What octane?

[mcbrew]

Not looking for approval.

In case you have trouble with the math, my last tank cost LESS per mile on 89 octane than my previous 8 tanks on 87 octane. Yes, the fuel cost more per gallon, but it was still CHEAPER in the end.

Is this going to keep happening? I don't know, yet. My current tank is only at 27.6 MPG, but I had to do some towing. We'll see if it goes up or not soon. By the way, I have the MPG display dialed in pretty well. It is set at -4% and the last tank was exactly the same as my calculated MPG.

I don't care why it is happening. It may have less ethanol in it. Either way, the apparent increase in MPG makes it CHEAPER to drive on this more expensive gas. I will continue o report my findings so that the trolls don't starve.

[Mr.Steve]

Originally Posted by mcbrew

Again (and this is where people stop listening), I am not out to prove anything. I just have a lot of accurate data points, so I am sharing them. I am not trying to prove that 89 octane is better than 87 octane. I have used 87 octane for all fill-ups until now. I am trying out the 89 to see what will happen, since some other folks were reporting better fuel economy with 89. If it is worse, then I will post that. So far, the mileage is looking better. I have nothing to gain by making false statements. So far, there are just my facts and other people's opinions in this thread.

If you save a few bucks that's great and you should keep doing it.

I listen just fine, I think your experiment is just a complete waste of your time that's all. This thread started out as "What octane?" so people reply. If you want to argue with every single person that replies 87 then by all means knock yourself out.

[mcbrew]

Originally Posted by Mr.Steve

If you save a few bucks that's great and you should keep doing it.

I listen just fine, I think your experiment is just a complete waste of your time that's all. This thread started out as "What octane?" so people reply. If you want to argue with every single person that replies 87 then by all means knock yourself out.

I'm not arguing with anyone using 87. I'm arguing with anyone who says I'm not getting the results I'm getting.

[countryboy9799]

Used to be around here when you bought 87 and 89 octane one would have ethanol and one wouldn't. Cant remember which one but the one with pure gas would give you better mileage, not because of the octane though. That is changed now and it is hard to get straight gas in MN, when I do get straight gas it is 30 to 60 cents higher so it is not worth the extra mileage for that. I only use it in my small engines.

[lloyd123]

Originally Posted by countryboy9799

Used to be around here when you bought 87 and 89 octane one would have ethanol and one wouldn't.

This could be a possible reason mcbrew is getting better mileage with 89 octane. Perhaps his station's 89 octane tank still has a large percentage of the pre-winter non-ethanol gas - could be that his station doesn't move a lot of the 89 octane but sells mostly 87 octane which would probably be 100% of the winter blend by now. I really have no idea how big these tanks are and if this could happen but it would explain why this is happening and make all the higher octane critics happy.

[GuitsBoy]

Too many variables in the anecdotal evidence posted above. The facts are that higher octane fuel contains less energy, gallon for gallon. Octane is simply a measurement of the fuels resistance to auto-ignite (ping, detonation, pre-ignition, call it what you will).

High octane fuel is used in performance engines since you can increase compression and advance timing much further than the lower octane fuel, more than making up the difference for using a lower energy fuel.

For instance, e85 has only 70% the energy that a gallon of gas has, but because its rated at 110 octane, the serious turbo guys can really crank up the boost and make more power than they could on gasoline.

All other things equal, you will do best running the lower grade fuel that allows for an optimal timing curve. Usually, if you will ping if you go past the what your fuel will support, but this is not always the case. You can put a car on a dyno and see that sometimes if you push timing too far, you will start to lose power even before you start pinging. But for the average joe, as long as youre not pulling any timing, you're doing fine.

That being said, I run 87 in our fourth gen OB, and I have had the bluetooth scanner in a few times, and I have yet to see it pull any timing whatsoever.

[JacquesPere]

By pull timing do you mean retard the spark?

[GuitsBoy]

Originally Posted by JacquesPere

By pull timing do you mean retard the spark?

Yes, correct.

Strong knock will result in a couple of degrees less spark advance, but will fade out after a couple of seconds. The more severe the knock, the more timing gets pulled, but you rarely see more than 4 degrees pulled out.

Not sure if subaru has a low octane timing table like GM does, but if so, persistent knock will cause the computer to switch to reading from the low octane timing table, which of course has lower timing. After a a certain number of miles or filling the tank, the computer resumes using teh high octane table.

[awd-gg]

Originally Posted by mcbrew

I'm not arguing with anyone using 87. I'm arguing with anyone who says I'm not getting the results I'm getting.

since you have so much time.... I, for one do not believe the result you posted....
GO ahead, knock yourself out!

[JacquesPere]

Originally Posted by GuitsBoy

Yes, correct.

Strong knock will result in a couple of degrees less spark advance, but will fade out after a couple of seconds. The more severe the knock, the more timing gets pulled, but you rarely see more than 4 degrees pulled out.
.

After switching from 89 to 85 octane I see no change in timing. At idle I run about 14 or 15 degrees BTDC. At higher RPMs I've noticed it go to 30 or more degrees BTDC on my Scan Gauge. I assume this is normal.