do i really need premium gas for my new car?

hey all car mags,

i recently bought a 2006 subaru 2.5 xt ltd outback. it has the new 2.5l dohc intercooled turbo boxer engine (its fast!). the manual says that the turbo engines perfom best using premium gasoline (91 aka octanie or higher).

now, i have heard conflicting reports on weather or not premium fuel is worth the extra $ or really even does anything (as well as the other argumenst that your car runs better on higher octane fuel, etc).

what say you, car mags? premium gas is mucho dinero vs regular...

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Originally Posted by freshies

hey all car mags,

i recently bought a 2006 subaru 2.5 xt ltd outback. it has the new 2.5l dohc intercooled turbo boxer engine (its fast!). the manual says that the turbo engines perfom best using premium gasoline (91 aka octanie or higher).

now, i have heard conflicting reports on weather or not premium fuel is worth the extra $ or really even does anything (as well as the other argumenst that your car runs better on higher octane fuel, etc).

what say you, car mags? premium gas is mucho dinero vs regular...

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Tubrocharged = higher compression ratio = preignition of low octane gas (i.e. gas goes boom before the spark fires) = engine damage, but you'll save $3 per tank. Your call.

IMVHO, cut starbucks out once per tank and buy the good stuff for your car.

Not even $3, really, assuming that you'd otherwise use the mid-grade gas rather than regular.

I believe that Subaru's ECU attempts to adjust for lower octane gas by reducing available power (I don't know of a modern turbo that doesn't), but isn't that what you spent the extra money to get?

you have a turbo. get the premium.

ok, sounds like everyone here believes the premium is worth it....will do then. thanks guys

You have to run the good stuff. Lower octane gas will cause some knock and the ecu will pull timing to prevent any damage to the piston ringlands. This comes with a decrease in power as well. You dont want an oil-eating, cracked piston engine.

Enjoy your new car!

Here you are:

http://tetongravity.com/forums/showt...ghlight=octane

Some engines recommend premium in order to get the highest HP output; some engines require premium in order to avoid engine damage. The engine computer can adapt somewhat for premium vs. regular gas, but I guess there are limits to this -- hence the "required" premium on some cars. Go with what the user manual says.

Mrs. C.'s Tacoma is in the category of "recommended" premium, but user manual says regular gas is just fine -- she uses regular unleaded, and no knocking, no problems, 20K miles so far.

Dunno on the Turbo 2.5 from Suby... my observations.

I had a Taurus SHO for years with the Yamaha 3.0 DOHC normally aspirated and it ran fine on 87, or even 85 in the mountains, but it did run with slightly better power on 91.

http://www.geocities.com/uglymoney/Dscn0992.jpg

My Suby Outback with the 2.5 definetely, absolutely, picks up power when I put 91 in it. Very pronounced, especially at low rpm high throttle settings when it seems to be able to really take advantage of 91's lower flash point with much more aggressive timing, and in fact, I normally run premium when towing. When I switch back to 87 it pings for the first couple miles until it backs the timing off slightly, and I lose power.

Its interesting to note that while my SHO with the Yamaha which was supposed to run premium ran fine on 85 octane at altitude, my Subaru, which recommends 87 as the 'minimum' octane will not run on 85 without pinging uncontrollably at certain throttle settings even at very high altitudes.

So a lot of it is just marketing I think. A 2.5 Subaru without the Turbo is being marketed at cost conscious consumers, so they recommend 87, while the H6 (which I am relatively certain can run fine on 87) and the turbo (not certain) with their more upscale buyers gets the 91 recommendation.

Here is what I'd do though. Run your tank down to nearly empty, put five gallons of 87 in the tank. Take a drive, listen for pinging at various throttle settings. If you hear nothing you can probably burn 87, at least for highway trips. If it pings while holding the throttle at set positions, that means it has too much boost on top of its compression ratio (yes, lower than the normally aspirated) to burn 87 without pre-igniting the fuel diesel style. If thats the case, fill it the rest of the way up with 91 and never look back.

Go with what the manual says. If it says use 91, use 91. The manual for my Honda says use 87, so I use 87. It's not rocket science.

id say use the premium, but who knows if its just all the same anyway. coming back to boulder from michewaka in poudre canyon i passed two stations where all the octanes cost the same amount of money. WTF? And just saw a news report about how tons of gas stations havent been checked by dept of weights and measures in years and "water" down their shit

from someone who has FIRST HAND EXERIENCE tuning a 14:1 compression ratio racecar engine to 14,000 RPM here is the deal.

engines have a knock sensor that detects detonation.
if detonation it adjusts firing.
you will lose power, but thats all youll lose.
You will know if your engine is detonating if you hear a medium pitched "pinging" or "cracking" sound

If you're going to be doings lots of driving that will place your engine under higher load, ie mountain driving or racing, I would reccomend you use at least 89-91, but for driving around the city like a normal person, the cheapstuff will be fine.

also forced induction engines generally have LOWER compression ratios to account for the higher Volumetric Efficiencies attributed to the compressor (turbo\super chargers).

Does the increase power of 91 lead to better miles/gal for regular highway and around town driving?

Or does it only help when you are dropping the hammer like Freshies driving to Tahoe for a powder day?

