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This is a discussion on GAS - High Octane Truth or Myth? within the Everyday Impreza Talk forums, part of the Community - Meet other Enthusiasts category; the problem with ethanol is that while it has a higher octane rating, the amount of boom in the same ...

  1. #16
    Registered User wrxwalsh's Avatar
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    the problem with ethanol is that while it has a higher octane rating, the amount of boom in the same volumes of gas and ethanol is a big difference.

    octane ratings are simply under what presure the fuel explodes, but it says nothing about BTU's. the BTU of gas vs. ethanol is a pretty big difference.

    and moral of the story-

    gas > ethanol


    Energy density - Wikipedia, the free encyclopedia
    /\ pretty cool article explaining energy density.
    Gasoline- 34.6 MJ/L
    Ethanol- 24 MJ/L

    Matt
    Last edited by wrxwalsh; 06-08-2008 at 02:10 PM.
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  3. #17
    fre
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    Yes, you have to run roughly 30% more E85 than gasoline to produce the same power, but I am making more power on E85 than I ever did on race fuel and while I get only 16-17mpg, it costs $3.30/gal and race fuel costs at least $7.00 a gallon for 110.

    The decision is obvious for me. Of course if your car is a daily driver and not just a fun car, then pump gas is definitely more efficient (I get around 23-25mpg on pump), but certainly not as fun

    When you do an actual comparison $3.30/gal @ 16mpg vs. $3.90/gal @ 24mpg. Lets say you go 100miles with both so 100/16 = 6.25 gallons * $3.30 = $20.6
    for gas it would be 100/24 = 4.167 gallons * $3.90 = $16.25

    So is $4 per 100 miles worth the extra 100+ horsepower and torque I get... ummm yes.
    Last edited by fre; 06-08-2008 at 02:22 PM.
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  4. #18
    Registered User Cheers's Avatar
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    a guy I know in Vermont blew his head gasket after ~40 K miles...he ran his WRX on regular...that was the best "I told you so" ever
    "Winners generally are not nice people, they try to be but are immensely selfish, immensely arrogant, have total belief in their own ability and nothing else matters when they are at work." -Eddie Jordan

  5. #19
    Registered User SVTorWRX's Avatar
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    I vote use the recomended gasoline...

    Subyman34 - you are using 87octane because its cheaper right? but you say you've noticed a 2mpg drop, which negates your "it's cheaper" mentality; do the math, find out your wrong and then buy the right gas... yes modern engines do have the magical knock sensor, but you fail to realize taht while that knock sensor is adjusting your timing, its effecting your fuel economy & performance. where I live gas typically jumps 10 censt per grade.. i.e. 87=x 89=x+.010 91/93=x+0.20; I drove a 2003 SVT focus and averaged 34mpg on 91/93 octane (high test required because of 10.2 cr & 7300rpm redline), I figured out that 1mpg roughly equaled 10 cents per gallon; so if you get 2mpg better on the recommended fuel, then you break even if the fuel costs 20 cents more per gallon, not to mention you don't have to worry about your knock sensor failing and destroying your engine, turbo, and all those other expensive components under the hood; a knock sensor is not designed to be actively removing timing from your engine on a constant basis; so how bout you pay the extra 3 bucks a tank and save yourself some headaches down the road? If you guys really want to save money on gas, just stay out of boost when you are driving to work, or better yet buy an economy car and give my your WRX, its an AWD turbocharged flat four that weighs more than 3000lbs; what about that leads you to believe it would be a cheap car to operate?
    I'm sorry, did I just jack your thread?

  6. #20
    Administrator RayfieldsWRX's Avatar
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    I fill my WRX roughly twice every 4 weeks. If I were to run the cheap gas in it, it would save me <$5 a month. I'll give up a couple of Starbucks coffees a month to know that I'm running the fuel for which my car was engineered, and for the knowledge that my engine is running optimally. Why buy a nice car, and then intentionally set up the engine to run crappy?

    Just as a tip for you... anecdotal information by a retired engineer probably won't help when/if you tell the buyer of your car that you ran the wrong gas in the car. Most educated buyers will take this to mean that you're cheaping out in other ways, as well, and won't want anything to do with your car at that point.
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  7. #21
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    [QUOTE=SVTorWRX;2302169]Subyman34 - you are using 87octane because its cheaper right? but you say you've noticed a 2mpg drop, which negates your "it's cheaper" mentality; do the math, find out your wrong and then buy the right gas... yes modern engines do have the magical knock sensor, but you fail to realize taht while that knock sensor is adjusting your timing, its effecting your fuel economy & performance.

    Yes it's cheaper. No i didn't say that I noticed a 2mpg drop. I said it's hard to tell because 35 miles of my commute changes daily because of construction. Some days I can sail through at 50mph other days it's stop and go the whole way.

    I did the math and it's obviously cheaper if I maintain the same mpg on cheaper gas. I'm not debating the lack of cost savings down the road because of internal engine damage because I have never found any proof of it's existence. Just as anecdotal was my comment about my fathers career so is telling me about a friends blown wrx because of low octane gas (not your post).

    btw I owned a 2003 Focus SVT- Hated it, no low end balls at all and wretched in the snow.

