What octane Fuel is safe to run?
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What octane Fuel is safe to run?

  1. #1
    Senior Member holorinhal's Avatar
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    Default What octane Fuel is safe to run?

    I am clueless when it comes to compression ratio and the proper octane to run in in an engine.
    In particular, I want to make sure I am running the proper fuel in my engine. I have ben running 100 octane, but would like to know if it would be safe to run as low as 91.
    Below is the specs for my engine.

    489 c.i.d. BBC Full roller motor.

    .030" over bore

    Scatt 4340 forged crank, 4.25" stroke

    Splayed main caps /w ARP main studs

    Eagle, 6.135" forged H beam rod's

    SRP forged pistons/17cc dome volume

    Block has been zero decked

    Bullet Racing Cams solid roller cam

    Morel pressure fed solid roller lifters

    Cloyes hex adjust double roller timing set

    AFR 325cc as cast aluminum heads/117cc chamber/ ARP head studs
    (Head specs and flow chart. Air Flow Research

    Crane gold rockers,1.8 int.,1.7 ex

    Edelbrock TR2 tunnel ram

    two 600 cfm Holley carbs

    As good as the heads are I had a little more done to them. A little more port work to smooth out a couple bumps and a little shave to get a little more compression.
    I am unsure of the head gasket quench, but the builder said it was knocking on the door of 11:1.
    I also used the AFR intake gaskets which match the head's intake ports much better than the felpro and the intake has been portmatched to the head's.

    Timing is set at 20* initial and 32* total.

    Here is the Cam card with all the cam specs .

    Last edited by holorinhal; 06-18-2013 at 06:35 PM.

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  3. #2
    Loose Nut on the Wheel michaellone's Avatar
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    http://www.summitracing.com/expertad...ion-calculator

    This is what I came up with (guessing at the head gasket). As far as the cam, that is still black magic voodoo to me.
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  4. #3
    mo balls than $cents$ IMPATIENT 1's Avatar
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    10.5 to 1 can run on 91 or 93 no problem.im lucky to have a station on way to the lake with 93.only station around with over 91 but its priced like 91

    Dare to be different, if it turns out great you can claim you planned it that way.

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    gn7
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    Quote Originally Posted by michaellone View Post
    http://www.summitracing.com/expertad...ion-calculator

    This is what I came up with (guessing at the head gasket). As far as the cam, that is still black magic voodoo to me.
    That calculator is kind of lacking. I makes the assumtion that the head gasket has the same bore size as the block which very seldom the case. There are no 4.280 bore head gaskets. 4.370 bore gasket is more like it. Plus .040 is more common thickness.
    The AFR chamber size is usually 118 or 122 with the CNC option.

    The compression for that thing is closer to 10 to 1 or 9.7 with a 122 CNC'd chamber.



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    Bostick Racing Engines six-oh-nine's Avatar
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    Obviously, I'd check with your engine builder as he would know all the details of the build before going and doing anything... BUT, with that said, based on the numbers you've given, and doing an assumption of a 4.38 bore and .041 thick gasket, you are looking a lot more like 10.2ish:1, and considering the cam, compression, and aluminum heads, I'd say you were way good on 91 octane... and to be honest... I'd start where you are at on timing, do a plug check, and adjust for what the engine likes... But like I said, take some of the info you get here and run it by your builder first, as he's the dude that put it together... and it never hurts to ask questions.
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    gn7
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    Quote Originally Posted by holorinhal View Post

    As good as the heads are I had a little more done to them. A little more port work to smooth out a couple bumps and a little shave to get a little more compression.
    I am unsure of the head gasket quench, but the builder said it was knocking on the door of 11:1.
    I also used the AFR intake gaskets which match the head's intake ports much better than the felpro and the intake has been portmatched to the head's.
    Quote Originally Posted by six-oh-nine View Post
    Obviously, I'd check with your engine builder as he would know all the details of the build before going and doing anything... BUT, with that said, based on the numbers you've given, and doing an assumption of a 4.38 bore and .041 thick gasket, you are looking a lot more like 10.2ish:1, and considering the cam, compression, and aluminum heads, I'd say you were way good on 91 octane... and to be honest... I'd start where you are at on timing, do a plug check, and adjust for what the engine likes... But like I said, take some of the info you get here and run it by your builder first, as he's the dude that put it together... and it never hurts to ask questions.
    I agree. I didn't even notive that this was built by someine other than the OP.
    A "little shave" ?? Thats open ended!!! If its really knocking on the door of 11 to 1 that was more than a little shave!



