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Thread: High Compression 400 sbc

  1. #1
    Senior Member DYNO OPERATOR
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    High Compression 400 sbc

    what is the highst comp ratio that should be used in a 400 sbc 817 block?

    I have a std bore block low mile I want to build to a 420cid with 3.875 stroke.All opt are open

    this will be for drag racing only.

  2. #2
    Senior Member RACING JUNKIE
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    We're running 14.5 with cosmetic gaskets in a 434" motor no issues as of yet, with close to 200 passes.

    Zip.

  3. #3
    Senior Member DYNO OPERATOR
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    I was told the cyl would bollon in time.

  4. #4
    Senior Member RACING JUNKIE
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    We fill the block up to the bottom of the water pump inlet with hard block and use total seal rings. Leakdown is still within 2 to 3%. I'm sure that an after market block will seal better, but it's what works for us.

    Zip.

  5. #5
    Junior Member APPRENTICE
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    Hello, with a factory block, especially a GM 400, the real question is, what is the target horsepower figure you are trying to accomplish with this build? If you intend to go with a higher compression ratio and the cubic inch that you specified, I would estimate with a properly matched induction combo (roller cam/intake/heads), you will be pushing somewhere around 600+ horsepower depending on how well the combination is put together. I personnaly don't reccommend using the 4 bolt main 400's, simply because the main webbing where the outer main bolt is drilled and tapped through during factory procedure makes a great breeding ground for stress or cracking. If you must use a factory GM 400 block search for a 2 bolt main and have a reputable machinist install splayed 4 bolt caps. Please keep in mind after all this trouble and machine work/exspense, you could have probably bought a reasonably priced aftermarket cylinder block such as Dart's new entry level SHP (there are many posts about this block on the forum). Aftermarket cylinder blocks seem out of reach on a tight engine budget but machine processes to get that old iron suitable for higher horses will put you into the $1000+ range, unless you have a good engine machinist that owes you a favor. Long story short factory iron is good for between 500-550HP MAX, not to say you can't put more into these blocks, many will tell you they have with great success, as have I but with great preparation put into the block. Years and years ago I built a 670HP alcohol 377cid out of a factory GM 400 with splayed caps, I wouldn't go to that trouble again though, deck plugs, splayed caps, grout block fill, pan rail clearancing for alum. rods, etc., etc. I hope this sends you in the right direction and you are doing what any good racer does,....homework. Thanks.

  6. #6
    Senior Member DYNO OPERATOR
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    I have brodix track 1's no port work on a 360 cid hv intake 1050 dom and was looking to put more cube's under it.260/[email protected] 630 lift cam,c 12 gas

    the block i have is a two bolt std bore. cyl look beutiful dont need to be bored.Was just going to have it honed for piston clearance and deck

    I know i'm going to need a small base circle cam
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  7. #7
    Senior Member EXPERT BUILDER
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    You don't have to run a small base circle cam if you use Eagle Stroker Rods...That is what I have in my motor...

  8. #8
    Senior Member DYNO OPERATOR
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    love the info,13.5 to 14.0 would be the most i'd want to go.not looking to set a record just go a lil faster.
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  9. #9
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    You'll need to consider that, IMO, even a 406 will be DONE by 6500rpm with those Track 1's. A 420"er will be even lower. It'll make great mid range power but won't like rpm. You'll need to regear to keep it in the power band.....

  10. #10
    Senior Member DYNO OPERATOR
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    I was thinking a power glide would make good use of that power

    right know I shitf at 6500 and go through the lights at 68 to 7300 depending on the weather.
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