  8. #22
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    Quote Originally Posted by SVTorWRX View Post
    Subyman34 - yes modern engines do have the magical knock sensor,
    Also, there's nothing magical about them

    Timing Basics
    Ignition Timing is determined by the ECU as follows:

    Total Timing = Base Timing + Knock Correction Advance + Other timing compensations

    Other timing compensations = other compensations including those for IAT, ECT, per cylinder, among others.

    Knock Correction Advance = (Timing Advance Maximum * (IAM/16)) + feedback knock correction + fine learning knock correction

    Note: The IAM (ignition advance multiplier) used in the formula above is the raw value (ranging from 0-16) for the 16-bit ECU. For the 32-bit ECU, the IAM will range from 0 to 1 (substitute "IAM" for "IAM/16").

    From the formula above, you can see that Knock Correction Advance (KC) consists of three elements. How these values are determined and how they interact with one another will be discussed below.

    Although exactly how the ECU determines knock (based on input from the knock sensor) is not fully understood, the result is quite simple. The knock signal is either set or clear. That it, as far as the ECU is concerned, there either is knock or there isn't. There's no level of severity that is stored or used for knock control.

    Feedback Knock Correction (FBKC)

    Feedback knock correction (FBKC), as applied, is always either zero or negative. It is active when engine speed and load are within the ranges specified by the 'Feedback Correction Range' tables (although it can be disabled temporarily as described below). If enabled, the ECU looks at the knock signal. If set (i.e. knock), the FBKC value is decreased by about 2 degrees("Feedback Correction Retard Value"). Each time this portion of the code is executed, 2 additional degrees will be pulled if the knock signal is still set, up to a limit of about -11 degrees("Feedback Correction Retard Limit"). However, if FBKC is negative and the knock signal then becomes clear (i.e. no knock), FBKC does not immediately reset to zero. Instead, there is a delay("Feedback Correction Negative Advance Delay") in which the current correction remains until the knock signal is clear over the delay period. If so, feedback correction is increased by ~1 degree("Feedback Correction Negative Advance Value") and the feedback delay counter is reset. If not, and the knock signal is set over the
    delay period, the counter is reset and 2 degrees will be pulled. This process continues and FBKC will eventually reach zero as long as the delay period is satisfied enough times to increment the correction back to zero. Of course, "delay" is in the terms of the ecu, so this all happens very rapidly.

    As noted above, FBKC can be disabled even when load and RPM are within the ranges specified by the "Feedback Correction Range" tables. In fact, this is necessary as the two other elements of knock control (changes to the IAM and changes to fine learning knock correction, described below) cannot be active unless FBKC is disabled. ALL of the following conditions must be met in order for FBKC to be disabled. If any of the following are not met, FBKC is enabled within its load/rpm ranges:

    * Coolant temp is greater than 140F
    * A/C is off or A/C was not just turned on (an extremely brief window of time after the A/C is switched on)
    * If in rough correction mode (described below), load and RPM are within the "Rough Correction Ranges".
    * If in fine correction mode (described below), load and RPM are within the "Fine Correction Ranges"
    * Immediate change in load is less than about +/- .05 g/rev.
    * ECU is not in test mode
    * ECU is not in limp-home mode due to the triggering of specific groups of CELs.
    * An unknown timing compensation based on throttle change is not active (i.e. no compensaton).

    Rough Correction

    Rough correction involves manipulating the IAM (ignition advance multiplier) due to knock. This has the result of correcting timing advance across the board (timing advance maximum * IAM/16). The IAM can range between 0-16 for the 16-bit ECU.

    The ECU has two modes of operation - rough correction or fine correction. That is, the ECU is either poised to possibly make corrections to the IAM (rough correction) or to possibly make changes to the fine learning knock correction table (described later), but not both at the same time. But, just because the ECU is in either of these modes, does not mean changes to the IAM or the fine learning knock correction (FLKC) table will be made. Therefore, there are two sets of thresholds for each mode. One set determines when to switch between modes (different depending on the current mode) and another set to determine whether changes to the IAM or FLKC table will be made in their respective modes. NOTE: After an ECU reset, the ECU defaults to the rough correction mode.

    To exit from fine correction mode to rough correction mode, ALL of the following requirements must be met:

    * Engine speed and load must be within the ranges specified by the 'Rough Correction Range' tables.
    * Timing advance (maximum) map value is greater than 4.9 degrees.
    * Some FLKC value change (positive or negative) occured last execution.
    * The last FLKC applied value (|x|) is greater than 3.9 degrees (that is, the absolute correction -> ex. -4 = 4)
    * The last FLKC raw difference (|y| * 2.84) is greater than last timing advance (maximum) map value.
    * (IAM > 1) or (IAM <= 1 and last applied FLKC was positive).

