using higher octane gas

[98outbackpdx]

can anyone confirm that using a higher grade of gas like using 89 octane instead of 87 octane will improve your fuel economy?
when i put plus in "89 octane" my mileage will jump a few mpg's.
does anyone know if it is actually doing something for my Outback.
my moms Chevy Astro van gets about 14 mpg on the highway using 87 octane and using 89 octane it gets about 17 mpg.
i was just curious if anyone has a similar experiences with higher octane gas.

[jduldula]

Well, from my experience with my BD, running a richer octane gas promotes cleaner burn, i.e. less prone to detonation/knock and all the other bad stuff from crap gas, but im thinking since your ecu is still running the same a/f map as like 87, it wont have much of an effect. Improving driving habits (not speeding, taking it easy on the accelerator from a stop) would get your better mpg than anything.
Just my .02

[Grafton]

imho anything over 89 is a waste, but i see a lot of ignition advance with 89, i peak @ 50 degrees advance

[impreza_GC8]

I don't think it will have a noticeable impact at all. I put midgrade in my NA car, I guess because I like to waste money LOL and I always run 93 (NOT 91) in my turbo car.

[mike-tracy]

The way a shadetree mechanic explained it to me, the older a car gets, the looser components become, especially in the valve train. Thus older or higher mileage cars can see some benefits from the midgrade, to mitigate the effect of the sloppy valves or other components. Notice, though, no brand new car (I've ever heard of) calls for midgrade.

[FlatBlack]

Use the lowest octane available for your stock N/A car.

My 1986 300ZX Turbo was factory tuned for 87, and it's... turbo.

I use 91 in my Turbo'd 280Z, only because that's the highest we have here. I also have to run a very conservative timing table >_< My old timing table was giving me audible pinging.

I was told by many NON shade tree people that running any more than 87 is a waste. There is no "cleaner burn" - it is just less prone to detonate.

The only reason I could see using 89 was for a slightly higher bump in CR on a N/A, let's say if you were using flat top pistons or your heads were shaved, thinner head gasket, etc.

You are seeing the bump in gas mileage because you are altering your driving.

[ouch1011]

Here are some misconceptions that need clearing up...

The higher the octane rating, the LOWER the amount of energy in the gas because chemicals (like ethanol) have to be added to the gas to increase the octane/anti-knock rating. Higher octane fuel is less volitile, which means it is harder to vaporize, and gas only burns if it is in vapor form. Higher octane fuel burns slower and is less volitile which means it takes more pressure and more heat for it to ignite, which makes it ideal for engines that run very hot or very high pressure (i.e. very high compression or forced induction). Some N/A cars recommend running 91+ octane because they run more ignition advance, which increases combustion pressures and can cause lower octane fuels to pre-ignite and cause detonation. But on a car that is tuned to run on 87 octane fuel (like the Legacy, aside from the early 2.5s and the turbo models) running more than 87 is a waste because the car is not tuned to run on higher than 87 octane. Running higher octane fuel in a car not designed for it can actually cause running problems because the fuel will burn too slow/cold. It can also cause carbon deposits and can actually increase emissions for the same reason.

[jey]

Notice, though, no brand new car (I've ever heard of) calls for midgrade.

Plenty of new cars call for premium and I have even seen one ask for mid grade. Higher octane gas isn't around just for people with old cars that have so much carbon buildup in the cylinders that they have altered their compression ratio :lol:

[mike-tracy]

out of curiosity, what cars do call for mid-grade?

[decke48]

out of curiosity, what cars do call for mid-grade?
3.5 Chryslers do

[Grafton]

i disagree



Here are some misconceptions that need clearing up...

The higher the octane rating, the LOWER the amount of energy in the gas because chemicals (like ethanol) have to be added to the gas to increase the octane/anti-knock rating.

