Myrdhyn

iTrader: (3)

Ok, I have a different sort of "What octane to use?" question.

Here's my situation:

-I understand all about higher octane = slower burn = less gas mileage but safer
-I'm not worried about saving money.
-I have both 91 and 93 available in my area.
-My car is basically stock (soon to have free mods and lid). it has stock tuning
-My car does not knock on 91 octane even at WOT or when it is being pushed hard

My question is, taking all of the above into consideration, am I leaving anything on the table in terms of power, gas mileage, something else I haven't thought of, etc. if I run 93 octane? Or will I see a small increase in mileage and/or power running 91?

91RS383

First off, why would you get lower MPG from higher octane gas? Unless your right foot get's heavier. I get better fuel economy on higher octane due to the computer running the optimum spark maps. The increase in fuel economy pays for the increase in octane.

2nd, take a quick poll of what stations offer 93 vs. what stations offer 91. Typically your discount stations will offer the lower quality and lower octane gas. Buy the quality gas and your injectors, valves, etc. will stay cleaner.

My '02 is picky. She runs much slower (0.1-0.2 sec) at the track on 91 octane gas.

Myrdhyn

iTrader: (3)

Higher octane gas burns slower, and therefore can potentially less completely making less power per giving volume of gasoline per stroke, which translates to less gas mileage. Conventional wisdom says for maximum MPG you run the lowest octane gas your car can handle. This can be confirmed on hypermiling forums all over the internet, and I myself have tested it in both my cobalt and my firebird. The cobalt gets SUBSTANTIALLY less gas mileage running 93 vs 87 (talking 5+mpg difference), the firebird gets slightly less (~1-2mpg difference) running 93 vs 91. Also, I have 91 and 93 available to me both at reputable gas stations, so I'm not worried about the quality of the gas.

I'm also not particularly worried about the injectors/valves/etc b/c I keep up with my maintenance including seafoam/injector cleaning. I guess my real question is how much difference in spark advance, and therefore power, does the stock PCM tuning make when using 93 vs 91? It would seem to me that if you aren't knocking (causing the PCM to pull timing) it is going to run the same spark advance regardless, it isn't like the car has a "magic octane detector" that automatically advances spark timing extra when you put octane with a 2 higher value. At least that would be my assumption.

I know this seems like I'm stressing over minutia, but I'm curious if I'm leaving anything on the table power wise when I run 91, or if I can save the extra drive and few extra cents and get the 91 that is closer to my house without sacrificing anything.

91RS383

Sounds like you've done your homework then and your car likes 91. Stick with that.

Myrdhyn

iTrader: (3)

It likes both, I'm trying to decide which to make my "regular" gas. I like to pick a gas station and stick with it for years for the majority of my gas purchases after I move (bought a house a few months ago), get to know the attendants/owners/etc. Yeah, I know....I'm a freak...whatever, anyway if the stock PCM tuning really can compensate noticeably in spark advance going from 91 to 93 octane, then it would be worth the extra few mins driving down the road to get 93, so that's what I'm trying to find out.

91RS383

I hear what you're saying. I log every drag race pass in a log book and use a weather computer with an o2 sensor to predict e.t. at the track. I can tell you without a doubt that it makes a diference on my car. If your car gets better fuel economy on 91 I would use that.

Myrdhyn

iTrader: (3)

Ok, I get you now. I don't have any drag passes (ever) to compare it to unfortunately.

TooLateVTEC

iTrader: (2)

Stick with the 91 since your basically stock unless your going to be racing it, then I would use 93.

Myrdhyn

iTrader: (3)

Yeah, I figure after boltons and whatnot, I'll have it tuned for 93 octane specifically, but for now 91 is what I think i'm going with.

mikedamageinc

iTrader: (49)

I would try a dozen tanks of "brand A" and a dozen tanks of "brand B" and compare mileage of each tank taking into consideration driving habits and see if there is a noticable difference in mileage, power, etc.

