5.7 or 6 inch rods for 383

Discussion in 'Engine Topic' started by CMitchell, Nov 12, 2008.

  1. CMitchell

    CMitchell Veteran Member

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    Jun 1, 2008
    Southeastern N.C.
    Someone school me on rod lengths.
    I am going to build another 383 for the z28 and I would like a little info on rod lengths. The guy that does my machine work told me that I will probably have less trouble if I go with 5.7 rods. Which way is actually better? There is not but about $100 difference in the price.
    Here is what I am looking at putting together:
    3.75 stroke, 5.7 rods, -5cc flat 11.8:1
    Comp cams 292 501/501
    world sportsman 2 heads (crap I know but I have them already)
    holley street dominator intake
    holley 770 cfm carb

    Somebody school me so I dont make a mistake please
     
  2. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

    8,963
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    Nov 4, 2004
    Ringgold, GA
    Common theory is to go with the longest rod you can fit or afford. One of the benefits is less side push on the pistons, reducing friction & increasing longevity. Down size is clearancing the block for the longer rods and with longer rods, it pushes the pin further up into the ring pack. Six inch rods are the preferred way to go if you're going to do it right. IMHO
     
  3. Mwilson

    Mwilson Veteran Member

    Ive heard 6" is better but im running 5.7's
     
  4. onovakind67

    onovakind67 Veteran Member

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    Jul 8, 2001
    Fairfield, CA
    Side push on the pistons also increases force on the crankshaft, the basic way an engine produces torque. At any time in the power stroke, a short rod has more leverage on the crankshaft, and produces more torque.
     
  5. LILRED496

    LILRED496 Veteran Member

    1,208
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    Jul 14, 2008
    FLA
    I had 6 inch rods in my 406 don't know of any benefits other then maybe lighter pistons.
     
  6. Camaro1194

    Camaro1194 Veteran Member

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    Sep 11, 2008
    LaSalle, Colorado
    6 inch rods will get the pin up in the rings, requiring buttons or rails, I run 5.7s in mine just to avoid that hassel, it wont make a big difference in HP either way.
    I wouldnt call those heads crap, I know lots of guys running 11s with them.

    Mike
     
  7. Mwilson

    Mwilson Veteran Member


    Yeah I ran 11's with them on my 355 (11.62)
     
  8. Bikefixr

    Bikefixr Veteran Member Lifetime Gold Member

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    Mar 13, 2006
    Google what Smokey Yunick says about long rods. Long rods make more torque and horsepower. The rods are the levers,,,longer levers can do more work. Long rods decrease piston side loading. Long rods also allow the piston to dwell at TDC for a longer period allowing the combustion pressure to build and generate more downforce on the piston crown. The downside is the ring-pack intrusion and camshaft clearance issues. The rod bolts might hit a normal base circle cam. Long rods will not rev as high, but won;t be an issue on a street engine. Smokey wrote the book on long rods.
     
  9. camertom

    camertom Veteran Member

    There you go, justifies em right there. My reciprocating assembly has Scat $280.00 at the time, 6" rods which are 20 grams more than the 5.7" rods. The Probe piston I run is well over 100 grams lighter than the 5.7 type. Net gain 80 plus grams. Thats easier on the assembly and revs snappier to boot.
    The lower friction thing is cool too!
     
  10. onovakind67

    onovakind67 Veteran Member

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    Jul 8, 2001
    Fairfield, CA
    A connecting rod is not a lever, it has no fulcrum. If you side load the piston, what does the rod push on in order to achieve this load? What's the coefficient of friction of oiled aluminum on cast iron?

    Smokey wrote the book, but Darin Morgan of Reher Morrison did the dyno tests:

    "Most people tend to overgeneralize this issue. It would be more accurate to compare different rod-to-stroke ratios, and from a mathematical stand-point, a couple thousandths of an inch of rod doesn't really change things a lot in an engine. We've conducted tests for GM on NASCAR engines where we varied rod ratio from 1.48- to 1.85:1. In the test, mean piston speeds were in the 4,500-4,800 feet-per-second range, and we took painstaking measures to minimize variables. The result was zero difference in average power and a zero difference in the shape of the horsepower curves. However, I'm not going to say there's absolutely nothing to rod ratio, and there are some pitfalls going above and below a certain point. At anything below a 1.55:1 ratio, rod angularity is such that it will increase the side loading of the piston, increase piston rock, and increase skirt load. So while you can cave in skirts on a high-end engine and shorten its life, it won't change the actual power it makes. Above 1.80- or 1.85:1, you can run into an induction lag situation where there's so little piston movement at TDC that you have to advance the cam or decrease the cross-sectional area of your induction package to increase velocity. Where people get into trouble is when they get a magical rod ratio in their head and screw up the entire engine design trying to achieve it.The rod ratio is pretty simple. Take whatever stroke you have, then put the wrist pin as high as you can on the piston without getting into the oil ring. Whatever connects the two is your rod length."

    4500-4800 ft/sec piston speed is about 7800-8300 rpm with a 3.5" stroke.

    1.48-1.85 r/s ratio would equal rods from 5.55" to 6.94" in a 383 SBC.

    Let's say you want peak cylinder pressure 16° ATDC to produce max power in your 3.75" stroke SBC. If you swap from 5.7" rods to 6" rods, what is the difference in piston position at peak cylinder pressure?

    A - .00012"
    B - .0012"
    C - .012"
    D - .12"

    You guessed right, it's B. There is about one thousandth of an inch difference in piston position. What's the difference in rod/bore angle? What's the difference in rod/crank angle? Would you want the rod/crank angle to be closer to 180° or 90°?

    Gere Stahl did an essay, as well as Rick Draganowski:

    http://www.stahlheaders.com/Lit_Rod Length.htm
    http://www.rustpuppy.org/rodstudy.htm
     
    Last edited: Nov 13, 2008

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