Must i use 91 octane?

[Moosedaddy]

...I have not seen the '07 Spec.B Legacy yet. Who knows, maybe I'll like it, but based on the features list, its missing some things. I'll compare it to the new '07 Infiniti G35 when thats available in Nov'06. I want to replace my old Subaru Legacy GT, so it could be a Spec.B, or maybe the new G35. Or maybe a Corvette...

 

Dude, the new G35 6MT Sport Sedan is HAWT. It's gonna be a close call between that and a Spec B IMHO.

[Vimy101]

I have a 2004 FX35 and a 2006 M35x.

Back in early '04, I went to look at a G35x, and drove home with an FX35. There were things I didn't like about the G, and the salesman let me take out an FX35 that had Nav and rear camera, etc, etc. I didn't know I needed those things until trying them out. The FX isn't light, but it does feel tossable and very composed and stable at near double the speed limit, even on curvy highways. Its been a good reliable vehicle so far. The M35x is a good mix of luxury and sport, with AWD. I was leaning toward a short wheelbase Audi A8, and then I heard about the new S8, and the fact that there would be a bunch of improvements for the A8 for model year '07 (such as rear camera, etc), so the '06 M35x was the next best thing (not to mention that it'll probably be a lot more reliable than the Audi product).

 

To keep some Subaru content here, I have to comment that I wish Subaru had

1) a much more loaded (sport and luxury) Legacy GT/STi/Spec.B (or whatever else they want to call it), OR

2) the rumored "executive sedan", that is a little larger than the Legacy (such as Audi A8 short wheelbase size or Infiniti M35/45 size), with all the goodies of an A8/S8 or loaded Infiniti M.

 

Years ago, I used to search for coins under my seats, just to afford gas to go out for an evening. Eventually I got older, and had some success, and now I can afford to buy more expensive cars, and the price of gas is not a worry for me anymore; but Subaru hasn't grown with me. I'd really rather buy a Subaru product, but Subaru doesn't have what I'm looking for. I have not seen the '07 Spec.B Legacy yet. Who knows, maybe I'll like it, but based on the features list, its missing some things. I'll compare it to the new '07 Infiniti G35 when thats available in Nov'06. I want to replace my old Subaru Legacy GT, so it could be a Spec.B, or maybe the new G35. Or maybe a Corvette...

 

 

Infiniti was what I was first looking at and will probably own again at some point but I was curious about AWD and the Subie seemed the better bang for the buck. I also wanted to go down in size and displacement and again SOA was the choice. Turbo power was also something I wanted to try and the turbo boxer is killer.

 

I initially wrote the Legacy off when DDing cars because it didn't really stack up as well to a G or M as far as highway feel to me. It was an excellent car for the money but a step down in terms of road manners. The Spec. B was different. The suspension is grown up. That said, it is not as luxurious certainly as an M. It's much more of a sports car in that regard. Moreso, IMHO, than the G sedan yet is still a good tourer. If the more sporty G coupe came in AWD, I might have gone for it but I prefer a sedan so here I am. The Spec is also a sleeper which I dig and you don't see many of them or even many LGTs on the road.

 

I think Subaru is on to something with the new Legacy and I also would like to see a true luxury offering in their line up. However, I think the LGT is a little too small for that application. I guess SOA has their game plan worked out so we'll see how it goes.

[rporter]

If the AWD G35 sedan would have been available with the 6-speed, the LGT would not have been a consideration.

[The B4]

Don't forget about altitude. Up here at 6500', 87 octane performs just fine.

 

are you nuts? running a lower octane in a higher altitude is worse!

[NSFW]

You know why these threads run on for pages and pages? Because half the people posting to them don't know what they're talking about, and it's tough to tell who is who.

 

Posts like "Up here, it performs just fine" and like "are you nuts? that's worse!" both suck (for those reading along at home) because they lack:

1) any indication that either post has tested both options, and MEASURED THE RESULTS.

2) any theory - any demonstration of understanding - that would help the home audience guess which poster actually knows WTF he is talking about.

 

Any poster who points out commonly misunderstood aspects of high-octane fuel gets bonus points for cluefulness. Any poster who can point to test results and provide a reasonable theory of why the results differ gets actual respect. (note, a "reasonable theory" must not appeal to guidance from the owners manual.)

