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Old 08-02-2002, 10:50 PM   #1
DrBoy01
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Default 93 octane gas OK for a 2002 2.5 RS AT?

I've been having this debate with my friends over this, and it's really beginning to bug me.

I have a friend who puts 93 (premium) octane gasoline in his '00 Nissan Xterra. He claims he gets better milage, acceleration, etc. I've started doing the same thing with my MY 02 2.5 RS AT, and I haven't been seeing the same kind of benefits.

Should I reset my ECU to take advantage of the higher octane fuel? Should I go back to 87 octane? 89? Should I impliment a "learning period" for the ECU (drive carefully & deliberately, rather than "spirited") so it learns the optimum fuel/air ratio?

Has anyone found an optimum octane fuel for the 2.5 RS (not FAI)?
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Old 08-02-2002, 10:58 PM   #2
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Be prepared for responses from both extremes and everything between.

Personally I always use 92 in my NA 2.2 and I know that at certain times it makes a difference. You have to realize that even if the car is not tuned for the higher octane you will get some timing retard. Running a higher than needed octane fuel further guarantees that the car will run un-retarded (?!) more often if that makes sense.

For example, fill the car with 87 on a very hot day and while standing at a light slam the accellerator to the floor for a second. Hear the knock? Now do the same with the 92. I bet it will be less.
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Old 08-02-2002, 11:47 PM   #3
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What's the problem with 93 ?

I've running with 93 or 94 since I got my car (2002 RS AT) and there's no problem so far. I'm quite happy with it especially after I replaced stock filter with a K&N.

I don't know why everybody is saying that we should use 87 for our RS ? Generally, for a car with compression ratio 10:1, it's better to use higher octane such as 93 or higher. In some country, they ( government and gas supplier ) even suggest using 98 for a car which has compression ratio of more than 10:1.

The only reason I can think of for using 87 is it's cheaper...

Last edited by aek; 08-02-2002 at 11:53 PM.
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Old 08-03-2002, 12:48 AM   #4
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Quote:
Originally posted by aek
The only reason I can think of for using 87 is it's cheaper...
... and it's what Subaru specifies for the 2.5L engines, but what would they know? Hell, they only spent millions of dollars developing and extensively testing the drivetrain, what could they possibly know about my engine that I don't?

Quote:
Originally posted by aek
I don't know why everybody is saying that we should use 87 for our RS ? Generally, for a car with compression ratio 10:1, it's better to use higher octane such as 93 or higher.
There are all sorts of factors that determine what octane should be used in an engine, and the static compression ratio is only one of them. Our engines are all aluminum, which means more heat is transferred away from the combustion chamber, which means you can run more compression without causing pre-ignition. The dynamic compression ratio, which depends on valve timing, is really what determines what kind of pressures the combustion chamber sees, and when it sees them. Etc, etc, etc...

Quote:
Originally posted by aek
What's the problem with 93 ?
There is no problem with using 93, it's just a waste of money. The engine just doesn't need premium gas to operate normally. If you do a search for "octane" in this forum you'll find a number of discussions about this subject. Higher octane gas doesn't give you better gas mileage, it doesn't improve acceleration, it doesn't make the engine run "cleaner" unless there is some problem with your engine that prevents it from running properly on the specified 87 octane! In that case, yes, going to higher octane gas might show noticeable benefits, but that just means you're hiding a problem rather than fixing it.

Pat Olsen
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Old 08-03-2002, 01:16 AM   #5
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Old 08-03-2002, 02:03 AM   #6
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Patrick Olsen: The engine may not need 93 to run properly, but it does run optimally with it.

So what if it costs more, say you fill up with 15 gallons and the price difference was 20 cents. That is only 3 dollars.

nukeyfish: realize that just because the government says so it doesnt mean its true. I can easily demonstrate the difference between 87 and 92 in my car. Sure cruising down the highway the 87 and 92 are exactly the same. Its when you put extra strain and run at wider throttle that the timing retardation occurs.