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    Last edited by gn7; 06-17-2013 at 11:07 AM.

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    Senior Member Futs's Avatar
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    Those heads started with 119 cc chambers. You have 116 in your detailed list. Is that your combustion chamber cc after the "little shave"?
    My 540 has the same heads and am at 10.2:1 and i run 91 with 38* timing

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    Loose Nut on the Wheel michaellone's Avatar
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    Quote Originally Posted by gn7 View Post
    That calculator is kind of lacking. I makes the assumtion that the head gasket has the same bore size as the block which very seldom the case. ....
    Sorry about that, here is a better one:

    Engine Compression Ratio (CR) Calculator

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    gn7
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    Quote Originally Posted by michaellone View Post
    Sorry about that, here is a better one:

    Engine Compression Ratio (CR) Calculator
    This one lets allows you to calculate dynamic CR

    United Engine & Machine Co. Incorporated



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    Loose Nut on the Wheel michaellone's Avatar
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    Quote Originally Posted by gn7 View Post
    This one lets allows you to calculate dynamic CR

    United Engine & Machine Co. Incorporated

    NICE! Bookmarking that bad boy.

  13. #11
    Senior Member holorinhal's Avatar
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    Quote Originally Posted by Futs View Post
    Those heads started with 119 cc chambers. You have 116 in your detailed list. Is that your combustion chamber cc after the "little shave"?
    My 540 has the same heads and am at 10.2:1 and i run 91 with 38* timing
    Sorry, a miss print on the combustion chamber size.
    I think I was told these are the as cast heads. According to AFR, they come with 117cc chambers. The heads were cut .030"

    I'm not here for a long time, I'm just here for a good time!
    If your not on the edge.......your taking up too much room......Drive it like you stole it!

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    Bostick Racing Engines six-oh-nine's Avatar
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    Quote Originally Posted by holorinhal View Post
    Sorry, a miss print on the combustion chamber size.
    I think I was told these are the as cast heads. According to AFR, they come with 117cc chambers. The heads were cut .030"
    Then IF they were actually 117cc's... and were cut .030... you'd be looking at 112-113ish.

    at that point, you'd be approximately 10.4-10.5

    Either way... with the cam you have listed, and such, I'd still say 91 octane should be fine... and where you are at on timing would be a fine place to start, and go from there after reading your plugs... but again... if you are not sure, especially of EXACTLY what you have going on in your build, ask your builder or at least run it by him... it doesn't hurt or cost anything to ask an opinion, and any engine builder would be happy to answer any questions or ideas a customer may have concerning their engine.
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    Senior Member holorinhal's Avatar
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    Thanks for the replies fellas. It is nice to know that I can run on 91. Much easier on the pocket book.
    Being that I know very little about octane, what would be the benefit of running a higher octane, say 100 and above, on this engine? is there an octane rating that would be to high and unsafe for an engine of this level?
    I am going to do some research on octane and performance engines, to better educate myself. Thanks....... Hal

    I'm not here for a long time, I'm just here for a good time!
    If your not on the edge.......your taking up too much room......Drive it like you stole it!

    73 KONA Restoration http://www.performanceboats.com//showthread.php?t=10678
    '81 Spectra 20 Facelift http://www.performanceboats.com/user...-facelift.html

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    with the info given and the altitude of las vegas you will be fine. If you drop close to sea level i would go back up on octane. the octane rating is a basic referance to the ability of a fuel to resist pre ignition in the simplest form. Running to high of rating is just a waste and can be problematic.
    Last edited by bottom feeder; 06-18-2013 at 07:29 PM.

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