SBC 4 7 swap
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SBC 4 7 swap

  1. #1
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    Default SBC 4 7 swap

    I,ve read about swaping 4 and 7 on a SBC. What is the advantage and whats involved is it as simple as swaping wires or is there more?

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  3. #2
    mo balls than $cents$ IMPATIENT 1's Avatar
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    better be a liteeeeee boat with a c or d-cut

  4. #3
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    You also need a cam with the 4 7 swap ground in. It's suppose to smooth out the harmonics in the crank the way I understand it.

    Tim

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    1)The advantage is in the fuel distribution onto cylinder #2. In the conventional firing order 5 and 7 pull fuel to the back of the engine and then 2 pulls to the front at the opposite end. This became a problem in BBC because the number 2 and 7 cylinder ports are long. 5 helps 7 pull to the back then 2 tries to pull forward and gets lean. You do not see this on 1 and 8 because they are short runners. By revising order (swap 7 and 4), 8 helps 7 and 4 helps 2. Thus, your fuel distribution is more balanced between cylinders.
    2)Helps equalize EGT across the board.
    3)Works best with 500+cid/7000rpm+ packages. +/- 1% on the dyno.
    4)Reduces crankshaft deflection (jurys out on that one)
    5)Has been used in Cup/Busch/Craftsman but unconfirmed information has it that it has been outlawed, stock firing order only now. Don, perhaps your contacts can shed some light on that.
    6)I have information that some restricted intake class builders have had very good success with the 4/7 cams. That seems to go against the grain but I have heard it from more than one source.
    7) Flatter torque curve.

    (Taken off the web.)

  7. #5
    Bostick Racing Engines six-oh-nine's Avatar
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    Quote Originally Posted by hotrod56cars View Post
    1)The advantage is in the fuel distribution onto cylinder #2. In the conventional firing order 5 and 7 pull fuel to the back of the engine and then 2 pulls to the front at the opposite end. This became a problem in BBC because the number 2 and 7 cylinder ports are long. 5 helps 7 pull to the back then 2 tries to pull forward and gets lean. You do not see this on 1 and 8 because they are short runners. By revising order (swap 7 and 4), 8 helps 7 and 4 helps 2. Thus, your fuel distribution is more balanced between cylinders.
    2)Helps equalize EGT across the board.
    3)Works best with 500+cid/7000rpm+ packages. +/- 1% on the dyno.
    4)Reduces crankshaft deflection (jurys out on that one)
    5)Has been used in Cup/Busch/Craftsman but unconfirmed information has it that it has been outlawed, stock firing order only now. Don, perhaps your contacts can shed some light on that.
    6)I have information that some restricted intake class builders have had very good success with the 4/7 cams. That seems to go against the grain but I have heard it from more than one source.
    7) Flatter torque curve.

    (Taken off the web.)
    And then I can show you dyno sheets where it made absolutly no significant difference what so ever... and the only difference was it made the motor idle a bit smoother. It's voodoo man... not a one size fits all, cure for the common cold and diahreah patent medicine. We tried it in a single carb big compression small block olds with seriously worked batton heads and it really helped out in the mid to upper rpm... tried it on my injected t-ram doughnut motor and didn't do jack. It's like anything else... combination and application will dictate what works and what doesn't. IMO
    Last edited by six-oh-nine; 01-15-2009 at 10:15 PM.
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  8. #6
    Senior Member Chuck Wagon's Avatar
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    Larry Minor runs the 4-7 swap in some of his small block sand jeeps with pretty good results not to sure how a boat application would work. Call Larry nice guy to talk to he is out of Hemet. Larry Minor motorsports

  9. #7
    Senior Member jetboatperformance's Avatar
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    Heres some info for you to digest re 4/7 Tom

    http://www.hotrod.com/techfaq/113_07...ams/index.html

  10. #8
    steelcomp was here
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    4/7 swap might make a difference in an engine where every last little detail has been addressed, but on most builds, it's more hype than it's worth. You can pick up more power just making sure you've got the basics covered, like correct valve train geometry, proper cylinder finish and ring choice, a good oil system, timing and fuel curves optimized, etc. On most builds, these are areas where there is a ton of room for improvement that will make significant increases in power.
    JMO
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    Acts 2:38, the perfect answer to the perfect question.

  11. #9
    steelcomp was here
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    It's like anything else... combination and application will dictate what works and what doesn't. IMO
    +1
    If God is your co-pilot, change seats!
    Acts 2:38, the perfect answer to the perfect question.

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