And NONE of you can argue that the knock sensor in the subaru engines is not over reactive. If anything the subaru engines benefit more from the higher octane than most.



I wish we could view the timing map that is generated after normal driving. I bet two identicle cars, one running 87 and one running 92 would show a significant difference.

Realize that for the majority of public they could even run 87 in the vehicle that asks for 92 and not notice.
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Old 08-03-2002, 01:08 PM   #7
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OR,,,,,,,,,,,,,, you could find a happy medium like 89. I use it, runs no differently than 92, or 87 for that matter.
Plus, it's only like $.05 more than 87.
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Old 08-03-2002, 01:33 PM   #8
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I filled up with 89 today. Running a little more normal, I think. Thanks for the advice, guys I don't want to waste the cash on a more expensive octane fuel that my car really doesn't need.
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Old 08-03-2002, 06:24 PM   #9
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Car and Driver did a test with three different cars: the M3, the Accord 4 cyl, and some other car. (I can't remember which) The tired different octanes for each car and then ran them on a dyno. The accord ran fine on regular but received no benefit at all from premium, the other car (I cant' remember what happened.), the M3 ran sluggish on regular (the ECU retarded the timing to prevent knock) but went back to normal on premium. Pretty much they came to the conclusion to never use lower octane than what's recommended for the car (owners manual) but the high octane won't hurt, or help, the performance of your car. They did state that if the car was designed to sense what octane the car was running, it could adjust timing to take advantage of the higher octane. Problem is, that the ECU's arn't programed with that in mind.

So you want to run higher octane and see a real performance boost? Buy a performance chip. They are designed to sense and take advantage of the higher octane gas.
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Old 08-03-2002, 08:46 PM   #10
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Default 92

I had been running 87 in my Legacy just fine until I filled up with 92 in prep for nitrous feed. Pulling out of the gas station my certified butt dyno reported a "felt faster" difference. Whatever. I run 92 now all the time whether on N2O or not. Yes, the factory says 87 is fine but some of those $ millions are spent balancing pros/cons and compromising for mass sales. For my personal values, the extra money for the higher octane is worth it. For others, they need to decide themselves.
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Old 08-03-2002, 10:35 PM   #11
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Default Everyone relax

As you can see DrBoy01, ciper was right. Here comes the "I'm a such n' such" or "Your engine must be bad" or "It's a waste of money".

As for me. It took alot of trying different grades and brands to see whay my car liked best. So far it likes the taste of Exxon or Gulf 93 the best. Big deal if it costs me a little more each week,, it's my car & my $$$. Sunoco and Mobil just never did it for some reason,, but oh well.

Well that's my .02 cents.

Ken
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Old 08-03-2002, 11:19 PM   #12
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And just so everyone knows, 93 octane will make you run slower than 87 octane and will decrease gas milage (all else being equal)

Eric
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Old 08-03-2002, 11:30 PM   #13
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with my RS

if found that switching back and forth between brands made much more of a negitive differance than any thing else... stick with one brand

i ran 87 90% of the time ... when the air temp went over 100 .. i switched to 89
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Old 08-04-2002, 03:44 AM   #14
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Living in L.A. and going into the mountains from time to time the extra $$$ for 92 is totally worth it...no knocking at all on steep grades hauling bikes or camping gear or both.

One look at the inside of a motor that has been suffering from long term chronic detonation and you'll think it's worth it too.

-Milan
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Old 08-04-2002, 08:11 AM   #15
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Lightbulb A challenge....

Higher octane characteristics cause the fuel to burn slower. High octane is for PERFORMANCE engines....TURBO engines.....SUPERCHARGED engines.

I've heard and read that you are wasting fuel by putting in 93 for an 87 car....and guess where that ends up? In your catalytic converter and out your tail pipe.

If you guys who think the higher octane fuel makes a difference....why not fill up with 110 octance and make the car perform even better. While your at it, dump a can of paint thinner in your tank to clean them injectors too!

