question was run by a Chevron chemist friend last year. Yes the Rovers will load up if you dog it around town for 50 plus miles, it will still be dirty inside after a 40 mile run on the highway at 70 mph unless you give it full throttle up to 4,500 rpm. I'm running a 4.6 5-speed in a D1 with a RPI Tornado chip that will run richer than a stock chip and yes it will blow dark gray for 4 seconds on "clean out" in 3rd at 4,500 rpm. Carl.
...............................................................................o&o>...............

 

[ayavner]

It makes sense to me that they would engineer a little bit of tolerance into their system... as they market these as global exploration vehicles, it seems like they wouldn't expect you to get Chevron 93 Octane w/ Techron in Borneo...

 

[Sergei]

Carl, you wrong. Application of flat formula isnt working well in organic chemistry. 2D is what you need to really check out how things working.

Octane rating represented (100-rating, i.e 87 octane is actually 87 octane + 13 heptane) amount of heptane in particular fuel. Heptane is thingy that compressed pretty badly, and thus ignites faster. Then rest of things getting into play. So higher compression engines sort of requiring higher octane rating, overwise you will get pre-ignition and knocking.

Now, next bad thing is that heptane, while helps ignition is weak C-H link, and when getting burned - you getting free carbon flying about, which bonds with O and H bonds with O & etc..
More flying carbons in engine that have timing set up to have longer burning - as you may guess will sit down on walls around.

Thats second thing.

 

[beemer]

right or wrong?

Sergei, true you can't do much with just straight 100% gasoline, it's the additives. Many people state Shell is better than Arco, only Chevron with their additives makes a difference. Last June at the camping party i spoke to a Chevron chemist and what i was told was in the above post. Sorry if it's misleading, i'll **** him next June. ....................o&o>..............................

 

[sydeem]

Possible last word?

Finally struggled through the entire paper at:

http://www.faqs.org/faqs/autos/gasoline-faq

End result is that I should be using Premium gas but can probably get away with Regular because I live and mostly drive at 7,000 feet altitude. Octane requirement drops with altitude.

Apparently the engine management system takes care of us if we play with Octane levels. The result is the truck will run fine but at slightly less than optimum design. I respect the stories of carbon depsoits, etc. as to some extent there could be a minor build up as Preimum does have an aditive to help control intake valve deposits.

BUT - it sounds to me like one should more worry about the manufacturer than the octane. I am about ready to switch to Cheveron Premium and save money some place else.

A couple of paragraphs the paper:

6.14 Does low octane fuel increase engine wear?

Not if you are meeting the octane requirement of the engine. If you are not meeting the octane requirement, the engine will rapidly suffer major damage due to knock. You must not use fuels that produce sustained audible knock, as engine damage will occur. If the octane is just sufficient, the engine management system will move settings to a less optimal position, and the only major penalty will be increased costs due to poor fuel economy. Whenever possible, engines should be operated at the optimum position for long-term reliability. Engine wear is mainly related to design, manufacturing, maintenance and lubrication factors. Once the octane and run-on requirements of the engine are satisfied, increased octane will have no beneficial effect on the engine.

6.16 What happens if I use the wrong octane fuel?

If you use a fuel with an octane rating below the requirement of the engine, the management system may move the engine settings into an area of less efficient combustion, resulting in reduced power and reduced fuel economy. You will be losing both money and drive ability. If you use a fuel with an octane rating higher than what the engine can use, you are just wasting money by paying for octane that you can not utilize. The additive packages are matched to the engines using the fuel, for example intake valve deposit control additive concentrations may be increased in the premium octane grade.

 

[discomike]

Do yourself a favor and don't screw with your engines fuel, they run bad enough up there and all you are going to do is save a couple of $'s a tank full to drive a truck with less power, worse gas mileage and gain alot of carbon that will bite you in the butt down the road.
Mike J.