Fuel Octane Experiment 87 vs 89

[warrior]

I've always put in 87 octane since I've purchased my Legacy in April 2006. I've always got really crappy gas mileage, but after I did my spark plugs/wires, it improved slightly.

At half a tank, I'd always be at around 250km. For no reason at all, I decided to fill up with 89 octane. At half, I've already traveled 300km. That's 50 more km! I kept filling up with 89 octane for a while and managed to squeeze 550km out of the gas tank which isn't too bad at all in my books.

In the last month or so, I went back to 87 octane.. and guess what, back to 250km at half. With 87 in the tank, the needle would be on 'E' at around 430km. I did this for the next two fill ups, and even drove like a grandma. This is all mixed highway/city same route everyday.. but mostly highway.

Two days ago, I filled back up with 89 octane and again, at half I'm at 300km.

My question is, why am I getting over 100km on a full tank every time I fill up with 89?? :-?

[MCarp22]

If the car is knocking on the 87, it might be pulling timing / running rich that would decrease your mileage.

[d1giPhux]

Mark will be the first one to jump on this and say it doesnt help at all to put higher octane.. but, after putting 87 in my car for the past month or more, ive been getting really crappy gas mileage as opposed to when i put 93 in it. YES, i know it does not help the performance.. but i was getting better mileage. Sad part is.. i cant afford 93 now that the gas has jumped like 40 cents.. fun fun!

[shazapple]

whats the price of 89 compared to 87?

[warrior]

whats the price of 89 compared to 87?

a whole $0.05 more a litre! So thats like $3 for an extra 100km


If the car is knocking on the 87, it might be pulling timing / running rich that would decrease your mileage.

That makes sense.. but mines an N/A.. why would it be kocking? Hmm.. I need to do a tune up.

The only thing I didn't replace last summer for my tune up was the fuel filter.. oopsies :oops:

[peugoby]

Funny this topic was brought up. I had been running 87 in my car ever since I bought it. The other day I was going up a steep hill, with my windows down under full load, and swear I could hear it pinging.
Ever since I've been running 92 and have noticed a slight increase in gas mileage, especially in city driving.

When I'm doing deliveries in my Legacy, my mileage will drop to as low as 20mpg, ( I live in a very hilly area), but on the highway I can easily achieve 30mpg. The higher octane doesn't help at all when driving on the highway, only in the city.

Thoughts?

[Sarra]

My car knocks on 87, so I run 89. I got almost 30 mpg on a 400 mile trip. 390 miles of cruise control. ;)

[studum]

Winter played a huge role in my gas mileage. Most I got in the winter was ~450km to a tank and that whole tank was @ 80km/h. Avg winter tank was ~410 to ~430 km full to gas light.

I know winter is pretty well over because I just got 485km before the light came on and I wasn't even trying. I only use 87 octane.

Here it is when I got to school
http://i39.photobucket.com/albums/e197/studum/DSC00009.jpg

Gas light came on, on the way home.

Why did I take a picture? I have no idea... wasting time before class playing with my phone.

[Wiscon_Mark]

If you're knocking on 87, it makes perfect sense that 89 will work a lot better.

And if you've been running premium fuel for a long time, and go back to 87, it's likely that you will get poor mileage because your ECU won't be used to it.

That being said, running higher octane fuel still doesn't make sense on a stock vehicle that is running in good condition (not knocking, etc).

A little tidbit about 89 octane - on some fuel stations (probably most) 89 is just the overflow between the 87 and the 91/93 so it's usually not a perfect 89 octane, but more likley closer to 87 than anything.

Another thought: They might not be oxygenating the premium fuels and still delivering winter gas for the 87 octane - that would certainly explain why 87 is delivering bad fuel economy.

[studum]

A little tidbit about 89 octane - on some fuel stations (probably most) 89 is just the overflow between the 87 and the 91/93 so it's usually not a perfect 89 octane, but more likley closer to 87 than anything.


