Just got '06 wrx wagon - wow!! What octane?
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    Just got '06 wrx wagon - wow!! What octane?

    I just bought an '06 wrx wagon and I can't wait to get out of the break-in period! What a car!!!

    The manual states that I must use at least 91 octane. Ok, 91 is fine with me, but what's the effect of using 93 octane?

    I know on engines that take 87 octane, using 91 is actually bad (And I understand why, as well) but I'm not sure how 93 would affect my wrx. I'm guessing I should stick with 91.

    The main reason I ask is that there's a station nearby that discounts 93 on tuesdays and thursdays, so if it's better to get 93, I'll do so there.

    Thanks!

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    Registered User wrx wagone's Avatar
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    More octane = better.
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    Is in Love With the Admins Here 20WRX04's Avatar
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    Quote Originally Posted by wrx wagone
    More octane = better.
    +1
    Go with the 93 at all times.
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    Registered User Robert46123's Avatar
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    It won't hurt to go 93 octane, but you won't see any gains from it. If you modify the car later on and go with Cobb AccessPort for your EM you'll see gains from 93 octane as they have maps specifically built for it.
    Last edited by Robert46123; 09-26-2006 at 02:20 PM.
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    Registered User tayhadar1's Avatar
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    Quote Originally Posted by Supagoat
    I just bought an '06 wrx wagon and I can't wait to get out of the break-in period! What a car!!!

    The manual states that I must use at least 91 octane. Ok, 91 is fine with me, but what's the effect of using 93 octane?

    I know on engines that take 87 octane, using 91 is actually bad (And I understand why, as well) but I'm not sure how 93 would affect my wrx. I'm guessing I should stick with 91.

    The main reason I ask is that there's a station nearby that discounts 93 on tuesdays and thursdays, so if it's better to get 93, I'll do so there.

    Thanks!
    If you have the ability to get 93 do it. I can only get 92 where I live
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    Registered User blahblah's Avatar
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    my autos teacher told us once that if your car is rated for 87 for example and you put 93 in for a while, then go back to 87 it will be bad cuz the cars used to the 93. he had a lot of bs to throw around but i think he may be right on this one
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    Registered User D0WNxSH1FT's Avatar
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    If you have 93oct as an option, then use it. Higher octane will give you better MPG and better performance (plus more protection from knock).

    Once you start modding you will consistanly beat 91oct guys on the dyno mod for mod.
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    Registered User Damon_Chambers's Avatar
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    Quote Originally Posted by Robert46123
    It won't hurt to go 93 octane, but you won't see any gains from it.
    que? not gonna see any gains from it?

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    Registered User D0WNxSH1FT's Avatar
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    Quote Originally Posted by Damon_Chambers
    que? not gonna see any gains from it?
    If u have the 93oct option, use it. It will be benefitial.
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    Administrator Trainrex's Avatar
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    Using 93 in our cars is a good idea. It wards off detonation, and allows the ECU to advance the timing as far as it can go.

    Using 93 in an average non-turbo beater will do nothing for you but empty your wallet quicker.

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    Registered User D0WNxSH1FT's Avatar
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    Quote Originally Posted by Trainrex
    Using 93 in our cars is a good idea. It wards off detonation, and allows the ECU to advance the timing as far as it can go.

    Using 93 in an average non-turbo beater will do nothing for you but empty your wallet quicker.
    When i had my RSX, it was rated at 87oct and 27/31 mpg. I used to put in 93oct and I would consistantly see 33mpg around town.
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    Registered User goBoating's Avatar
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    Congrats

    Congrats on the Wagon! I'm partial to the OBP Wagons myself.

    Last edited by goBoating; 09-28-2006 at 01:22 PM.

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    Administrator Trainrex's Avatar
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    Quote Originally Posted by D0WNxSH1FT
    When i had my RSX, it was rated at 87oct and 27/31 mpg. I used to put in 93oct and I would consistantly see 33mpg around town.
    Coincidence.

