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Old 07-19-2004, 10:12 PM   #1
Geoffman72
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Default What octane do you use in your stock 2.5 engine?

I've always run 87 in my 2.5 RS and always noticed a bit of pinging at certain points and times and switched to 93 a few weeks back. The car runs smoother, pulls harder, and the pinging is gone.
The confusing part is, I know Subaru rated this thing for 87. I guess its just a case of using what your car runs best on.
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Old 07-19-2004, 10:33 PM   #2
Tim Sanderson
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I've tried higher octane only to find my mpg plummet. I may try 91/93 again soon to see if anything has changed.
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Old 07-19-2004, 10:39 PM   #3
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Though your car will run much smoother and with less noise. You are actually losing power because your car is designed to burn 87. 92/93 octane fuel burns much slower and is ideal in force fed engines (turbo, supercharger, etc.) So basically, all that extra pull is in your head. Don't waste your money. Just fill up with a bottle of techron every 5-6 tanks to clean your system and stick with the 87.
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Old 07-19-2004, 11:05 PM   #4
NC2.5RS
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Just stick with 87.
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Old 07-20-2004, 01:34 AM   #5
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I use teh best 87 my rubles can buy... And since I'm in wunderful Calif... Its probably 85 octane... since the 91 octane is a joke... lol

If I'm hitting any canyon's my car seems to appreciate 89 octane.. only because I really push my car...

Plus I'm a gas snob... I'll drive outta the way to find a Chevron. lol

Chris.
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Old 07-20-2004, 01:35 AM   #6
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93 b/c i got advanced timing
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Old 07-20-2004, 07:39 AM   #7
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I guess I need to do a good check up on the engine. As for 93 being ideal for forced induction engines, this is definitely true, but its also a pretty good idea in engines where the compression ratio starts in the double digits.
I've always thought Subaru really conservatively tuned the NA 2.5 from the factory to be able to run on 87 with the compression ratio it has. Maybe I'll look more into ECU reflashing...
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Old 07-20-2004, 08:21 AM   #8
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Some cars will advance their own timing a bit until they hear knock, I don't know if Subies do this or not. Your pinging is due to carbon build-up in the combustion chamber area most likely. It gets hot enough to cause pre-detonation. Clean her out real good and switch back to 87, or run 89.
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Old 07-20-2004, 09:43 AM   #9
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Anything under 89 without ethanol, anything above that is a waste of $$$. Brian
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Old 07-20-2004, 10:28 AM   #10
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I run 91 out here in Colorado because of my Torque II Chip. I still get around 325-400 miles on a tank of gas with mostly highway but a lot of mountain driving. I also honestly believe that the 91 out here is not 91 is probably closer to 88-90 like California.
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Old 07-20-2004, 11:33 AM   #11
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87 and I just deal with the detonation during the warm days. It's just tip-in anyways, so it's not a real big deal since the motor is under little load at that point.
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Old 07-20-2004, 01:53 PM   #12
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I ruled out carbon deposits as I run Redline fuel system cleaner every 5,000 miles. I dont believe we have any station that has no ethanol fuel here in Western NY.

