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What are the benfits to using longer rods?

Discussion in 'The Garage' started by Emmettology 101, Oct 28, 2003.

  1. Emmettology 101

    Emmettology 101 3/4 ton status

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    I have been running some ideas through my head for the engine in my 89 p/u. Looking at pistons I see that some are made to use 5.70" rods and others are made to run 6" rods. If the stroke is the same then what are the benefits of running a longer rod? The piston would be designed to have the wrist pin farther up int he piston but the length from the crank to the piston top would remain unchanged, correct?
    So what would be the benefit of the longer rod?
     
  2. Mudzer

    Mudzer 1/2 ton status Author

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    In the dragracing Scene, a larger Rod to Sroke ratio will net higher torque at a lower RPM. I think there are several benefits:

    Lighter piston because the wrist pin is higher or closer to the top of the piston. The added weight because of the longer rod does not affect the overall gain as drastic as lightening the piston - in this case.

    Higher torque at a lower RPM.

    Excerpts from the Web:

    True, average piston velocity is higher with the longer stroke, as is rod angularity. These may seem like steps in reverse, but they aren't all bad. Piston velocities across top and bottom centers are faster with the longer stroke but are relatively slower through the mid-part of each stroke because it is an inescapable fact of mechanical life that exactly one revolution of the centre, to bottom centre, and back to top centre again, regardless of the rod length or the piston stroke. However, the faster piston velocities across top and bottom centres have the most significant de-sensitising effect upon valve timing because the effective valve opening and closing points usually fall within the range of these faster piston velocities. This is sometimes convenient because it allows the small luxury of a small error to be made in effective duration without the usual penalty of feeling that the vehicle is stuck to one spot in the pavement.

    And a link for ya!

    http://www.chevymania.com/tech/rod.htm
     
  3. Emmettology 101

    Emmettology 101 3/4 ton status

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    Cool Beans.. Thanks Mudzer... That would be a good route for me if i build this engine then since I am looking for low end torque for towing and such... /forums/images/graemlins/thumb.gif
     
  4. JK5

    JK5 1/2 ton status GMOTM Winner

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    What are thinking of...doing???
    A LT1 Stroker!!!! /forums/images/graemlins/wink.gif
     
  5. Emmettology 101

    Emmettology 101 3/4 ton status

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    Nope, what I was telliin you on the PM.. 406 sbc with Trick Flow heads for the p/u... /forums/images/graemlins/smile.gif Just thinkin right now though.
     
  6. mudjunkie 82

    mudjunkie 82 1 ton status GMOTM Winner

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    A longer rod moves the pin up Closer to the top of the piston which means you can have a shorter skirt on the piston which equals less drag Which equals more H.P
     
  7. Stroked72Blazer

    Stroked72Blazer 1/2 ton status

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    Also with a longer rod you have less rod angle which alows for higher RPM. And to clearify what Mudzer wrote (It confused the hell out of me but I know the principle)

    With a longer rod you slow the piston at TDC & BDC alowing more options for cams. It also speeds the piston up in between providing a more consistant piston speed and reduceing jerking which will improve bearing life. You can get a much better explenation of the benifits in an old Hot Rod Magazine dated somewhere in late 94 early 95.
     
  8. 70~K5

    70~K5 1/2 ton status

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    Smokey Yunick also said that with a longer rod the piston stays at TDC longer which lets the combustion pressure work on the piston longer.
     
  9. ntsqd

    ntsqd 1/2 ton status

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    A while back Circle Track mag did a dyno test of long rod vs. short rod in an otherwise identical 355. The short rod had more pronounced curve to the torque curve (think 2 stroke) whereas the long rod's torque curve was much flatter. Peak Torque rpm did NOT change though, peak torque for both versions of the engine occured at the same rpm. Flat torque curve = easy to drive.

    Now consider this: with a long rod the piston has more dwell time at TDC. No brainer there. If this is a work truck or one that gets lugged on occasion the extra dwell time can easily put the engine into detonation where a shorter rod would not result in detonation because of the shorter dwell time.
    So if it's a play toy engine a long rod would be a consideration. If it's a work truck (tow rig etc.) I would give some serious thot as to whether to go down that path or not.
     

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