AV Gas to 91 Octane jetting changes
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AV Gas to 91 Octane jetting changes

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    Senior Member 74Bonneville's Avatar
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    Default AV Gas to 91 Octane jetting changes

    I've read enough to realize I do not want to run straight AV gas anymore. I've probably run less than 50 gallons total through this engine. I read from a GN7 post you use about 7% more fuel when running straight AV. I may still mix some AV & 91, but not much AV. Currently the engine is dialed in on the jet size running straight AV gas.

    My question is how many jet sizes should I go down? I'm thinking 2 sizes to start and then go through the process. Plug reading, how it runs, changing jet size ...

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    Senior Member EVILFORCE's Avatar
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    On a blower set up I just did was 3 steps up from stock on 750's. I went from gas to av gas.

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    gn7
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    Without knowing why you were running AV to begin with, but you are willing to pull AV out of you mix now, I am guessing you thought it would make more power even if the engine did need the octane. If that was the case, you were wrong, it will not make one HP more than 91 in an engine that does not need the octane. In fact, it was possibly costing you power depending on the tune up.

    If this is some 9.5 to 1 engine, and you really were tuned to AV, you would need to pull more then just jets out. Your timing is probably too high as well. Just a guess.

    If you're going to just splash in some 100LL, the stock jetting on the carb from Holley will be a very good place to start.
    A little AV is not going to mess with the jetting that much. Standard pump gas fluctuates that much from season to season. The compound in AV that makes it desirable as a octane improver for pump is toulene, and the toluene content of pump gas goes up and down with the seasons in pump gas. You don't need to make much of a tune change from pump, to AV + pump up to about 50%, except maybe a timing change.

    I would check your carb jetting against Holleys stock jetting, and start there.



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    Last edited by gn7; 05-09-2013 at 02:04 PM.

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    Senior Member 74Bonneville's Avatar
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    Quote Originally Posted by gn7 View Post
    Without knowing why you were running AV to begin with, but you are willing to pull AV out of you mix now, I am guessing you thought it would make more power even if the engine did need the octane. If that was the case, you were wrong, it will not make one HP more than 91 in an engine that does not need the octane. In fact, it was possibly costing you power depending on the tune up.

    If this is some 9.5 to 1 engine, and you really were tuned to AV, you would need to pull more then just jets out. Your timing is probably too high as well. Just a guess.

    If you're going to just splash in some 100LL, the stock jetting on the carb from Holley will be a very good place to start.
    A little AV is not going to mess with the jetting that much. Standard pump gas fluctuates that much from season to season. The compound in AV that makes it desirable as a octane improver for pump is toulene, and the toluene content of pump gas goes up and down with the seasons in pump gas. You don't need to make much of a tune change from pump, to AV + pump up to about 50%, except maybe a timing change.

    I would check your carb jetting against Holleys stock jetting, and start there.
    This is the engine from the "Guess my Horsepower" post. BBC 468, 8 to 1 with Procharger 9psi pulley, QFT carb. I did think I needed it, now I understand I don't. I'm going to leave the jets alone and run it this weekend. I'll look at the plugs to see the change. Timing is at 34 degrees. The engine runs really well right now, the only change I'm making is the fuel. I have a selection of jets so I can make adjustments while I'm there.

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    gn7
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    Quote Originally Posted by 74Bonneville View Post
    This is the engine from the "Guess my Horsepower" post. BBC 468, 8 to 1 with Procharger 9psi pulley, QFT carb. I did think I needed it, now I understand I don't. I'm going to leave the jets alone and run it this weekend. I'll look at the plugs to see the change. Timing is at 34 degrees. The engine runs really well right now, the only change I'm making is the fuel. I have a selection of jets so I can make adjustments while I'm there.
    the straight AV could explain the issues you were having when you pumped up the boost. If you ever pump it up again, try 50%. Your jetting shouldn't cahnge because you put more boost to it. Hass runs pump at low boost with a blow thru and switches to race fuel on the fly without making any changes and it works just fine.

    If you pump the boost you might pull the timing back couple degrees to be safe. Hass has a timing retard as the boost climbs.



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