How Can I run 87 Octane Fuel?


46 replies to this topic
  • ghrocketman

Posted June 17, 2008 - 06:28 AM

#41

There is NO such thing as Pre-Detonation !!!!!!
Every time I see that term tossed around, I cringe. and yes I AM an engineer.
There is such a thing as Pre-IGNITION, and there is Detonation which are not the same thing, but Pre-Detonation does NOT exist.
Pre-Ignition is generally caused by hot-spots in the combustion chamber and can, but not usually be caused by too low fuel octane.
Detonation results from too much spark advance for the octane of the fuel used or conversely not enough fuel octane for the desired spark advance.
Both are loosely described as "spark knock" and tend to sound similar when audible, but are NOT the same event.

  • grayracer513

Posted June 17, 2008 - 08:31 AM

#42

There is NO such thing as Pre-Detonation !!!!!!

That's what I was alluding to in the last paragraph.

Detonation, as I said, is the instantaneous combustion of the entire remaining unburned fuel charge in the combustion chamber caused by any condition, as opposed to the controlled, progressive burning of that fuel, ignited by a spark at a chosen time. Since detonation is virtually always the result of heat and/or pressure beyond the fuel's ability to resist ignition, it normally never occurs before the spark happens. This is because compression pressure is never greater than the pressure created by the combustion cycle itself.

In normal combustion, the flame is ignited before the piston reaches TDC because it takes time for the flame to spread and produce enough pressure to push the piston down the bore as forcefully as it should.

If the ignition event happens too soon, it can lead to detonation by because the building pressure of combustion will reach a high level before the piston is far enough over TDC to be able to move downward without excessive resistance, forcing pressures to rise farther still. Detonation may be the result.

Thus, detonation can be caused by timing that is too far advanced, fuel that burns too rapidly, even though its octane may be high enough, or by pre-ignition.

Pre-ignition is the condition in which the fuel is ignited by an ignition source other than the spark. The fuel will initially burn in a normal manner, and in minor cases, no detonation occurs. In that situation, there is no obvious sign that pre-ignition even happened other than reduced power. If it happens early enough, detonation does occur, first noticeable as "pinging".

Unlike detonation, pre-ignition is frequently caused by compression heat, but not directly. Typically, the heat of compression will add to the heat of an already hot spot in a carbon deposit, and cause it to glow and become an ignition source.

  • steve86

Posted June 17, 2008 - 07:02 PM

#43

Whether one has been out of school for one year or 41 years one should never make the mistake of rejecting empirical data because it doesn't fit with pre-conceived theoretical notions. Were all "misfit" data to be considered invalid some of the greatest scientific advances ever would not have occurred. Another error I saw above is to attribute inconsistencies (with theory) to "phantom" variables, without being able to prove the dependence on the level of the variable in the specific experimental case. Physicists are familiar and on the look out for these errors of interpretation in the context of experimentation.

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  • grayracer513

Posted June 17, 2008 - 08:57 PM

#44

...one should never make the mistake of rejecting empirical data because it doesn't fit with pre-conceived theoretical notions. ...

Another error I saw above is to attribute inconsistencies (with theory) to "phantom" variables, without being able to prove the dependence on the level of the variable in the specific experimental case.

Empirical data that ignores all but one characteristic of a complex chemical compound, and then attempts to connect that characteristic to performance attributes that are known to be unrelated is of no particular value.

When a person tests a fuel about which the only thing that is known is its octane number and then attributes any and all performance gains or losses to the octane number, when virtually none of those gains or losses have anything to do with octane, it is as spurious as claiming that one tire wears faster than another without knowing the air pressure they were run with. In fact, some of the things complained about here are no more related to octane than tire selection is related to poor headlamp life.

The simple, plain, provable fact is that octane is and is only a measure of a fuel's resistance to detonation, and absolutely nothing more. There has rarely been anything so well understood within an industry, and so widely misunderstood by the public at the same time. If you like, you can continue to believe that octane does something that it doesn't, but the information is established and there for anyone to absorb. Try this:

http://www.faqs.org/...line-faq/part3/

If you really want to get a better understanding of gasoline and the combustion process, read the other 3 parts, too.

  • MountainMax

Posted June 18, 2008 - 02:08 AM

#45

I know we are suppose to run 91 octane fuel however i have to drive 480km round trip to the nearest (and only one in my province of Labrador) gas station that sells Supreme. How can I mod the bike to run regular fuel instead? thicker head gasket? lower compression Piston? retard the timing? colder plug? what can I do??????



Man, what a can of worms I opened with this question, lol

  • beezer

Posted June 18, 2008 - 07:20 AM

#46

And no answer to your question.

  • grayracer513

Posted June 18, 2008 - 08:48 AM

#47

Man, what a can of worms I opened with this question, lol

There's a lot of fantasy afoot regarding octane number.

And no answer to your question.

If you can run 87 without pinging, then your problem is solved already, if not, perhaps the most practical way to alter the engine would be to shim the cylinder upward. It won't take much, only about .040" or so, to reduce the compression ratio to about 10.5-11:1. Using a heavier material for the base gasket is what I would do. Keep in mind the O-ring seal at the coolant passage in the front of the cylinder flange needs to be compressed well to seal.




 
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