I’m seeing the difference between 89 and 91 around $0.06 with 89 costing around $3.20. So, if I could get even 1 more mph out my cars I would come out ahead. Cars: 2004 XC70 and 2006 Forrester

If youre driving around town regularly, chances are you arent loading your engine enough to create a scenario where it will detonate and wont make any difference. Also depending on how each engine manufacturer programs their ECU, it could retard fuel as well as spark to compensate for detonation and the possibility exists for lower fuel consumption.

^^^^ Yep, what pechelmand said.

The energy content of 85, 87, 91 is the same (with seasonal variations). The energy content of any of those fuels with an ethanol oxygenate will be lower by 2-3 percent depending on percentages, so using ethanol would make a bigger difference than octane.

Bottom line is this though. If, in order to run a lower octane fuel than what is recommended, your car has to retard the timing in order to avoid detonation, then yes, you will lose fuel economy. For instance, my Subaru running 85 pings constantly, so the engine is retarding the timing as far as it can, lowering the engines efficiency, and I am losing mileage. But going from 87 to 91, especially for routine driving, would do nothing for my mileage since the engine is able to run pretty aggressive timing at most 'routine' throttle/rpm settings even on the 87.

I believe I gain power in my engine by running 91 at very high throttle settings because the engine is able to be so aggressive with the timing, so perhaps, in some applications, 91 would get me better mileage, but it would be on top of already very poor mileage from driving so aggressively.

Short answer: doubt you'll save fuel by going from 87 to 91 unless your driving like crazy to catch first lift on a powder day^^^^ in which case you might go from fucking terrible mileage, to really terrible mileage.

huh, I always heard that you will get worse mileage runnning 91 in a car designed for regular than you will if you just use the regular. More power but less mileage. Just what I've heard, though.

edit: well I just read that any mileage difference would be negligible, both regular and high-test contain about the same amount of energy per gallon. High test simply resists detonation so it can be used in higher-compression engines and hte extra power is from the higher compression not the gas itself.

I believe in the hi octane. Here's what my turbo'd SAAB's done for me over the years (as of April) thanks to a couple bucks more each week or two.
Attachment 14875
1/3rd of a million & still going. Almost all mountain driving. Plan on takin' it to a half mil.
Been the cheapest car ever in the long run.

FWIW:
Subaru 2.5 Turbo = 8.2:1 comp. ratio
Subaru 2.5 NA = 10.0:1 cr

If you're not getting "knock" with regular, it's OK to run it. You probably won't really notice the difference as far as performance or MPG. As the ambient temp increases, you may need to switch to a higher octane. Also, some "Unbranded" stations sell some pretty shitty gas from time to time.

2001 Outback H6 3.0 Liter Wagon

Book says to use 91. I use 89 and every tenth tank of so, 91.

No problems, no ping, no power loss.

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Originally Posted by FrankZappa

Here's what my turbo'd SAAB's done for me over the years (as of April) thanks to a couple bucks more each week or two.

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And that, my friends, is a classic 900 Turbo dash.

Frank, which vintage? SPG, by chance?

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Originally Posted by Keoni

FWIW:
Subaru 2.5 Turbo = 8.2:1 comp. ratio
Subaru 2.5 NA = 10.0:1 cr

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Because the turbo pressurizes the intake air, requiring less compression to get to the same air/fuel mixture.

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Originally Posted by alpinedad

And that, my friends, is a classic 900 Turbo dash.

Frank, which vintage? SPG, by chance?


Because the turbo pressurizes the intake air, requiring less compression to get to the same air/fuel mixture.

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Not meaning to be a smartass but, air/fuel is a control of the ECU. Just pointing out that an NA motor will have a higher compression ratio than the turbo version.

Quote:

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Originally Posted by alpinedad

And that, my friends, is a classic 900 Turbo dash.

Frank, which vintage? SPG, by chance?

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85. First year they went to the 16 valve & the last year they didn't have to put that silly brake light in the rear window. Nay on the SPG. - which has been OK with all the snow I've driven in I guess.

You're on the original turbos? If so that's sick.

We had an '86 900s for a long time, didn't get the turbos because we thought they'd be a maintenance issue. I finally killed it by running into a manhole cover that was six inches too high and covered by leaves. 45 to zero in a milisecond and a bent frame/unibody later, we had to move on, so...

Then we had a '94 900S convertible, nice car but it was never the same after it got parked with the roof open for the thunderstorm of the century.

I dug both cars though, SAABs are (or at least were) pretty cool.

now i know why our fuel is so expensive ;)
..... the average gasoline is 95 octane here and you can get premium gasoline with 100.... even the cheap has 91.

And i thought it was the same worldwide. Interesting.

*this is just a random reply.

Quote:

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Originally Posted by El Chupacabra

Some engines recommend premium in order to get the highest HP output; some engines require premium in order to avoid engine damage. The engine computer can adapt somewhat for premium vs. regular gas, but I guess there are limits to this -- hence the "required" premium on some cars. Go with what the user manual says.

Mrs. C.'s Tacoma is in the category of "recommended" premium, but user manual says regular gas is just fine -- she uses regular unleaded, and no knocking, no problems, 20K miles so far.

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Tacos recomend premium? new to me. my dad's 02(?) has never seen premium, and has almost 80k on it.