Connecting Rod question

Discussion in 'OT Driven' started by Original Sin, Jan 31, 2007.

  1. Original Sin

    Original Sin I'm so hood.

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    Would increasing a rod length have any real impact on power? I mean just replacing the rods themselves, not the crank?


    Backstory:
    I just picked up a 400sbc and just planned to go .030 over, but a friend of mine was telling me to use 6" rods in my build and somehow I'd end up with a 412 :confused: Now, I can see if I increased the stroke to 3.800, but I'm failing to see how swapping rods alone would do that. I feel like I'm missing something, as this guy knows his shit usually (built a 406 powered, all steel street legal 69 Camaro that runs 10.54).
     
  2. 311-420

    311-420 New Member

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    uhhhh...yea...im pretty sure increasing rod length will increase torque. simple physics right?
     
  3. matrix243

    matrix243 My body, is ready.

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    :nono:
    think about, that would just give you a higher TDC and BDC in the stroke, and give you a higher CR. Then you would definitely need to check for a .080 valve to piston.

    increased stroke = increased main journal center to rod journal center distance, and counteracting with a shorter rod to keep a safe piston to valve clearance.
     
  4. Original Sin

    Original Sin I'm so hood.

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    That's what I was thinking, but I kept figuring I was missing something somewhere.
     
  5. Junkie

    Junkie re-tarded OT Supporter

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    note that this is NOT specific SBC info, but I see no reason why it wouldn't apply to them.

    Longer conrods get you somewhat different engine characteristics, although I don't remember exactly what. To go with them you end up using pistons with wrist pins closer to the surface - so for a given piston face design and CR, the distance from the crank to the piston surface is the same at TDC with long rods vs without. For example, if the rod was 5mm longer the wristpin would have to be 5mm closer to the piston face.

    So, when people talk about long rod engines, they are NOT talking about raising CR. In addition, rod length has no effect on stroke.
     
  6. alltracman78

    alltracman78 New Member

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    Longer conrods give you a lower rod:stroke.
    Which means less stress on the rods.
     
  7. MAD PUNK inDC

    MAD PUNK inDC Sic Semper Tyrannis

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    if I remember correctly a longer rod typical gives you more dwell time, as well.
     
  8. It won't increase your displacement, but as mentioned before, your rod length to stroke ratio will be better. Also, you'll have to find pistons designed to be used with the 6 inch rods and the 3.75 inch stroke. Another option is to go with the standard smallblock 5.7 inch rod (still longer than the original 400 rod) you'll still need special pistons, but it's a much more common setup and you'll have an easier time finding them.
     
  9. nitrane

    nitrane OT Supporter

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    if you have a 400 block drop in a 350 crank and use (i think) a 6" rod.

    long rod and short stroke means you can rev the shit out of it, we call them 'destrokers' here
     
  10. Original Sin

    Original Sin I'm so hood.

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    I'm actually going the other route...more bore and more stroke. I'm just not sure how much more stroke yet. It'll either end up a 420 or 434.
     
  11. 377 ftw
     
  12. nitrane

    nitrane OT Supporter

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    awesome proven combo down here, the 380 cuber in the dragster i crew on pulls 6.31 @ 219.5 and still has heaps left in it... keeps up with the KB's :cool:
     
  13. nitrane

    nitrane OT Supporter

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    buddy runs a 434... good combo but i believe clearencing on the machining was a bastard
     
  14. Original Sin

    Original Sin I'm so hood.

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    yeah, I'm a little worried about that. Most places are recommending an aftermarket block if I want a 4" stroke, which is why the 420 is a serious consideration to me. Hell, I might even drop it down to a 412 pending advice from my machine shop.
     
  15. nitrane

    nitrane OT Supporter

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    leave it as a 406 and you'll make big power with a stock block

    i've got a 3.85" stroke and that's as far as i'd ever go
     
  16. Unless you are building something for high RPM use, like a road race engine it's stupid to de-stroke an engine. People usually want more displacement, not less. If you're drag racing or using the car on the street you'll be happier with the higher displacement engine.
     
  17. flyinbrian

    flyinbrian New Member

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    There is no replacement for displacement. Always go as big as you can when drag racing.
    The only thing is, I would rather go 434 big with an aftermarket block rather than a stock casting.
    IIRC, the earlier factory two bolt main 400s have more meat in the block which allows for the machining needed for splayed main caps.
     
  18. nitrane

    nitrane OT Supporter

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    what car is this going in? weight? transmisson?

    street or strip?

    blown or NA?



    This is what will decide what combo you should use. Personally if it was a NA street car I'd agree with getting as many cubes as you can safely get...
     
  19. Original Sin

    Original Sin I'm so hood.

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    '84 Monte Carlo SS. ~3500lbs. 200R4
    Street, with occasional track time.
    NA

    On the plus side, looks like I'm picking up a set of Dart Sportsman heads :wiggle:
     

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