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Thread: A Basic Rookie question.....

  1. #1
    Senior Member Hotyacht's Avatar
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    A Basic Rookie question.....

    Well as the title says, I am standing in my undies asking a dumb question........

    I have always owned European cars and they like to run on high Octane petrol (or as you guys call it...gas), and here in New Zealand most stations offer 3 different octane ratings - 91, 95 and 98, and the price reflects the octane level. Most Japanese cars seem happy to run on the 91, unless they are higher performance models in which they run on 95, and most European cars run on 95 or 98.

    This Roadster build is my first foray into the world of Merican V8s and Hotrods and so it has been a world of learning the language, and the tricks

    I am 2 weekends away from my first start and so I am about to fill my new engine with fluids - I have the break-in oil and the antifreeze all set to go........but what Octane fuel do you guys generally use in you V8s? My engine is a 427W stroker with an 8 stack EFI from Inglese. The guys at the Engine factory who built it said to use fuel of at least 87 octane. Gas is not cheap here....91 is US$5.80 per US gallon and the 98 is US$7.92 per US Gallon and I am figuring on this beast drinking it's fair share. I obviously want the best performance, but maybe V8's prefer a certain diet? All of our fuel is unleaded. You can get 100 octane here as well, but you generally buy it by the drum and store it somewhere handy.

    What are you collective thoughts/wisdom regards the best gas to use? Do you use any additional additives?

    cheers in advance.
    FFR MkIV 8419 - Right Hand Drive
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  2. #2
    Boydster's Avatar
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    If the builder says to use at least 87 octane, then its most likely a friendly compression and conservative, or standard, timing. You can try 87 for the first few times, see if there is any detonation or preignition. This will generally show up at low rpm, hi load times. If you want to be safe, just run the next highest octane and it will most likely be fine.

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    Please keep in mind that different countries rate their octane numbers differently. In the US and Canada, the pump octane is (MON + RON)/2. MON = Motor Octane Number and RON = Research Octane Number. The generally differ by 8-10 points. As you might have guessed by the numbers, other countries just use the RON so it is typically 4-5 points higher than the North American pump octane.
    FYI, iso-octane fuel has an octane number of 100 while n-heptane fuel has an octane number of 0. The anti-knocking properties of the tested fuel is compared to that of an iso-octane and n-heptane blend to obtain the octane number. To be clear, fuel contains a huge blend of many different organic compounds, not just iso-octane and n-heptane.

    So why do we have MON and RON? Well, the wonderful thing about standards is there are so many to choose from. I remember hearing that in the early days of fuel standards, the engine makers were using MON and the scientists from oil companies were using RON that resulted in some big issues with expected performance of the fuels. They agreed to split the difference, hence (MON+RON)/2

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    Comes down to compression ratio. If yours is under 9 to 1you can use low octane. 9 to 9.5 to 1 use mid grade. 10 and over use High octane. if you get detonation you can back up the distributor a couple of degrees and slow down the full advance curve ...da Bat

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    Senior Member CraigS's Avatar
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    Can you look through your engine info to see what the compression ratio is? I suspect that your 91, 95, 98 corresponds to our 87, 91, 93 but I don't know that for a fact. I also suspect that you will want your 98, but the comment from EF may indicate you could run the 91 if the CR is low enough..
    FFR MkII, 408W, Tremec TKO 500, 2015 IRS, DA QA1s, Forte front bar, APE hardtop.

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    Senior Member MisterAdam's Avatar
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    Quote Originally Posted by j.miller View Post
    Comes down to compression ratio. If yours is under 9 to 1you can use low octane. 9 to 9.5 to 1 use mid grade. 10 and over use High octane. if you get detonation you can back up the distributor a couple of degrees and slow down the full advance curve ...da Bat
    my coyote is 11:1 and ford recommends 87

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    What you want to know is Dynamic compression, rather than mechanical compression. Dynamic takes in to account things like intake valve closing point and altitude.

    But if your local builder recommends 87, I wouldn't use anything less.
    .boB "Iron Man"
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    There's a lot more to what fuel rating you use than strictly compression ratio for preventing detonation. It has to do with cylinder head material, combustion chamber design, ignition timing, air/fuel ratio, coolant temperature, intake air temperature. But we tend to simplify all of that and make rules of thumb.

    If the manufacturer said 87 then I'd put your 91 in (which is going to provide similar performance) and it should be fine. Maybe put something higher in if you're going to run it on a road course where you'll be at high power for an extended period of time. There's a difference in running at high power for a few seconds vs. being on it for a long period of time. I'm assuming that the EFI and ignition timing are also set per a spec or tuned on the dyno to make sure you have appropriate air fuel ratios and timing. But I would also talk to the manufacturer of the engine to understand their recommendations in your situation and what sort of dyno testing they may or may not have done.