Thats what I like to do and it works for me, some cars/gas brands make a difference and some dont.

99 ss

iTrader: (12)

just wondering is 91 oct just as good as 93 couse i used to use 92 oct then thay went to 91 i hope my car wont ping on 91

01ssreda4

iTrader: (96)

That is just plain incorrect. By your theory we should all run 0 octane, power would be infinite. Seriously, with an internal combustion engine and higher octane fuel you can run more timing so indirectly higher octane DOES make more power. Your car has fuel/spark maps for this and will use them if you use premium fuel. Lower octane with a lower octane fuel map will not make more power then higher octane with the higher octane fuel map.

Once again, another rediculous piece of information The cobalt is an economy car. Non performance models of all cars are designed to run off 87. Period. They get less mileage from premium fuel bc they arent designed to run off them. The manufactures and your government dont want every econo-box running 93 octane gas. Higher octane creates more emissions too btw. All the DD here in this house get 87 octane and thats what they are made to run on. The camaro gets 93 every fill up. <<--- the reason for that is that I no longer have low octane fuel/spark tables in my computer so my car is forced to run the more aggressive tables whether it wants to or not. So I buy my fuel accordingly.

Have a nice day

jimbo98z

iTrader: (3)

My question is if you're not worried about saving money, and the only thing you might possibly lose would be 1-3 mpg's which only translates to a little more money due to slightly more often fill-ups, and you know it WON'T cost you anything at all in terms of performance, why wouldn't you just run the 93 and call it a day?

Myrdhyn

iTrader: (3)

01ssreda4: I meant to say "burn less completely" and I'm sorry if it sounds like I'm being rude because I'm absolutely not trying to be, but it is not a theory, that's how it works or at least it is what I've always been told. You are forgetting to think about WHY econoboxes get less gas mileage off of premium not just that "they aren't designed for it". It burns slower, and the spark tables aren't advanced enough to take advantage of the slower burn and pre-ignition resistance, so they both make less power due to less advanced spark tables and use more gas due to slower burn. That's what I meant to say before, I'm sorry if I implied that lower octane directly = more power on its own outside of all the other variables. At least that's how I've always understood/read/experienced it to be. Either way though, my twisted brain is enjoying the conversation.

Jimbo98z: Good point, and something I hadn't considered because I have to overcomplicate everything

baalic

iTrader: (8)

My 98 Mecury Tracer gets better fuel mileage on premium fuel....as does my v6 Camaro, as does my z28. Also, if your "higher octane" fuel is higher octane because of methanol you WONT get better gas mileage you will get worse mileage.... but your internals will probably stay looking new longer....

Try NON ethanol gasoline at higher octane...if your still getting worse mileage.....well...I dunno cause on every car I have, prem fuel = higher mpg..... On that note...the best octane in this **** hole of Texas I live in, is 91 octane.......

Myrdhyn

iTrader: (3)

That's also a very good point, I haven't been able to find any non-ethanol gas, and I've been looking for a long time.

Vik

I run 1/2 a tank of E85 and 1/2 tank of 93. I noticed smoother idle and acceleration as well as a more aggressive exhaust note. I can't really tell if the car is any faster or not, all I do know is that it still hauls ***. E85 has an octane rating of 100–105 and in combination with 93, I not only am saving a bit of money than if I were to get all 93, but I'm also running a higher overall octane rating. Its a neat little trick I've been testing with. Feel free to question or flame. =]

baalic

iTrader: (8)

I tried the same thing actually, ...14 mpg highway....yuck......LOL but your right, maybe for a "track day" it would be great

Vik

Really? you had that low of an MPG? I noticed mine was a bit lower but not by THAT much. Haha, I guess I have a lighter foot than you. Im prolly at 18 or even 20 MPG highway. But I'll exchange the 4-6 more MPG for the higher octane benefits and the nicer note.

91RS383

The reason our cars are not flex fuel vehicles is because the fuel lines were not designed for the increased levels of ethanol. I would not run anything more than 10% ethanol on our older cars.