Thread: Why you shouldn't be tempted by 87/89 Octane in your GTP

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  1. #1 Why you shouldn't be tempted by 87/89 Octane in your GTP 
    GTP Level Member Impala131's Avatar
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    Hello all, I know a lot of people ask why you can't use regular gas in your GTP, and some insist that you can if you stay out of the boost. Well, you can... but it's not worth it. Although it's been said before, I'd like to provide actual proof rather than just being one of the GPF "common knowledge" parrots. This is going to be fairly lengthy, so you might just want to glance through it.

    I started thinking about using 87 octane when I recently visited a small town in Tennessee where I will be working in the near future. Unlike MI where I was from, the difference between regular and premium is greater (25-35ยข per gallon), and many areas where you can't obtain it at all unless you drive out of your way. Some people have said that as long as you stay out of the boost, you can run whatever grade you want. As I understand it, the only difference between octane grades is resistance to detonation. Since I have HP tuners, I decided to conduct a loose experiment to find out how viable 87 octane is.

    I own a 99 GTP. I started by putting a 3.8" pulley on my MPS. The other mods are headers, 605 plugs, and an open cone intake. I reflashed the PCM back to a stock tune. It's probably safe to assume that if I got knock on my setup... you will most certainly get the same or worse on a stock setup. I let my fuel tank reach nearly empty (the needle was completely below the E mark) and filled up with 10 gallons of 87 octane at my local Shell. Perhaps you can nitpick over the small amount of 93 octane in my tank. At the time of tuning, it was 42* outside, with IATs of 60-70*. My ECT stayed around 185-190*

    I then drove my car carefully and followed our forum's MAF tuning method. I found out relatively quickly that the "as long as you stay out of the boost you're fine" theory didn't hold any water for my vehicle. On flat surfaces, I was fine. However, slight uphills, shifts, somewhat sudden throttle changes... all produced small amount of KR. I once peaked 4* on a shift. This was all at very low throttle levels. (Less than 20%) I responded by reducing the timing advance by subtracting from all the cells that had KR. When my part throttle/MAF tuning was done, I had somewhat butchered my High Octane timing table already. I was, however, able to add some timing to the cruise tables that effect the highway cruising gas mileage the most. It seems that my car had the most trouble with the high airflow with low engine speed scenarios.

    For the WOT tuning, I knew I would have problems, so I cut back the timing for WOT by quite a bit (And cleared the adder tables). I also ran much richer than I would ever really want... I got it to about 11.2. I purposely ran rich though so I could get away with as much timing as possible for the sake of this experiment. The most timing I could push was a bit less than stock levels, which, most L67 people will tell you is bad, because according to them, stock timing should be considered a minimum. To put it in perspective, my car (93 octane) ran 17* of timing on a 3.6" pulley and 11.5 AFR with zero knock prior to this experiment.

    I continued to cut timing from all areas that experienced knock. I'll link it below for you to see for yourself. Again, remember that the timing adder tables are zeroed out. Anyone familiar with tuning these cars will tell you that this table is pretty bad.



    Now, from a cost standpoint...both the cost of gas per gallon, and fuel economy, are the important factors. From a purely cost savings perspective (lets ignore the wear/engine damage from knock, say you have a relatively safe 87 octane tune) all you care about it how much it costs you to drive a certain distance. If you divide cost per gallon by miles per gallon, that eliminates the "gallons" from the equation... and gives you cost per mile. It makes the most sense to read it in pennies. Then you can compare apple-to-apples the cost savings of running one grade over another.

    I created an excel chart to make this easy. Just fill in the green box with the most accurate information you can, and look at the blue box. Suddenly, regular gas doesn't net much of a savings. Although I was able to get approximately the same gas mileage on both grades on the freeway, other members on various forums reported a 2mpg loss as typical, so that's the default in the table. I used the extreme Tennessee example for the table's defaults.

    Excel

    So, to summarize, I just can't see enough justification to switch to regular. Now, mid-grade (which already fairly close to the cost of premium anyway) may allow you to run stock timing in most of the non-boost cells on a car with headers and rockers/cam, although if you consider the point of a mod like a cam, it just doesn't make sense. And, keeping out of the boost is not enough to protect it from this more volatile fuel. That $4 we save over the course of 1000 miles is just not worth it, IMO. Now I get to drive like an old lady for the holiday season until I get it out of my system and fill with the correct grade.
    Why you shouldn't be tempted by 87/89 Octane in your GTP Attached Images
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  2. #2 Re: Why you shouldn't be tempted by 87/89 Octane in your GTP 
    The Blue One blueguy's Avatar
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    While I agree it can be done...and I see no problem with it if you are knock free...