    Higher octane fuel is more resistant to burning. If you don't have a detonation problem from a super high static compression ratio, or forced induction, you won't make any more power, or improve economy with higher octane fuel.

    If 93 octane bumped up your mileage to 33mpg, 100 should net your more!

    Incorrect. You will see no benefit.

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    Registered User drummerboy's Avatar
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    Quote Originally Posted by Trainrex
    Coincidence.

    Higher octane fuel is more resistant to burning. If you don't have a detonation problem from a super high static compression ratio, or forced induction, you won't make any more power, or improve economy with higher octane fuel.

    If 93 octane bumped up your mileage to 33mpg, 100 should net your more!

    Incorrect. You will see no benefit.
    I'd have to disagree on this one. Higher octane fuel is not more resitant to burning, actually it burns even better, more efficiently, completely and smoothly leading to a more energetic burn. What a lot of people mistake as "resistance to burn" is acutally its tendency to burn predictably which is why higher octane prevents knock. All the cars I've seen that consistently use higher octane see some kind of increase in mileage: my brother's Protege5, my dad's Pontiac G6, even his old '89 Tempo.

    Oh, and welcome to the club! 91 is fine for the WRX... that's what's recommended... 93 won't hurt either.
    Last edited by drummerboy; 09-28-2006 at 03:22 PM.
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    Administrator Trainrex's Avatar
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    Quote Originally Posted by drummerboy
    I'd have to disagree on this one. Higher octane fuel is not more resitant to burning, actually it burns even better, more efficiently, completely and smoothly leading to a more energetic burn. What a lot of people mistake as "resistance to burn" is acutally its tendency to burn predictably which is why higher octane prevents knock. All the cars I've seen that consistently use higher octane see some kind of increase in mileage: my brother's Protege5, my dad's Pontiac G6, even his old '89 Tempo.

    Oh, and welcome to the club! 91 is fine for the WRX... that's what's recommended... 93 won't hurt either.
    Copied and pasted. I didn't write this.

    Effects of octane rating

    Higher octane ratings correlate to higher activation energies. Activation energy is the amount of energy necessary to start a chemical reaction. Since higher octane fuels have higher activation energies, it is less likely that a given compression will cause knocking. (Note that it is the absolute pressure (compression) in the combustion chamber which is important - not the compression ratio. The compression ratio only governs the maximum compression that can be achieved).

    It might seem odd that fuels with higher octane ratings burn less easily, yet are popularly thought of as more powerful. The misunderstanding is caused by confusing the ability of the fuel to resist compression detonation (pre-ignition = engine knock) as opposed to the ability of the fuel to burn (combustion). However, premium grades of petrol often contain more energy per litre due to the composition of the fuel as well as increased octane.

    A simple explanation is the carbon bonds contain more energy than hydrogen bonds. Hence a fuel with a greater number of carbon bonds will carry more energy regardless of the octane rating. A premium motor fuel will often be formulated to have both higher octane as well as more energy. A counter example to this rule is that ethanol blend fuels have a higher octane rating, but carry a lower energy content on a volume basis (ie per liter or per gallon). The reason for this is that ethanol is a partially oxidized hydrocarbon which can be seen by noting the presence of oxygen in the chemical formula: C2H5OH. Note the substitution of the OH hydroxyl radical for a H hydrogen which transforms the gas ethane (C2H6) (which is an alkane) into ethanol (which is an alcohol). Note that to a certain extent a fuel with a higher carbon ratio will be more dense than a fuel with a lower carbon ratio. Thus it is possible to formulate high octane fuels that carry less energy per liter than lower octane fuels. This is certainly true of ethanol blend fuels (gasohol), however fuels with no ethanol and indeed no oxygen are also possible.