Octane rating does not relate to the energy content of the fuel. It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner. there is no significant difference in energy content between 87 and 93


Higher octane fuel is less volitile, which means it is harder to vaporize, and gas only burns if it is in vapor form. Higher octane fuel burns slower and is less volitile which means it takes more pressure and more heat for it to ignite, which makes it ideal for engines that run very hot or very high pressure (i.e. very high compression or forced induction). Some N/A cars recommend running 91+ octane because they run more ignition advance, which increases combustion pressures and can cause lower octane fuels to pre-ignite and cause detonation. But on a car that is tuned to run on 87 octane fuel (like the Legacy, aside from the early 2.5s and the turbo models) running more than 87 is a waste because the car is not tuned to run on higher than 87 octane. Running higher octane fuel in a car not designed for it can actually cause running problems because the fuel will burn too slow/cold.

The higher the octane, the harder it is for the fuel to ignite. The only time you will see a cylinder temp. drop is when you run a richer mixture or have a better than stock cooling system. In general terms...the more fuel you put in it, the cooler it runs, NOT the more OCTANE. The stoich level of race fuel is also a little different though. NA (naturally aspirated) stoich levels of say 110 octane is around 13.7 (depending on brand) instead of the industry typical 14.7 for 93 pump. So it can be confused with running cooler because the race fuel inherently burns at a richer (cooler) level than pump gas with no computer changes.

It can also cause carbon deposits and can actually increase emissions for the same reason.

Carbon deposits are formed from overly rich AFR or oil leaking into the cylinder, and if an engine has a LOT of carbon build up running higher octane will net some of the performance back to that car by resting knock that might have been caused by the carbon build up in the first place

[ouch1011]

Higher octane rating in general doesn't ALWAYS mean that the fuel has less energy content, but with pump gas, 91+ octane does have less energy than 87. Its not as significant as the difference between pump gas and E85 (which is about 104 octane, btw) but it is there.

Higher octane fuel is harder to ignite because it doesn't vaporize as easily, and liquid fuel doesn't ignite. I'm not talking about race fuel, it is different than typical pump gas.

Cars tuned/designed to use 91+ typically have higher compression and/or more timing advance, which causes the fuel to burn faster (which is the reason they use 91+ octane in the first place). Using 91+ in a car designed for 87 means the combustion process will happen too slow or too late and result in the carbon deposits. Same thing can actually happen by using 87 in a car designed for 91+ because the PCM will pull timing in response to detonation.

Think of it this way. You take two sources of energy that have nearly the same energy content, but one burns very very rapidly and disipates all of its energy very rapidly, and the other disipates the same amount of energy slowly...which one is burning hotter?

I thought I'd mentioned it in my other post but I guess I didn't. Older cars with high mileage can benefit from higher octane fuel due to carbon deposits and general wear causing detonation. I think the OP pretty much is proof of that, both his and his mom's cars have knock sensors and are high mileage. The difference in mileage could be due to the fact that the PCM has to pull timing when running 87 octane due to the condition/wear of the old engine, but doesn't as much with the 89. But the same engine in better condition would most likely not see the same benefit.

[decke48]

different octane ratings have no difference in energy (in a controlled state) as long as the mix is at the same in gas, ethanol, methanol, ect. octane is rated as how much compression it take to detonate the gas. now gas at stations is never as high of octane as its rated, because of the contamination, in travel and the stations tanks sediments and ground water. and usually they drop the rating 1-2 points. which is legal.
this will effect cars in different ways depending on the condition of the engine, where you get gas, and how you drive. so some may benefit from a different grade. but the only way to know is to try and will vary per person. and will not work for everyone

[StatGSR]

since it hasnt been touched on yet. there is a huge difference between oxygenated and non-oxygenated gas. which is where i think most of you guys are getting all fussy about energy content.

if you can get non-ox, get it! I'm glad i can still find the stuff by me, 91 non-ox ftw!

[goodnight1985]

I have a 97 2.5 GT with 220,000 miles on it. It still feels strong and has plenty of pull. I have done a little experimenting in the past with fuel to see what differences I could get out of different octanes.

With a full tank of 87, I usually go 370-380 miles on a tank. With a full tank of 89, only about 340-350 miles per tank. But I also watched the ignition timing with a computer and the 89 octane allowed for more ignition advance and the car feels like it has a little more go.

