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Thread: Big Block Chevy

  1. #1
    Join Date
    Dec 2003
    Giddings, Texas

    Default Big Block Chevy

    I took my 396 block to the machine shop last week. I got a 402 back. They had to bore it .030 over. My next concern is compression ratio. 396 pistons .030 over will give me the stock 10.25:1 ratio. I think that is a bit high for todays gas, but I like the idea of the power it will make. Stock 402 pistons will give me around 9.0:1 compression. Good for power, but I think I will always wonder 'what if?". Any ideas or suggestions?
    Remember if you didn't build it you can't call it yours.

    6.2 powered M715, 5 M1009's, M416, 2 M101's, 2 M105's, 3 M35's, M1007 6.5 turbo Suburban project called Cowdog.

  2. #2
    Join Date
    Mar 2004
    northern Arizona


    Getting proper fuel for a 10.25 motor might be a pain, especially if it's not fuel injected. I know I would always be asking what if as well, so don't know what to tell you....sorry.

    67 M725 67 M715 68 M715

  3. #3


    My buddy runs his truck at 11:1, but we are at 5000 ft, and wheel up to 9500 ft where the motor can really use the extra stroke space for the thin air. I'm sure it would ping like a mofo at lower altitudes- gonz
    Come and take it
    Go work at joann fabrics if you can't shoot a gun

  4. #4
    Join Date
    Jan 2004
    NorthEast Texas


    You could go with 396 pistons and get a little bigger cam to bleed off some of the compression. But for every day use I would probably use 402 pistons. Are you using early 396 heads?(larger washer seat spark plug) if so then 402 pistons will have more than 9:1 comp as they were designed for later open chamber heads. (small taper seat plug)

  5. #5
    Join Date
    Dec 2003
    Giddings, Texas


    I have the 98cc closed chamber oval port heads. Good point and thanks.
    Remember if you didn't build it you can't call it yours.

    6.2 powered M715, 5 M1009's, M416, 2 M101's, 2 M105's, 3 M35's, M1007 6.5 turbo Suburban project called Cowdog.

  6. #6
    Join Date
    Dec 2003
    parsons Kansas

    Default Rollie in Bellingham

    mill the heads run 13:1s roller. Id go back and tell them you want your 396 back.LOL!you could have had a four bolt block.
    stock 396 with 10:25 was a 325 horse motor and they ran pretty good on 91 octane. may have to back timing down a few degrees but I dought it. My 396/375 horse ran all the time on premium unlead (91) and I only had a problem once and that was when I ranway over 38 deg. total advance racing it and burnt a hole in a piston that time.
    67 M725, 67 M715, 68 M715

  7. #7


    i have a 396 bored to 402 in my daily driver k-30 chevy... 9.5:1 compression, runs fine on mid-grade, runs like absolute crap on regular. works good for me, but the cam is not in the right RPM range, so i need to change that. it has a 270 or 280 (don't remember), but i'm going to put in a comp extreme 4x4 cam for that low end grunt. also going to put in pete jackson timing gears and MAYBE roller rockers.

    i also need to mess with my advance weights because my MSD billet HEI distributor came out of small block in a fast-running car, not a BBC in a big 4x4, and i think the advance goes too high... i want all my advance by 4,500 RPM... not 6,000 like in a car.

    i'm also going to swap in a TBI setup down the road...

    have fun with your new motor! I guess my buildup all comes down to wether or not i buy a diesel dodge... if so, the M-715 is going on that chassis...

  8. #8
    Join Date
    Sep 1998
    North Central Wisconsin


    Pete Jackson gear drive.....nice that!!
    Lord send your Holy Ghost into our hearts and make the desire of our hearts Your Will.

    Pro-choice, that's a LIE, babies don't choose to die!!

  9. #9
    Join Date
    Sep 1998
    Winlock, WA

    Default Rockwell pinion brakes and brackets.

    Actually, you want your full advance in around 2,000 to 2,500 rpm for help with the low end grunt.
    The percentage power gained by the higher compression will probably not be worth the extra money you have to spend to safely run it like that. You have to keep your timing in check, your fuel has to constant (fuel injection makes this point mute though), and other cooling issues that arise as well.
    Run a roller cam to get the best low end grunt.

  10. #10

    Default BRIAN!!

    With a sb 9.5 should be the max cr with iron heads, but with the larger bore of the bb you gain about .5 to around 10:1. Add thick head gaskets and you can knock .2 off the cr. Or you could just get the 402 pistons, they cost about the same.
    my .02

    भगवान तुम्हें प्यार करता है

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