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  1. #1
    Join Date
    Jul 2009
    Posts
    26

    Default How important is Rod Length?

    I was wondering how much Rod Length really affects an engine.
    What is better for what?(torque/horsepower)
    What is better on longer track or shorter track?
    I am talking about 5.7, 6.0, 6.125 Rods for a SBC motor I know a lot of other factors go into motor too. I know what I think and what I have been told, Just wondering everones elses opinion, and if anyone has any data to back it up. Trying to Figure what I want to build
    Thanks in Advance

  2. #2
    Join Date
    May 2007
    Location
    PA
    Posts
    844

    Default

    Longer rods allow for more cyl. fill time at higher rpm, this helps the H.P. curve hold on longer past it's peak. This would be better for restricted carb classes, or where your geared to turn well past the peak h.p. such as on a short track, where you usually turn more rpm then you would on a big track. 6.0" is the most common, and is very versatile.
    Josh K.

  3. #3
    Join Date
    Jan 2008
    Location
    Canton, GA
    Posts
    75

    Default

    Rod length analysis NASCAR 358cid

    4500 rpm
    %
    Bore in 4.185 4.185 1.0000
    Stroke n 3.250 3.250 1.0000
    Rod length in 5.700 6.250 0.9120
    mean Piston speed m/s 12.260 12.260 1.0000
    max Piston speed m/s 20.250 20.120 0.9936
    max flow cfm 367 355 0.9673
    reversion flow cfm -61 -80 1.3115
    cumulative flow cfm 16726 16727 1.0001
    Piston side force int N 1066 947 0.8884
    Piston side force combined N 4541 4147 0.9132


    9000 rpm
    %
    Bore in 4.185 4.185 1.0000
    Stroke n 3.250 3.250 1.0000
    Rod length in 5.700 6.250 0.9120
    mean Piston speed m/s 24.510 24.510 1.0000
    max Piston speed m/s 40.500 40.240 0.9936
    max flow cfm 715 709 0.9916
    reversion flow cfm -123 -159 1.2927
    cumulative flow cfm 33704 34012 1.0091
    Piston side force int N 4264 3788 0.8884
    Piston side force combined N 5745 5090 0.8860

    Look at the CFM #'s
    Last edited by lockwire; 01-16-2011 at 09:06 AM.

  4. #4
    Join Date
    Dec 2007
    Location
    northwest,ar
    Posts
    74

    Default

    i dont know about all those numbers and i dont think they mean much to most people what i do know is if you build a 6in rod motor you will never want a 5.7 motor again very big differance especialy in a limited class where you have flat top pistons and low lift cams i have never ran anything longer would like to see if makes as much differance as going from 5.7 to 6in

  5. #5
    Join Date
    May 2007
    Posts
    1,289

    Default

    seems like guys that swear by shorter rods for better torque also are running cams to big for their application. makes you wonder what the real problem is.

  6. #6
    Join Date
    Sep 2007
    Posts
    207

    Default

    do you have more specifics on low lift .we must run 410 lift limited 10.5 compression

  7. #7
    Join Date
    Dec 2007
    Location
    northwest,ar
    Posts
    74

    Default

    the low lift i was refering to is 480 lift we have a couple classes that have that rule around here the other low lift classes are 425-450 lift but they have rules that say 5.7 rods so cant try it unless your cheating so i cant help with that, i do know it free's up the engine some, easyer on pistons because it changes angle at which the rod pushes the piston and they are lighter pistons also thats a plus also

  8. #8
    Join Date
    Oct 2007
    Posts
    904

    Default

    if you built two 400s one 5.7 and one 6" with all the same parts other than rods and pistons, i bet you would never be able to tell the difference, hell i bet a dyno couldn't tell the difference

  9. #9
    Join Date
    Aug 2010
    Location
    Mendenhall MS
    Posts
    543

    Default rods

    Right on Bush.
    K.I.S.S.
    Meaning, stay with a worked out combo, unless you have a lot of time/money to kill.
    1. Pick the biggest bore size your rules allow.
    2. Get the stroke to make the CI's you want.
    3. Get a piston made with a good CH (around 1.200). Too long (1.550), makes a heavy piston, too short (1.000), and the piston is "pivoting" off the ring pack, hurting ring seal and ring life.
    4. Buy a rod that will fit the parts above.

    Or........buy the rods first, because they were a deal on E-bay.
    Wind up with some too long, which the affects are explained above, or too short, and spend major bucks adding heavy metal to the crank because the counterweights had to be shortended to clear the pistons. TB

  10. #10
    Join Date
    Aug 2010
    Location
    Mendenhall MS
    Posts
    543

    Default rods

    Oh yea, rods that are too long make the cam selection difficult. Matter of fact, call 5 cam guys, and you'll get 5 different answers on the cam for a long rod engine.TB

  11. #11
    Join Date
    May 2008
    Posts
    574

    Default

    Very seldom any reason to run any sbc with a rod over 6.0. No power is gained. 5.7 versus 6.0 the difference is minimal at best. Cam selection plays the biggest part in the equation.

  12. #12
    Join Date
    May 2007
    Posts
    1,289

    Default

    i would never opt for a long rod because i thought it made more hp. for me the benefits are a lighter piston and less side load on bores.

  13. #13
    Join Date
    Nov 2007
    Location
    Wash state
    Posts
    93

    Thumbs up

    http://www.stahlheaders.com/Lit_Rod%20Length.htm

    build the engine to suit your needs.
    pick a piston, pick a bore, pick a crank..
    then pick the rod that connects them..
    Outlaw #8
    22R Celica GT
    www.outlawcar.com

  14. #14

    Default

    i've found just about every "untouchable" motor around here is 6.200"

  15. #15
    Join Date
    Oct 2007
    Location
    Winfield, IA
    Posts
    981

    Default

    This is just my opinion and from the experience I have had over the last 23 years......

    I've built engines for others and driven hobbies to late models with everything from 5.585 to 6.3 length rods.....and I have found the 6" rod works about the best. Never really thought there was any HP difference with any combination. More of a driver choice really. Seems like the 6" rod was much more drivable and is easier on the engine because it changes the angle of how everything reacts in the cylinder. Too long of a rod makes the engine lazy and too short tends to shove them right out the side of the block.

    However once I found something that worked, customers liked it, durability was good, and guys were winning races/getting top 3's....I never ventured away from it. The only thing that changes is the camshaft because every driver has a different feel for his car and where he wants the power at on the track.

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