 

We've all heard the conventional wisdom of high-octane fuels vs. low-octane fuels. Repeating that stuff (especially the more urban-legendary aspects of it) doesn't help the discussion. It just promotes tit-for-tat debate from people who have heard different versions of the conventional wisdom, and again, nobody laying along at home as any idea which of y'all (if either) really knows what's going on.

 

If you want to see this discussion resolved:

1) do a couple tests and post some fccking numbers

2) explain why the numbers from test A differ from those in test B.

 

Note that the same person need not do both. If you don't know why octane matters, you can still fuel up with 95, run 'til the light comes on, measure milage, refill with 87, and drive the exactly same route again (if traffic matters (and varies a lot) do the second run at the same time of day on another day (and lets us know if the weather changed). Then post your mileage figures. And post some on-board performance data logs if you got 'em. And let us know how the engine sounded (that's not objective data, but it might be interesting).

 

If you haven't got the time to run such a test, but you know how octane works, wait for such test results and explain them, or post your predictions and see if someone takes you up on a test run.

 

But, for the love of pasta, y'll gotta quit trading unsupported assertions as if they had a snowballs chance in hell of settling this issue. They don't. If you want to see this issue settled, do fccking experient and post the fccking results. That is how humans understanding of the world got to be where it is today - it'll work for Subarus too, I promise.

[The B4]

Holy Thesis Batman!!! You could have gotten your point across with a lot fewer words. And for the record, I live near the mountains, I'm in subaru country, and i've seen the effects of turbo motors vs. octane and altitude... including a friend running a Vishnu map on calling for 93 octane and running on 91 in Colorado in the mountains. He threw a rod. That's a more dramatic instance but higher altitudes and the wrong octane of fuel don't mix on the turbos.

[NSFW]

A Subaru Legacy GT fuels up at point A with 87, takes a long circuitous drive that takes the back to the gas station with the empty light on. Refuel with 93 (or whatever the local max is), note fuel pumped into tank, note trip mileage, compute gas mileage for the first trip (we'll call it T87). Take trip 2, refill, do math, get milage for trip 2 (call it T93).

 

I predict higher mileage during T87. The ECU will lower the boost pressure to accomodate the that the the lower-octane fuel ignites too easily can causes detonation. The turbo isn't doing much, fuel economy goes up from what you normally get from a GT - but of course this gain comes at the expense of horsepower. Horsepower and efficiency would be aapproaching Legacy-non-GT figures, though the higher-propression non-GT motor will still do better due to its higher compression (gimme-high-octane) design.

 

The question is - how much does fuel economy improve on T78 compared to T93? And how much does horsepower suffer? I'm guessing that the losses due to the mismatched octate/compression would be offset by the fact that the turn won't cut out entirely - in other words, you'd get a little more power than the stock non-GT car, and a little less mileage.

 

So someone can volunteer to do some testing, or we can all yak on for pags and pages about what should really happen. Perhaps some compelling theories will bubble to the top. More likely though, this will just go on for pages and pages until somebody does some friggin' tests.

[jPunanni1]

slegacy99, you are getting 32mpg??? and 27mpg on a bad day?? how are you doing that. even if i use the high test gas i only see about 24mpg. what am i doing wrong?

[SLegacy99]

slegacy99, you are getting 32mpg??? and 27mpg on a bad day?? how are you doing that. even if i use the high test gas i only see about 24mpg. what am i doing wrong?

 

You have a turbocharger and I dont. Therefore, youre not doing anything wrong.

[STG]

Not with a turbo it wouldn't.

 

 

Yes, it does.

[STG]

Holy Thesis Batman!!! You could have gotten your point across with a lot fewer words. And for the record, I live near the mountains, I'm in subaru country, and i've seen the effects of turbo motors vs. octane and altitude... including a friend running a Vishnu map on calling for 93 octane and running on 91 in Colorado in the mountains. He threw a rod. That's a more dramatic instance but higher altitudes and the wrong octane of fuel don't mix on the turbos.

 

Congratulations on not understanding anything in the post you are bitching about.

 

Just because your friend threw a rod in his modified car at altitude on 91 RON doesn't mean altitude or 91 RON gasoline had anything to do with it.

 

Read about octane and altitude on the Chevron website in an American Petroleum Institute paper. (Naturally, you and your Vishnu running buddy know better).

 

http://www.chevron.com/products/prodserv/fuels/gas_qanda/api_octane.shtml

 

You can also try reading this. But it's full of big words and numbers, so you probably won't understand the relation between altitude and octane.