You want proof higher octance isn't necessary for NA factory engines? Go for emissions testing.

Once with 87 octane the first time and the second time with 93.....compare the results.
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Old 08-04-2002, 11:34 AM   #16
Kevin Thomas
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Default Octanes

I remember running 87 octane in my Outback Sport awhile back and I posted the results. I ran at the same track on two different days but same conditions. I actually ran slower with the 93 octane (which I have been running since because I want to be able to hit the juice when I want).

There wasn't a BIG difference in my 1/4 mile times. Maybe a 1/10 or so but that was good enough. I bracket raced my Outback Sport A LOT so there's not saying that maybe it ran faster this night with the 87 due to it being colder, etc, etc. I did not expect those results and I never tested it again. What's odd is that the 93 octane felt faster than the 87. Wierd!

Perhaps more torque down low with lower 87 octane (because it's burning better) and slightly more hp up top with 93 octane (because it's not retarding timing as much). Who knows!
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Old 08-04-2002, 11:43 AM   #17
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Default Re: A challenge....

Quote:
Originally posted by OutbackMack
Higher octane characteristics cause the fuel to burn slower. High octane is for PERFORMANCE engines....TURBO engines.....SUPERCHARGED engines.

You want proof higher octance isn't necessary for NA factory engines? Go for emissions testing.
you talkin about every n/a car?? what about the integra type r, nsx, celica gts, rsx type s, ect...
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Old 08-04-2002, 12:02 PM   #18
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Default Re: Re: A challenge....

Quote:
Originally posted by BlackWagon
you talkin about every n/a car?? what about the integra type r, nsx, celica gts, rsx type s, ect...
I think it's safe to say that Mack was referring to Subarus, seeing as this is a site dedicated to Subarus and this a discussion about what octane to use with a 2.5L Subaru motor . Obviously there are cars that come from the factory with a higher state of tune, which is reflected by the fact their manufacturers spec premium gas for those cars.

Pat
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Old 08-04-2002, 07:45 PM   #19
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Default Re: Re: Re: A challenge....

Quote:
Originally posted by Patrick Olsen

I think it's safe to say that Mack was referring to Subarus, seeing as this is a site dedicated to Subarus and this a discussion about what octane to use with a 2.5L Subaru motor . Obviously there are cars that come from the factory with a higher state of tune, which is reflected by the fact their manufacturers spec premium gas for those cars.

Pat
Like the 3.0 H-6 in the Legacy Outback which does require Premium Fuel according to the manufacturer (Subaru) and is N/A.
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Old 08-05-2002, 02:06 AM   #20
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Default

This argument is getting rediculous and happens about once a month here. Like I;ve said before (even in this post). ALL ELSE BEING EQUAL, YOU WILL RUN SLOWER AND GET WORSE GAS MILEAGE WITH 93 OCTANE IF YOU RUN FINE WITH 87 OCTANE!!!

Jeez.

The higher the octane, the slower the fuel burns. That's why high compression or Forced induction engines use it. It helps deter detonation, not make you faster. If I could run 87 in my car, I would love to! And you are complaining about it? 87 Octane gas burns faster and therefor gives you more power but causes detonation earlier (all else being equal).

So, if you run fine with 87, use it. Save your money.

I use 91 (Highest here in AZ) but only because I have to. I used 87 in the car when it was NA and it ran fine.

Eric
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Old 08-05-2002, 11:36 AM   #21
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Imprezinator you are partially wrong.

Your description of the higher octane fuel is not really true. You are right that it will burn "slower" but not that it will reduce the amount of energy or the speed at which it is delivered to piston and ultimately the wheels.

The lower octane fuel when inside the combustion chamber can have or form molecules that will react before the flame front reaches them, creating more than one pressure wave. To get the most usable energy from the gasoline, the air fuel mix needs to burn evenly, in a uniform fashion from the spark plug out towards the top of the piston.