This is just my two cents and a little give in the data needs to be taken probably because I pushed the car harder on the 89, but it does feel like has a little more power.

[ScaryFatKidGT]

Running higher octane and compressing the fuel more allows the engine to get more power out of the fuel how ever it wont be significant enough to offset the extra fuel cost.

How ever when I go to wisconsin the 89 is the same price as 87 so I throw that in there and I seem to get 2mpgs better but I haven't thoroughly tested it yet.

I have a 97 2.5 GT with 220,000 miles on it. It still feels strong and has plenty of pull. I have done a little experimenting in the past with fuel to see what differences I could get out of different octanes.

With a full tank of 87, I usually go 370-380 miles on a tank. With a full tank of 89, only about 340-350 miles per tank. But I also watched the ignition timing with a computer and the 89 octane allowed for more ignition advance and the car feels like it has a little more go.

This is just my two cents and a little give in the data needs to be taken probably because I pushed the car harder on the 89, but it does feel like has a little more power.What do you use monitor and change timing?

[Grafton]

Running higher octane and compressing the fuel more allows the engine to get more power out of the fuel how ever it wont be significant enough to offset the extra fuel cost.

How ever when I go to wisconsin the 89 is the same price as 87 so I throw that in there and I seem to get 2mpgs better but I haven't thoroughly tested it yet.
What do you use monitor and change timing?

i use an obd scanner, of you only watch one output the update is not slow at all. in my area i seem to break even between the increased fuel cost and the improved mpg

[goodnight1985]

Make sure the OBD 2 scanner can do live readout, some of the smaller, cheaper ones can't, but I have one at work: A Matco CAN200 that did the read outs for me. Easy to use.

[ScaryFatKidGT]

I have a 97 2.5 GT with 220,000 miles on it. It still feels strong and has plenty of pull. I have done a little experimenting in the past with fuel to see what differences I could get out of different octanes.

With a full tank of 87, I usually go 370-380 miles on a tank. With a full tank of 89, only about 340-350 miles per tank. But I also watched the ignition timing with a computer and the 89 octane allowed for more ignition advance and the car feels like it has a little more go.

This is just my two cents and a little give in the data needs to be taken probably because I pushed the car harder on the 89, but it does feel like has a little more power.Did you advance the timming with an aftermarket EM or did the car do it on its own because I didn't think they could do that?

Make sure the OBD 2 scanner can do live readout, some of the smaller, cheaper ones can't, but I have one at work: A Matco CAN200 that did the read outs for me. Easy to use.Scan Gauge II???

[prodriver1]

I used 91 non-oxygenated gas in my car for like 5-6 fill ups in a row and started having fuel filter issues, i had to change 3 of them out as they kept getting clogged with what looks like charcoal, any thoughts on this?

[decke48]

Same station? Then Change gas station tank is probably contaminated. See if it helps
If not you might have deteriorated rubber lines?

[prodriver1]

Same station? Then Change gas station tank is probably contaminated. See if it helps
If not you might have deteriorated rubber lines?
ya its the only station i can find up here with non-oxy. and i switched to regular premium (at a different station) and the problem went away.

[rkrenicki]

I used 91 non-oxygenated gas in my car for like 5-6 fill ups in a row and started having fuel filter issues, i had to change 3 of them out as they kept getting clogged with what looks like charcoal, any thoughts on this?

Hmm.. Perhaps the charcoal canister has burst? I do not know how Subaru canisters fail, but the canister on my wife's Elantra failed in a spectacular fashion a while back where it dumped most of its contents into the fuel tank.

Also, I am not sure why this thread was seriously necro-bumped for something that is completely unrelated? Prior to this question.. the last post was over 4 years ago...

[prodriver1]

i was just going to mention the canister thing, i did some research on it and if you over fill the tank ie keep pumping it after it clicks off automatically, it goes into the charcoal canister and then into the tank. thats totally it! :smt023

and as far as the thread goes i looked for a relevant thread to put this in and it wasnt being used for 4 years as you mentioned so.... delete it if you feel so strongly idk :smt020