    I'm planning on building my engine around needing premium, but fuel is a lot cheaper here.

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    Member FlyingCobra's Avatar
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    Quote Originally Posted by MisterAdam View Post
    my coyote is 11:1 and ford recommends 87
    The Coyote has a much more modern design all around than the old things. With modern technology, it's perfectly doable in some situations to run over 10:1 on 87.

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    For break in and starting, I would step up an octane grade. After you get to drive it, you can see how it does with other octanes.
    20th Anniversary Mk IV, A50XS Coyote, TKO 600, Trunk Drop Box, Trunk Battery Box, Cubby Hole, Seat Heaters, Radiator hanger and shroud.

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  19. #11
    Senior Member Hotyacht's Avatar
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    Quote Originally Posted by CraigS View Post
    Can you look through your engine info to see what the compression ratio is? I suspect that your 91, 95, 98 corresponds to our 87, 91, 93 but I don't know that for a fact. I also suspect that you will want your 98, but the comment from EF may indicate you could run the 91 if the CR is low enough..
    The engine spec states a 10.5:1 compression ratio

    So I think I will start with 95 and see how I go. many thanks for your comments. I will post a video of the first start........hopefully nothing as exciting as an explosion or rod through the block!!
    Last edited by Hotyacht; 08-24-2018 at 05:35 PM.
    FFR MkIV 8419 - Right Hand Drive
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    Senior Member BEAR-AvHistory's Avatar
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    Quote Originally Posted by MisterAdam View Post
    my coyote is 11:1 and ford recommends 87
    FORD says it will run on 87. They also say all their performance & horsepower ratings are based on 93. The COYOTE's ECU will play computer games with the valve & ignition timing to reduce the dynamic compression ratio but the games also reduce the power output at the same time.

    Australia uses the European fuel rating system & 98 is 93, 95 is 91. Agree that 10CR+ you want premium in the car.

    FORD's notation on all their specs:

    "2018 Mustang | ford.com
    NOTE 1 Horsepower and torque ratings achieved with 93-octane fuel."
    Last edited by BEAR-AvHistory; 08-24-2018 at 08:12 PM.
    Kevin
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    Do they put ethanol alcohol in your fuel there? I put in shell 91 here in Canada as I know it doesn’t have ethanol. It does all kinds of bad things if your fuel system was not designed for it. I believe many aftermarket parts are still not ethanol friendly.
    The extra you will spend on premium will be very minor compared to any problem that arises due to detonation or alcohol eating rubber pieces and going bad in your tank.
    I suspect your 91 is our 87. We have 94 at one gas company but I was told they boost the octane with ethanol, not the same as 94 straight gas.

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  23. #14
    Senior Member Hotyacht's Avatar
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    Quote Originally Posted by Murd View Post
    Do they put ethanol alcohol in your fuel there? I put in shell 91 here in Canada as I know it doesn’t have ethanol. It does all kinds of bad things if your fuel system was not designed for it. I believe many aftermarket parts are still not ethanol friendly.
    The extra you will spend on premium will be very minor compared to any problem that arises due to detonation or alcohol eating rubber pieces and going bad in your tank.
    I suspect your 91 is our 87. We have 94 at one gas company but I was told they boost the octane with ethanol, not the same as 94 straight gas.
    Thanks for raising a good point, but no there isn't ethanol in our fuel except from one low budget chain. I also made sure that all of my fuel system components are able to handle ethanol.
    FFR MkIV 8419 - Right Hand Drive
    427W with Inglese 8 Stack injection - 560hp
    TKO600 5 Speed, 8.8 Detroit Trutrak LSD IRS
    VPM Adjustable Sway Bars
    Vintage Wheels 17" Pin Drive

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    Quote Originally Posted by Hotyacht View Post
    The engine spec states a 10.5:1 compression ratio

    So I think I will start with 95 and see how I go. many thanks for your comments. I will post a video of the first start........hopefully nothing as exciting as an explosion or rod through the block!!
    Our 302 is 10.1:1 and I’m running 91 octane pump gas per my engine builder. I will tell you though that the few times I ran 98 or 105 octane unleaded aviation fuel in it, it was tons of fun

    Ray
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    "If you can make black marks on a straight from the time you turn out of a corner until the braking point of the next turn, then you have enough horsepower."--Mark Donohue

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