 

http://www.faqs.org/faqs/autos/gasoline-faq/part3/section-1.html

 

Nah, you're better off jumping to conclusions without any scientific reasoning.

[SLegacy99]

Take it easy man.

[STG]

Take it easy man.

 

Whoa, dude! :lol::lol:

[mccorry]

People are funny

[DukeTrout]

A Subaru Legacy GT fuels up at point A with 87, takes a long circuitous drive that takes the back to the gas station with the empty light on. Refuel with 93 (or whatever the local max is), note fuel pumped into tank, note trip mileage, compute gas mileage for the first trip (we'll call it T87). Take trip 2, refill, do math, get milage for trip 2 (call it T93).

[snip]

So someone can volunteer to do some testing, or we can all yak on for pags and pages about what should really happen. Perhaps some compelling theories will bubble to the top. More likely though, this will just go on for pages and pages until somebody does some friggin' tests.

 

I think it works just the opposite. The lower the octane, the richer the ECU runs, in order to avoid detonation. Adjusting to lower boost pressures really only applies to WOT and high rpms. The rest of the time, it should be running richer, and thus burning more fuel, than on higher octane.

[mccorry]

Negative, Ghostrider...

 

Octane (R+M) is the measure of the fuel to resist detonation. Higher octane fuel burns more slowly to resist autoignition from cylinder hot spots.

 

Because it burns slower, the combustion process would be slightly less efficient than with a lower octane fuel.

 

The difference in milage will never be significant enough to mesure however.... compared to the other variables (wind, temperature, tire air pressure, etc).

[Metal2You]

Do I need to stop at red lights?

[Vimy101]

Because it burns slower, the combustion process would be slightly less efficient than with a lower octane fuel.

 

The difference in milage will never be significant enough to mesure however.... compared to the other variables (wind, temperature, tire air pressure, etc).

 

However, the resistance to pre-ignition allows for timing advance to maximize the power stroke.

[edkwon]

I think the OP is allowed to use whatever octane he/she wants. I say we let him use 89 for a prolonged period of time and report back to us if there is indeed any detrimental effects on performance and knock on an untuned engine. Who knows, maybe it wont be a problem with ordinary daily driving.

 

Ed

[Vimy101]

I think the OP is allowed to use whatever octane he/she wants. I say we let him use 89 for a prolonged period of time and report back to us if there is indeed any detrimental effects on performance and knock on an untuned engine. Who knows, maybe it wont be a problem with ordinary daily driving.

 

Ed

 

It might never cause a catastrophic problem as the knock sensor will retard timing to prevent pre-igniton. However, gas mileage may suffer among other things.

[rkl303]

I think most members here would kill to be in your situation.

[rporter]

I think it works just the opposite. The lower the octane, the richer the ECU runs, in order to avoid detonation. Adjusting to lower boost pressures really only applies to WOT and high rpms. The rest of the time, it should be running richer, and thus burning more fuel, than on higher octane.

 

Nope. The octane level has nothing to do with how rich or lean the engine runs. The ECU is not proactive, nor can it sense octane. It just has a knock sensor , and will back out timing when the sensor finds a problem.

 

In cold weather like we have now, running 89 or even 87 on a light-load freeway run could have zero effect on power or mileage, as the knock threshold is higher. How the engine reacts to octane is not a constant set of parameters.

[yeshia]

Ok... well I decided to take a new look at this thread that I started. All I wanted to know was wether or not running less than 91 octane was going to damage my car or not. Wasnt asking if it would hurt performance. Sure I spent a pretty penny on the car but why spend a pretty penny on a higher octane if its all a scam?

Basically, the answer to the original question I had asked is: NO, it will not damage the car. The car will adapt. You may (but most likely will not) notice a decrease in performace, and you may notice a decrease in MPG.

Really... 7 pages to say that? Lol...

 

Here is a decent article from USA TOday

 

http://www.usatoday.com/money/autos/2003-07-30-premiumgas_x.htm

[G.T.Subie]

dam been a while

[yeshia]

Yah.. I've been using premo, but now that its so expensive, I decided to rethink it again and I hadnt realized how long this thread got. Someone made a silly comment how this was a noobie question.. but I don't know that many noob questions whos answers take up 7 pages! Right? Still havent tried sub-91 gas, but soon I shall and Im confident I won't even notice.