wat does it do, and is it safe on our tbi cars?

 

Well if I understand it correctly the high octane is needed if you are running more advance on your timing. YOu will need the octane to cut down on knock. If you are not running more advance on your timing or high engine compression, then I don't think you will gain anything.

Some one please chime in if I am incorrect.

Bill

 

Octane refers to the amount of compression the atomised mixture can take before it ignites. Usually on our cars if you run more than 4* advanced you need at least 89 octane. Lower compression ratio cars can use less octane because the compression stroke does not generate enough heat to pre-ignite the fuel (knock). High compression motors need a higher ocatne rating to keep the fuel from burning before the spark sets it off. There are numerous factors that will also limit the amount of ocatane you can run based on ingnition set-up, flow rate of fuel, compression ratio, and timing across the power band. There is such a thing as to much octane. If you have too much ocatane the fuel becomes hard to ignite at its highest compressed state.

 

You would want to run the lowest octane that you can safely run without knock from sheerly a performance standpoint. That would make the most power, of course the differences are minimal but the difference in $$ are pretty big so use the lowest octane you can get away with.

 

If you really want to know how octane works go to the site www.howstuffworks.com and check it out. They have a really informative document on octane and what the 87 octane means when buying gasoline

 

well i run 8* advance with no knock with 89 octane sometimes 93 octane

 

Did you data log to make sure you are not getting any knock? The human ear can only hear serious knock. You should be okay though with 8* advanced and 89+ fuel.

 

I was using the good stuff (91 octane or so) on my then stock 305 TBI and it ran like crap. It took all I could do just to get it to chirp the tires. But after I went back to 87-89 octane, it runs MUCH better and I can peel out alllll day long. So dont waste your money if you dont need the good gas.

PS- howstuffworks.com is a great place to go!

 

I have only run and will only run 91 in my car. There was one time when Arizona had a "Gas Shortage" (please) and all there was left was 87 and when I used that, my gas milege dropped down 6mpg. Then entire time it just didn't seem like it was running quite right. I don't know, but yeah, my car only likes 91...

Bruce (90RS305)

 

ive usually run 93 octane and occassionally i have run 100 octane in my camaro with no problems. correct me if im worng but doesnt the higher octane keep the fuel passages a bit cleaner then lower octane..

 

Im pretty sure thats right ^

 

I've done a lil' research in the past and what I came up with is the higher octane fuel is just straight up more pure, so more of it ignites when the spark goes off, resulting in better power (sometimes). I didn't see anything about knock and compression abilities, so thats interesting to hear! For people that use 87/89 octane I hear it is recommended like every 3-5 tanks using the premium grade to help clean up the fuel system a bit. I know one really popular way of getting your car to pass emmisions down here in AZ is to use 91 octane and an octane booster. I've seen a car go from not passing to passing doing that alone!

Bruce (90RS305)

 

add an ethanol blend to the equation. i think we have 10% ethanol in out gas here. this allows more 02 in combustion and possibly more power. but i run a 2 stroke engine in speed runs so ethanol blend i believe helps with power but be carefull for a hole in piston if jetted too lean.

 

funny thing, i heard that ethanol is far less efficient than straight gasoline. Something like if you drove a car that got 30mpg on gasoline, the same car would get 20mpg on pure ethanol. Besides it takes so much electricty in the process of making ethanol that the whole thing seems counter productive to me. Sorry, i had to rant.

 

It is for a decrease in emmsions not for added performance. As the alcohol burns its components separate and combine with the harmful pruducts of normal petroleum combustion and turn them into harmless byproducts.

 

so you produce less emissions per gallon of gas/ethanol used but you are burning more gas. Still seems counter-productive to me...

Also, i believe the octane rating is a measure of resistance to cylinder pressure . As you said shifty, cylinder pressure is affected by comp ratio, spark timing and other odd factors. Like what your cylinder heads are made out of... i believe aluminum dissipates heat quicker, meaning combustion chamber temps are cooler, meaning that cylinder pressure is lower, which allows you to use more compression. Also, boost, clearly, increases cylinder pressure meaning that you have to use higher octane gas or find ways to lower cylinder pressure, usually by lowering comp. ratio. Even then, i guess, you still have to use high octane gas.

Someone correct me if i screwed that up.

 

Just For The Hell Of It...
I used to get a moderate but noticable enough to get pissed about knocking, when I used 87...so I started to use 89 and sometimes 93...and the result was a loss of the knocking.
-G

 

Yup you got it pretty much dead on. The only thing that I would add is that the pressure will still be the same with aluminum heads but because they are slightly cooler they are less prone to pre-ignite the fuel than cast iron. This is assuming equal chamber sizes with equal timing curves. Sometimes cast iron heads get so hot the heat itself combusts the fuel prematurly and causes detonation.

EDIT: Forgot to add about the alcohol. it is counterproductive because you are using it as a band aid or crutch to get you passed the sniffer. It just hides problem that still exists. It is just to help get you through the sniffer if you are trying to track down the root of your problem.

 

okay, i was thinking that as temperature decreases so does pressure. LOL, i was trying to use what i learned in physics

 

if ethanol allows more 02 in combustion process and the A/F ratio is enrichened to compensate would that not create more heat of combustion and hence more TQ and HP? can this be compared to N02 with added fuel result is more HP/TQ? detonation is less of an issue on a 10 second 1/4 mile run than a one mile run.

 

No you are right here but the temp difference in the combustion chambers during compression between the materials is minimal so is often neglected. This is not to be confused with the conduction properties of the two where aluminum is far supperior.