Car also has had a plug change with perfect gap like 4k ago and a new fuel filter about 7k ago. I've been meaning to do wires, perhaps those will help by some marginal amount.
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Old 07-20-2004, 02:04 PM   #13
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Not to sound like an a$$, but what do you mean by perfect gap? I have found varying numbers, with Subaru recommending a range, while the auto parts stores for some reason say 44 (Subaru lists .39 to .43). Ethanol is the devil.
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Old 07-20-2004, 02:28 PM   #14
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There's nothing scientific about the tests, but I've casually observed that 93 yields the best performance/MPG when I'm on a long trip and the miles are all highway. Around town, though, there doesn't seem to be much difference between grades, so I just get the cheap shtuff.
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Old 07-20-2004, 08:06 PM   #15
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I get the same MPG on whatever i use...i drive about 35000 miles per year.
I do notice pinging with the lower octane though, but i dont care, let it blow i have 4 more blocks waiting..
Nick
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Old 07-20-2004, 08:09 PM   #16
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Quote:
Originally Posted by projectrally
There's nothing scientific about the tests, but I've casually observed that 93 yields the best performance/MPG when I'm on a long trip and the miles are all highway. Around town, though, there doesn't seem to be much difference between grades, so I just get the cheap shtuff.
But does the better gas mileage ofset the added cost of the more expensive fuel?
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Old 07-20-2004, 08:21 PM   #17
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Perfect gap meaning I tripled checked to make sure each was within subaru specs and equal to each other.
Jeez 3BadHabbits, are they just RS blocks or?
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Old 07-20-2004, 08:29 PM   #18
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4 rs blocks with 45000 on all of them and an wrx block with 11000
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Old 07-20-2004, 09:50 PM   #19
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niiiiice!
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Old 07-21-2004, 01:18 AM   #20
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Quote:
Originally Posted by obyone
But does the better gas mileage ofset the added cost of the more expensive fuel?
Well, the price difference in grades equals out to about the cost of one extra gallon of fuel if filling the tank from empty. If I'm on a long trip, the 93 probably gets me pretty close to a free gallon over the span of a whole tank. So, it pretty much balances out, or gets so close that the cost savings are negligible.
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Old 07-21-2004, 04:39 AM   #21
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I use 91...

Art
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Old 07-21-2004, 10:55 AM   #22
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My dad bought a used Kia Sportage up here that was running like hell after only 25k miles. We put it on a 91 octane diet and put some, fuel system cleaner in the tank and after a few weeks the Kia was running great?! It currently sees 89 octane.. keeping in mind that its a Kia. At 87, it pings like hell, worse than any car I have ever heard.
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Old 07-21-2004, 11:35 AM   #23
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Quote:
Originally Posted by Tim Sanderson
I've tried higher octane only to find my mpg plummet. I may try 91/93 again soon to see if anything has changed.
odd, I get better mailage on higher octane but the offset in price evens it all out.

87 + cheap price + low mileage = 89 + higher price + high mileage

I'm back to 87 octane now.
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Old 07-21-2004, 06:07 PM   #24
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Quote:
Originally Posted by projectrally
Well, the price difference in grades equals out to about the cost of one extra gallon of fuel if filling the tank from empty. If I'm on a long trip, the 93 probably gets me pretty close to a free gallon over the span of a whole tank. So, it pretty much balances out, or gets so close that the cost savings are negligible.
Well then there really is no point, unless you're pinging. Still, I'll remain skeptical till an actual scientific test is done by another member.
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Old 07-22-2004, 08:10 AM   #25
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The rule of thumb is to run the lowest octane that you can get away with without pinging, if you're looking for best performance. Because higher octane gas is more expensive, general public assumes it must be better and that is not true. From the responses, I see you guys got more brains then that so it's all good, he he Gas quality is very important. Crappy stations have old gas and the additivies used in the gas are not very good, hence when I gased up at Getty, my car was pissed. I found best results with Mobil or Exon which are pretty much the same thing. Here are my observations from the past (I spent way too much time investigating this before):

1. I used to use 93 and driving was ok. Decent milage 25m/g, performance was not that great.

2. Later on, switched to 87 and the car ran great. Much better performance, I did not generate more knock with lower octane gas and my economy went down by about half a gallon to 24.5g/m.

3. I experimented with a mod where I would bypass the crankshaft vents away from the intake. 87 became a problem and my performance sucked. I switched to 89 and the car liked that a lot more. The economy remained around the same, 24.5g/m.

4. I removed the mod and I am back to 87 with an improved intake and right now the setup gives me the best performance. Even on a hot day, the car pulls very well.

After all of this time, one thing I have noticed is that every time I switched from 87 to higher octane, the first gas tank would produce a great economy rating, 26g/m or more but after that, the ecu adapts and the economy goes back to around 24.5m/g. I just want to throw this out there, but when you experiment, go at least 2 gas tanks before making the call. One is not enough to get a good judgment. By the way, I measured my ignition through an OBD2 scanner so I got real numbers to support my conclusions.
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