Wow, didn't know that at all. Sounds shitty.

While on this topic I might as well ask this question. My dad has always thrown in a tank of premium once in a while to "clean out" the motor. Whenever he borrows my car he usually fills it with it too as a thanks again its good to clean it out.

Is this a myth, like complete BS? Or should I keep doing this on my own?

[backpack09]

And if you are knocking, it is time to run some seafoam through your engine to clean it up.

[Wiscon_Mark]

Seafoam will do a good job of cleaning it out - cheaper than running a couple of tanks of premium too.

I think that whole "cleans the engine out" thing is just misleading marketing - it might be true, but it's probably about as effective as the turbinator :lol:

[Huffer]

There are more "detergents" in the higher octane fuels. They do need to burn cleaner.

Also - some places that serve 87 get it from a lower grade crude supply - so sometimes you're not even getting 87. If you are getting pinking/knocking from the 87, then run the 89.

It even says so in your owners manual.

[warrior]

This thread makes baby jesus happy!

Seafoam is such a hard thing to find here in Canada.. no suppliers have it. No one even know what it is.

Thanks for the info.. now I just have to hunt down seafoam!

[Sarra]

This thread makes baby jesus happy!

Seafoam is such a hard thing to find here in Canada.. no suppliers have it. No one even know what it is.

Thanks for the info.. now I just have to hunt down seafoam!

Order it off teh internet.

[scottzg]

higher mileage engines tend to have more carbon buildup on the pistons and cc's, raising the compression. this is a good thing, the carbon keeps the cc's hot, but higher compression requires higher octane gas. Really common for old cars to need good gas.

'cleaning the engine out' usually refers to the fuel system. a clogged injector can make for a lean cyl, which will cause the engine to pull timing. prem gas tends to have more corrosive stuff in it (more toluene, etc) so it can clean the injectors system out, sometimes. The junk that you pour in the tank tends to work better though.

The italian tune up is a similar idea- drive the snot out of the car and the injectors get hot and the fuel and oil system get really pressurized, blowing out any crap. Pretty common to see cars that get their whole rev ranges used to run better than the granny'ed ones. That said, it's harder on the accessories.

[backpack09]

There are more "detergents" in the higher octane fuels. They do need to burn cleaner.

Also - some places that serve 87 get it from a lower grade crude supply - so sometimes you're not even getting 87. If you are getting pinking/knocking from the 87, then run the 89.

It even says so in your owners manual.

I dont know about out in the sticks :), but here in the eco hippy state of taxachusetts. Everything is stricktly regulated. The fuel is tested every few months at every pump in the state, to make sure that they are both the right grade, and the accuracy of the pumps.

[AJM]

I've always done well on 87 with no problems. I can usually net about 35mpg ,but I also spend most of my 70 mi commute on the highway as well as having an upgraded igntion system.

[sansMYSTi]

Winter played a huge role in my gas mileage. Most I got in the winter was ~450km to a tank and that whole tank was @ 80km/h. Avg winter tank was ~410 to ~430 km full to gas light.

I know winter is pretty well over because I just got 485km before the light came on and I wasn't even trying. I only use 87 octane.

Here it is when I got to school
https://sl-i.net/FORUM/

Gas light came on, on the way home.

That is pretty much exactly what I am getting for gas mileage. I run on 87, and it really only occaisonally knocks under hard accelleration or heavy load (occaisonal crappy gas). When I first got the car, I actually ran acetone with 87, just because I had some acetone left over from my STi, and the car knocked with little provocation - so I figured running a higher octane wouldn't really be beneficial. It is always recommended that you run the octane to which your car is tuned for (either from the factory or other source). Sure you may get more gas mileage, but it depends on how your ECU can adjust - if you get knock on a higher (or lower) octane than your car is tuned for, then it is not worth it, regardless of extra performance or cost savings. Just IMO.