does octane change your afr?
 

ive been thinking this over and im unsure about this. how do you go about tuning different octanes? like ive heard of people tuning on multiple octanes and wondered what is different. thanks.

Re: does octane change your afr?
 

Won't really change your AFR. The biggest difference is that with a higher octane it's more resistant to detonation. This means that you can advance your timing more when running a higher octane to make more power. If you ran the same amount of ignition advance that you tuned for race gas and you ran it on regular premium, you would probably detonate.

Re: does octane change your afr?
 

hmmm, so if you tuned on say, 93 octane, then tossed in 100 your afrs would be the same? and youd just be able to advance more timing?

Re: does octane change your afr?
 

They won't stay exactly the same, but shouldn't be too off. And yes about advancing the timing.

Re: does octane change your afr?
 

For the current blends of pump and commonly available racegas, you are correct.

Keep in mind, though - octane/knock resistance has nothing to do with a fuel's burn rate and the ignition timing it likes.

Re: does octane change your afr?
 

can you explain this more?

Re: does octane change your afr?
 

a higher octane rating does this:
It changes the temperature at which that gas will ignite by the most minute amount.
however this gives the Gas a Small portion of a second longer before it ignites and helps in preventing preignition. Detonation is a result of the Flame Front in the cylinder. I can elaborate on that if you want me to colton.

Re: does octane change your afr?
 

No.

Octane is the rating by which you estimate the knock resistance of a given fuel. It has absolutely nothing whatsoever to do with (removed ignition temperatures - typed in haste, wrong), ignition rate, burn rate, etc. It is only a measurement of knock limit - everything else is a description because the fuel in question is composed of (insert chemicals here) and not due to the octane rating.

Slow ignition characteristics often - but not always - go hand in hand with good preignition characteristics. There's an argument to be made for uncontrolled hydrogen reaction in the first steps of combustion being the leading cause of preignition. Fuels like alcohol that form water vapor in the first few chemical reactions of combustion (as opposed to later, like gasoline) tend to cool the initial reaction and quench excessive hydrogen consumption.

Of course, we can dump a ---- ton of EHN or other catalyst into the fuel, increasing burn rate DRASTICALLY, with little effect on octane and minor impact on pre-ignition characteristics.

Re: does octane change your afr?
 

that ---- is confusing. here.
The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.

a good question is, what octane levels are good for different compression ratios? .. i've got notes on this somewhere,
somebody enlighten me.

Re: does octane change your afr?
 

No, that is Cetane rating, used for compression ignition engines aka DIESEL.

Spontaneous ignition in a low compression spark ignition engine ("high CR" spark ignition engines are very low compression compared to diesel) is not from adiabatic heating of charge upon compression - utter bullshit - but from pre-ignition/chamber hotspots which has nothing to do with octane ratings.

My ---- is not confusing, my ---- is correct. Ignore the noise.

Re: does octane change your afr?
 

..... :o

that is some important information, and good to know. Dang'!

Re: does octane change your afr?
 

http://auto.howstuffworks.com/question90.htm
This is where my facts came from. ::)

Re: does octane change your afr?
 

Looks like it's Joseph Vs. howstuffworks.com

Send em an email Mr. Davis

Re: does octane change your afr?
 

Ah, so when you advance the ignition timing it raises your compression, resulting in detonation. Now who's rolling their eyes? ::)

You do realize that most lay-explainations rendered to laymen are gross oversimplifications, which lead to gross miscalculations and outright errors when laymen attempt to get technical? Cylinder pressures are not compression. Consult any basic ICE text.

Stop citing stuff you googled to look smart, and start learning about the complete system. Build the picture in your mind and BE smart.

I'll post a HMT recommended reading list today, tomorrow, next week, or whenever I feel like it. Mostly stuff MGB passed on to me. There might eventually be .pdf piracies of some of these works floating around on the bad interweb - it won't help the signal:noise ratio in the automotive community much, although it might get my ass welded to my spine by the SAE, various technical presses, et al. Most of the reading is engineer-speak, and you need to be solid with math, and ideally know basic engineering physics. First year college classes for the typical engineering-wannabe or math type will do. I'd guess there are ten people on HMT right now who could digest all of it, and only three who'd care to try... but most of you HMT guys are still teenagers, or alcoholic early twentysomethings. Who knows what the future holds?

Re: does octane change your afr?
 

I for one would be up to the reading. I do agree with you with the over simplifications out there. Things should be made simple, but no simpler. (wonder what smart guy said that)