More often than not you will see an increase in performance AND fuel efficiency when switching to the next higher octane. If the car calls for 87 Id suggest 89. Personally I will continue to use 92.

Imagine it like this. Build a small machine that will accurately measure the amount of force applied. Say it was a hammer than would hit the side of a weighted sled.

If the hammer hit the sled for a short amount of time repeatedly equaling 100 units of work then pushed it in a single quick shove also equaling 100 units of work the sled would move farther the second time.

Now lets say the surface that the hammer was hitting was made of wood. If you hit it many times the wood would deform and absorb much of the energy into heat. However if you push it in a controlled fashion more of the energy used to move the object.

This is exactly why higher octane fuel is better for ANY vehicle. Itís just that some will show relatively small gains to the owner. Im in "tune" enough with my car to see the differences. When I let my brother in law borrow the car one time it had a half a tank. He was nice and filled it up for me with generic 87 octane. I could tell the difference. Sure much of it could have been in my head, not all though.


edit: the fully technical description and rude comments removed

The fuel property the octane ratings measure is the ability of the unburnt
end gases to spontaneously ignite under the specified test conditions.
Within the chemical structure of the fuel is the ability to withstand
pre-flame conditions without decomposing into species that will autoignite
before the flame-front arrives. Different reaction mechanisms, occurring at
various stages of the pre-flame compression stroke, are responsible for the
undesirable, easily-autoignitable, end gases.
During the oxidation of a hydrocarbon fuel, the hydrogen atoms are removed one at a time from the molecule by reactions with small radical species (such as OH and HO2), and O and H atoms. The strength of carbon-hydrogen bonds depends on what the carbon is connected to. Straight chain HCs such as normal heptane have secondary C-H bonds that are significantly weaker than the primary C-H bonds present in branched chain HCs like iso-octane [13,14].

end qoute
Which if I understand correctly means the higher octane fuels have a stronger bond to the hydrogen so they are less reactive, while still containing an equal amount of energy.

Also speed of burning or flame front speed is MOST effected by mixture ratio.

Another thing you have to realize is that this is a ron+mon average. Meaning that low octane fuel could be 82 octane or less under certain circumstances!!!!!! That is being nice, with a difference of only 10.

This is also why people prefer a certain gas. For example, I always have had good luck with chevron. Lets say their fuel is 89+85/2 to get 87. Now go down to El Cheapo and the gas is 92+82 to get 87. Now you tell me which you would rather have? Some gas has even greater sensitivity than this!

Last edited by ciper; 08-05-2002 at 06:39 PM.
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Old 08-06-2002, 01:22 AM   #22
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In practice, refer to those above that had data which ilustrates otherwise. I, too, have carefully collected data that concludes that 92 will yield poorer gas mileage as compared to 87.

Of course, I stick with Amoco exclusively, which I feel is a factor. I agree that the brand makes a differance. Geography does, too. The gas business is a complicated one, and from region to region, what gets put into gas - even for the same brand - will vary.

Bottom line - if you're engine doesn't ping on a lower octane, then stick with it (brand and octane). You will get better mileage and power. If it does ping, try changing the brand before you change octane. Don't buy off-brand crap gas. If it still pings, then try 89. If it still pings, take it in to the dealer, because there is something wrong with your engine.

(For the context-free ppl in here - I'm talking about the 2.5L NA EJ25 Subaru motor. )

- Steve
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Old 08-06-2002, 01:09 PM   #23
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Your suggestion for listening to pings does not work, especially in subaru engines. It has been mentioned many times even in this specific forum that the knock sensor is even over reactive. Besides the point of the knock sensor is to reduce the amount of knock just below audible levels, so even when you cant hear it some exists.

Once the ECU creates the custom timing map even if the octane is too low for the application you should not hear anything.

For those of you who primarily drive around in the city or set the cruise control on the highway I agree that premium will have little effect. I myself regularly hit WOT and put more than average load on the engine daily so I need more than plain 87. Maybe 89 would be good enough though.....
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