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rod length benefits? 5.7", 6", 6.2" whats up?

Discussion in 'The Garage' started by sled_dog, Aug 31, 2003.

  1. sled_dog

    sled_dog 1 ton status

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    rod length benefits? 5.7\", 6\", 6.2\" whats up?

    Whats the benefits of longer than stock rods? I'm always trying to expand my knowledge of engines and plan to build my first good horsepower motor this winter for my dad's circle track car. Anyone have some info for me?
     
  2. BowtieRed

    BowtieRed 1/2 ton status Author

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    Re: rod length benefits? 5.7\", 6\", 6.2\" whats up?

    higher compression but valve cllearing will be an issue with 6 or 6.2" rods. but get H be rods if they are that long, longer and higher compression means easier breakage.
     
  3. fatboy

    fatboy 1/2 ton status

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    Re: rod length benefits? 5.7\", 6\", 6.2\" whats up?

    that is one comment.........
    longer rod means the piston can stay at the top longer on the stroke which....will creat more TQ.
    My engine that i am buuilding has 6.0 rods with new pistons (forgot the part number) and 76cc heads which will give me 9.1 compression and 10.1 at the max if the heads need decked. I bought a 383 kit and told them I wanted 6" rods and 9.1 compression.
     
  4. mud390

    mud390 1/2 ton status

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    Re: rod length benefits? 5.7\", 6\", 6.2\" whats up?

    Can you run a 6" rod without stroking the motor? What would it take and what would the benefits be? Would it be worth the cost?

    Kris
     
  5. sled_dog

    sled_dog 1 ton status

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    Re: rod length benefits? 5.7\", 6\", 6.2\" whats up?

    you can run 6" rods without stroking. Cost would be rods, rod bearings, and bolts pretty much. I'm considering a 6" rod in the circle track motor and trying to weigh the advantages to cost.
     
  6. Beast388

    Beast388 1/2 ton status

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    Re: rod length benefits? 5.7\", 6\", 6.2\" whats up?

    I found this info here.

    Rod Length-to-Stroke Ratios

    In most large cubic inch engines it is hard to get a very high ratio. A Stock 350 Chevy has a 1.64:1 RL/S ratio, which is not very good. By increasing the rod length to 6" the ratio increases to 1.72:1, which is much better. You can squeeze a 6.1" rod in a 350 with little trouble, but longer than that requires plugging the piston pin bores after assembly to support the oil rings. It is not worth the extra expense for the little gains, so a 6" rod in a small-block Chevy has become common because everything fits right in. With endurance engines, longer rods are always better. Most endurance engines are using a RL/S ratio of at least 1.9:1 and some as high as 2.2:1. Before you go out and buy longer rods, let me just say that the gains are very small. This debate has been argued for years and will not end anytime soon. In my opinion, if are building an engine and need new rods and pistons, a longer rod will cost about the same. That makes the small benefits worth it. I would not waste my money buying longer rods if you have a good set of rods that you can use. Use that money to make more power elsewhere in the engine.

    Rod Angularity

    A longer rod reduces the maximum rod angle to the cylinder bore centerline. Less rod angle will reduce piston side loading; there will be less friction and less bore wear. Less rod angle also gives better average leverage on the crank for a longer period of time. A 5.7" rod with a 3.48" stroke (stock Chevy 350), will have a maximum of 17.774° rod angle. Switching to a 6" rod will reduce that to 16.858°, assuming that the wrist pin has no offset.

    Piston Pin Height

    A higher pin height will reduce piston rock and aid ring seal. Please do not email me arguing anything about a tighter ring pack, I am talking about pin height and pin height only. Moving the pin closer the the center of gravity of the pistons makes the piston more stable.

    Rings

    As the compression height is reduced, the space for the ring pack also get reduced. This can be a problem on some engines. It is good for power to have the top ring as close to the top as possible, but this is limited to the strength of the top ring land. As it becomes thin, it becomes weak. High output engines (especially nitrous engines) need a thicker ring land to keep the cylinder pressure from pinching the top ring. In my opinion, if you have to compromise ring location, it would be better to run a slightly shorter rod.

    Skirts

    Shorter skirts are usually combined with a shorter piston for a longer rod, but they are not really related. There is no reason to reduce the size of the skirts just because the pin location changed. A shorter skirts are used to reduce friction and lighten the piston. The cost is a little less stability, but it is arguable that a lighter piston with a higher pin height does not need the extra stability. For a street car, I would increase the rod length if it meant a reduction is skirt size. Most of the things listed here are for competition motors to gain a few hp, not worth a lot of effort for most street engines.

    Piston-to-Valve Clearance

    A longer rod decelerates toward TDC and accelerates away from TDC slower than a shorter rod, so piston-to-valve clearances are tighter with a longer rod. This may require deeper valve relief's in the piston (but probably not). A short rod is just the opposite, there is more clearance because the piston decelerates toward and accelerated away for TDC faster.

    Piston Velocity

    A longer rod reduces peak piston speeds slightly and delays peak piston velocity until the piston is further down the bore, which gives the intake valve more time to open more. Peak piston velocity is usually somewhere around 75° ATDC and since most cams cannot fully open the cam until at least 106° ATDC, it leaves the valve as a major obstacle when airflow demand is at its greatest. By delaying peak piston velocity, even if it's only 1 or 2 crankshaft degrees, it can allow the valve to open another 0.010-0.015", before peak airflow demand is reached. Not a huge help, but a step in the right direction. With a 350 Chevy, switching to 6" rods from 5.7" ones will delay peak piston velocity from 74.5° to 75.5°.