    Everyone although doesn't have a tuner, so running higher octane is common practice with boost.

    It's easier to just run 91+...and a few more bucks for each fill up?

    I'll just stick to the 91+.
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  3. #3 Re: Why you shouldn't be tempted by 87/89 Octane in your GTP 
    Turbo is the way to go. BillBoost37's Avatar
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    Seems like you started tuning fairly quickly on the fuel. The PCM likely didn't get a chance to see consistent knock and put you on the low octane timing table. That table exists for when the pcm senses low octane and it's significantly less timing. Which looks to be what you concluded in pulling enough timing.
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  4. #4 Re: Why you shouldn't be tempted by 87/89 Octane in your GTP 
    GTP Level Member Impala131's Avatar
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    That's a very good point Bill, I don't know how much knock is needed to force the PCM to use the low timing table. The much lower timing on the low octane table probably explains why I did not see a decrease in gas mileage on 87 octane, while typically, other members have in the past. I guess the question to ask is, how much knock in how short of a time period causes the car to use the low octane table?

    Maybe this experiment would have been more insightful if I had left the tables alone for awhile and logged the commanded timing advance, and then been able to see how much the car takes before dropping to the low octane table. What worries me is that the KR spiked significantly higher during high load conditions, and did not seem to switch tables due to these spikes. One could potentially get a very damaging spike of knock before the computer could react.
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  5. #5 Re: Why you shouldn't be tempted by 87/89 Octane in your GTP 
    GXP Level Member darkhorizon's Avatar
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    KR out of boost is far from dangerous, it is just in-efficent.

    Scan any stock L36 on 87 octane and you will see 4-10 degrees of KR most of the time the pedal is pressed.
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  6. #6 Re: Why you shouldn't be tempted by 87/89 Octane in your GTP 
    GTP Level Member Impala131's Avatar
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    Huh, I had no idea dark, I just never saw KR on 93 octane so I figured it was bad news. I have a Lesabre with an L36, I might have to try that out. If an L36 has KR most of the time though... why did they have timing that high from the factory?
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  7. #7 Re: Why you shouldn't be tempted by 87/89 Octane in your GTP 
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    I have a 2004 GTP Comp g and have burned regular 87 octane for 9 1/2 years and 190,000 miles. Every once in a while if I feel like racing a round a little I'll put in premium. With the 87 I can here the knock if I get on it at low rpm's and then I just back off. If this is bad for my engine I have yet to notice!!
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  8. #8 Re: Why you shouldn't be tempted by 87/89 Octane in your GTP 
    GTX Level Member HVACgtp's Avatar
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    very very bad from what i understand lol
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  9. #9 Re: Why you shouldn't be tempted by 87/89 Octane in your GTP 
    GTX Level Member HVACgtp's Avatar
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    its not the knock you hear thats scary, its the knock retard that you dont hears thats the worrisome part.
    2001 Silver GT; K&N Intake, 2.5in catback to flowmaster 40's, custom tune, led tails drag wheels and halos......SOLD
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  10. #10 Re: Why you shouldn't be tempted by 87/89 Octane in your GTP 
    GTP Level Member 01gpgt01's Avatar
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    if your hearing audible knock thats not so good lol
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  11. #11 Re: Why you shouldn't be tempted by 87/89 Octane in your GTP 
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    I believe you hear the valves rattle from the pre-detonation.
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  12. #12 Re: Why you shouldn't be tempted by 87/89 Octane in your GTP 
    GTX Level Member nos4blood70's Avatar
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    Quote Originally Posted by darkhorizon View Post
    KR out of boost is far from dangerous, it is just in-efficent.

    Scan any stock L36 on 87 octane and you will see 4-10 degrees of KR most of the time the pedal is pressed.
    Mine has seen up to 6 degrees of KR at WOT. With IAT's of 40*. :/
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  13. #13 Re: Why you shouldn't be tempted by 87/89 Octane in your GTP 
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    i run 93 the top octacne i can in my gtp so premium all the way for me.
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