    In the case of the alcohol fuels, like Methanol and Ethanol, since they are partially oxidized fuels they need to be run at much richer mixtures than gasoline. As a consequence the total amount of fuel burned per cycle, counter balances the lower energy per unit volume, and the net energy released per cycle is higher. If gasoline is run at its preferred max power air fuel mixture of 12.5:1, it will release approximately 19,000 BTU of energy, where ethanol run at its preferred max power mixture of 6.5:1 will liberate approximately 24,400 BTU, and Methanol at a 4.5:1 AFR liberates about 27,650 BTU.

    To account for these differences, a measure called the fuel's specific energy is sometimes used. It is defined as the energy released per air fuel ratio. For the case of gasoline compared to the alcohol fuels the specific energys are as follows.
    Fuel Net energy Units
    Gasoline 2.92 MJ/kg
    Ethanol 3.00 MJ/kg
    Methanol 3.08 MJ/kg

    Using a fuel with a higher octane lets an engine run at a higher compression without having problems with knock. Actual compression in the combustion chamber is determined by the compression ratio as well as the amount of air restriction in the intake manifold (manifold vacuum) as well as the barometric pressure, which is a function of elevation and weather conditions.

    Compression is directly related to power (see engine tuning), so engines that require higher octane usually deliver more power. Engine power is a function of the fuel as well as the engine design and is related to octane ratings of the fuel... power is limited by the maximum amount of fuel-air mixture that can be forced into the combustion chamber. At partial load, only a small fraction of the total available power is produced because the manifold is operating at pressures far below atmospheric. In this case, the octane requirement is far lower than what is available. It is only when the throttle is opened fully and the manifold pressure increases to atmospheric (or higher in the case of supercharged or turbocharged engines) that the full octane requirement is achieved.

    Many high-performance engines are designed to operate with a high maximum compression and thus need a high quality (high energy) fuel usually associated with high octane numbers and thus demand high-octane premium gasoline.

    The power output of an engine depends on the energy content of its fuel, and this bears no simple relationship to the octane rating. A common myth amongst petrol consumers is that adding a higher octane fuel to a vehicle's engine will increase its performance and/or lessen its fuel consumption; this is mostly false—engines perform best when using fuel with the octane rating they were designed for and any increase in performance by using a fuel with a different octane rating is minimal.

    Using high octane fuel for an engine makes a difference when the engine is producing its maximum power. This will occur when the intake manifold has no air restriction and is running at minimum vacuum. Depending on the engine design, this particular circumstance can be anywhere along the RPM range, but is usually easy to pin-point if you can examine a print-out of the power-output (torque values) of an engine. On a typical high-rev'ving motorcycle engine, for example, the maximum power occurs at a point where the movements of the intake and exhaust valves are timed in such a way to maximize the compression loading of the cylinder; although the cylinder is already rising at the time the intake valve closes, the forward speed of the charge coming into the cylinder is high enough to continue to load the air-fuel mixture in.

    When this occurs, if a fuel with below recommended octane is used, then the engine will knock. Modern engines have anti-knock provisions built into the control systems and this is usually achieved by dynamically de-tuning the engine while under load by increasing the fuel-air mixture and retarding the spark. Here is a white paper that gives an example: [3] . In this example the engine maximum power is reduced by about 4% with a fuel switch from 93 to 91 octane (11 hp, from 291 to 280 hp). If the engine is being run below maximum load then the difference in octane will have even less effect. The example cited does not indicate at what elevation the test is being conducted or what the barometric pressure is. For each 1000 feet of altitude the atmospheric pressure will drop by a little less than 1 inHg (11 kPa/km). An engine that might require 93 octane at sea level may perform at maximum on a fuel rated at 91 octane if the elevation is over, say, 1000 feet. See also the APC article.

    The octane rating was developed by the chemist Russell Marker. The selection of n-heptane as the zero point of the scale was due to the availability of very high purity n-heptane, not mixed with other isomers of heptane or octane, distilled from the resin of the Jeffrey Pine. Other sources of heptane produced from crude oil contain a mixture of different isomers with greatly differing ratings, which would not give a precise zero point.

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