    Once in rough correction mode, the following requirements must be met each time in order to make any change to the IAM. That is, even though the ECU is in rough correction mode, it will not always be adjusting the IAM:

    * Current timing advance (maximum) map value > 3.9 degrees ("Rough Correction Minimum Timing Advance Map Value").
    * Limp-home mode not active (IAM would already be 0 in this case)
    * FBKC is disabled.
    * IAM step value > 1 (explained below)

    If these are met, the following will occur, however, only after switching between the fine correction mode to the rough correction mode. That is, these will only be executed once each time the switch to the rough correction mode from fine correction mode occurs and right before a change to the IAM is going to occur:

    * IAM is set to the 'Advance Multiplier (Initial)' value
    * IAM step value is set to 4 ("Advance Multiplier Step Value", described below)
    * IAM learning delay counter set to 0 (explained below)
    * The entire FLKC table in ram is cleared.

    The knock signal is checked. If clear (i.e. no knock):

    * The IAM learning delay counter is checked. If the current value is less than the delay target specified by the 'Rough Correction Learning Delay (Increasing)' table then the counter is incremented (similar to the delay for FBKC). When the current value is greater than the delay target, then the IAM is increased. On the first run through, IAM is always advanced by 4 ("Advance Multiplier Step Value"). It remains 4 as long as it is keeps increasing the IAM (up to 16). However, if during the last execution the IAM was decreased, the IAM step value is reduced by 1/2. This occurs each time the IAM flip-flops. When the IAM step value is less than or equal to 1, the rough correction mode ends (after applying one last change to the IAM) and the ECU switches to fine correction mode. Basically, this is the way the ECU determines that the IAM has settled.

    If the knock signal is set (i.e. knock):

    * There is no delay for decreasing the IAM. However the counter value is cleared because knock was detected.
    * Other than no delay, the rest of the logic is basically the same as when the IAM is increasing except that the IAM is decreased.

    In addition, when the IAM is 0 or 16, after a slight delay of remaining at these extremes, the ECU will switch to fine correction mode regardless of the current IAM step value. This would be necessary if there was not enough "settling" before reaching these extremes to exit the rough correction mode.

    Note, that when the IAM has finally "settled", the ECU will switch from rough correction mode to fine correction mode. Fine correction mode will continue until the mode switch conditions listed at the beginning of this section are met again.

    Fine Learning Knock Correction

    Fine learning knock correction (FLKC) allows for positive or negative correction to KC based on knock. These values are stored in RAM and are stored and applied across specific load and RPM ranges. The ECU determines these ranges based on the 'Fine Correction Rows (RPM)' and 'Fine Correction Columns (Load)' tables. Although these tables consist of 7 values each, the ranges make up an 8x8 3d table. For example:

    If your 'Fine Correction Rows (RPM)' table is: 1400,1800,2600,3400,4200,5000,6000 - the ranges would be as follows:

    * Range 1: less than 1400
    * Range 2: 1400 to less than 1800
    * Range 3: 1800 to less than 2600
    * Range 4: 2600 to less than 3400
    * Range 5: 3400 to less than 4200
    * Range 6: 4200 to less than 5000
    * Range 7: 5000 to less than 6000
    * Range 8: 6000 +

  9. #23
    Administrator RayfieldsWRX's Avatar
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    So, to summarize, run the gas for which the car was engineered.
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  10. #24
    Geriatric Ginger Mod Rogan's Avatar
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    buhbye spammer
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  11. #25
    Registered User Feyd's Avatar
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    Figured I would add my 2 cents. I have an 06 STi bone stock. When I ran lower quality gas in it, at high rpm I got a Misfire code. Went to the stealership and they said nothing looked wrong with my engine and asked what Octane I was running and where I usually get my gas as it could be a gas issue. So I ran some chevron fuel system cleaner and only put in 92 oct in it, havent had an issue again so far. Oh and im in Oregon and we have the lame 10% ethonol here as well.
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  12. #26

    Flirty User That's Younger Than the Chunks of Corn in Brian's Crap

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    When we had the gas shortage months ago and we didn't have any premium gas, I had to use midgrade. Halfway through my tank, my car started shuddering and would accelerate really slowly. I haven't used anything but premium since.
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  13. #27
    Geriatric Ginger Mod Rogan's Avatar
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    in my wrx, i had to run 89 for a couple tanks (issue with paying 4.40/gal for 93), I reset the ecu, and was fine, for low boost/normal driving.. one high-load time, it rattled like rocks in a can, but normal driving was fine..

    When I went back to 93, i reset the ecu again, and was just fine.
    Rogan o_0
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    '01 Dodge 2500 CTD 6-holed hand-shaker - 3850# dual disk - 900 lb/ft - SOLD
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  14. #28
    Administrator RayfieldsWRX's Avatar
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    I had issues with paying $4/gallon too...but I didn't take it out on my car.

    And everyone calls ME a cheapskate.
    --Ray
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  15. #29

    Flirty User That's Younger Than the Chunks of Corn in Brian's Crap

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    Quote Originally Posted by RayfieldsWRX View Post
    I had issues with paying $4/gallon too...but I didn't take it out on my car.
    I just continue telling my employer that he needs to give me a raise so I can fill up my gas tank.
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  16. #30
    Registered User lytlec's Avatar
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    What about a car that says to use regular but then you reflash the computer and the tune says you need to run 91....will higher octane fuel hurt a car that isn't supposed to use the higher rating? Even though the tune says now you need to run 91...?

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