    Piston Acceleration / Deceleration

    Reducing piston acceleration / deceleration from and toward TCD will reduce tensile loading of the rod, the number 1 cause of rod failure. A Chevy 350 with 5.7" rods will have a peak piston acceleration rate of 101699.636 ft/sec/sec at 7000 rpm. Swapping in 6" rods will reduce that to 100510.406 ft/sec/sec at that same rpm. That is a reduction of 1189.23 ft/sec/sec.

    Intake Runner Volume

    Since it is easier for an engine to breath with a longer rod, less runner volume is needed. This allows more room for an intake system and, if the engine is raised, more room for an oiling system. This will be fractions on an inch, but can really help in some race classes, such as Prostock. A 1/2" lower hood scoop can make a large difference in aerodynamics. The reduced piston velocity makes for less pulsing of the intake charge and a smoother flow (this is a very small gain, but is real).

    Exhaust Gas Scavenging

    A longer rod is moving slower at TDC, which reduces the speed of the exhaust gasses during the overlap period. This reduces the scavenging effect at low rpm and reduces low rpm power slightly (makes the engine run more cammie). A short rod on the other hand moves faster past TDC and increases the scavenging effect and help low rpm power.

    Ignition Timing Requirements

    Due to the fact that the longer rod moves past TDC slower, it gives the charge a longer time to burn. So you need less timing for peak power. Using less timing also reduces the chance of detonation; so higher compression ratios can be used. Switching from 5.7" to 6" rods on a 350 Chevy can allow as much as 1 full point increase in compression. In other words, if you could only run 9.5:1 with 5.7" rods, you could run 9.6:1 with 6" rods.
    *****************************************************
    Longer Rod Pros


    Less rod angularity
    Higher wrist pin location
    Helps resist detonation
    A lighter reciprocating assembly
    Reduced piston rock
    Better leverage on the crank for a longer time
    Less ignition timing is required
    Allow slightly more compression to be used before detonation is a problem
    Less average and peak piston velocity
    Peak piston velocity is later in the down stroke
    Less intake runner volume is needed

    Longer Rod Cons


    Closer Piston-to-valve clearances
    Makes the engine run a little more cammie at low rpm
    Reduces scavenging at low rpm

    Shorter Rod Pros


    Increased scavenging effect at low rpm
    Helps flow at low valve lifts (a benefit if the heads are ported with this in mind)
    Slower piston speeds near BDC
    Allows the intake valve to be open longer with less reversion
    More piston-to-valve clearance
    Can allow for a shorter deck height

    Shorter Rod Cons


    More rod angularity
    Lower piston pin height (if the deck is not shorter)
    Taller and heavier pistons are required (again, if the deck height is not reduced)
    More ignition timing is required for peak power

    /forums/images/graemlins/thumb.gif /forums/images/graemlins/thumb.gif /forums/images/graemlins/thumb.gif /forums/images/graemlins/thumb.gif /forums/images/graemlins/thumb.gif /forums/images/graemlins/thumb.gif /forums/images/graemlins/thumb.gif /forums/images/graemlins/thumb.gif
     
  7. m j

    m j 1/2 ton status

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  8. sled_dog

    sled_dog 1 ton status

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    Re: rod length benefits? 5.7\", 6\", 6.2\" whats up?

    unless they are intake restricted? So wouldn't that mean its not a myth????? I think you just shot yourself in the foot on that one. We are head restricted. not gonna run anything more than some simple 1.94 non high performance heads and I have ot try and get the maximum power out of that. Why? because my dad is a stubborn man that will never listen to me or others and won't spend the money even if he realizes hes wrong. I'm sure many of you had/have dads like mine. When I was working on my k5's motor I port match teh heads cause they were nasty stock castings and he looked at me like I was a moron and seriouslly said "if you have a good bottom end(he means bore, stroke, and cam) you don't need a top end(meaning heads)." I looked at him and just said ok whatever dad, cause i know whatever I said after that wouldn't matter.
     
  9. m j

    m j 1/2 ton status

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  10. Z3PR

    Z3PR Banned

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    Re: rod length benefits? 5.7\", 6\", 6.2\" whats up?

    [ QUOTE ]
    for you to change the rod length from what the factory put in the chosen engine is entirely a waste of fawking time and $$
    is that simple enough for you
    you will gain "a 100 fold" putting same time/cash into other areas

    [/ QUOTE ] But what if you're doing a rebuild and need to replace the rods anyways ??? /forums/images/graemlins/thinking.gif
     
  11. mudjunkie 82

    mudjunkie 82 1 ton status GMOTM Winner

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    Re: rod length benefits? 5.7\", 6\", 6.2\" whats up?

    Stroke it!!!!!!!!!!!!!!!!!!!!!!!!!!
     
  12. Z3PR

    Z3PR Banned

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    Re: rod length benefits? 5.7\", 6\", 6.2\" whats up?

    [ QUOTE ]
    Stroke it!!!!!!!!!!!!!!!!!!!!!!!!!!

    [/ QUOTE ] Everyday. /forums/images/graemlins/eek.gif /forums/images/graemlins/eek.gif /forums/images/graemlins/ignore.gif No. I have a 400cid small block that I plan on using a 415 Stroker kit in when it comes time to rebuild. So we'll be strokin'. /forums/images/graemlins/thumb.gif
     
  13. sled_dog

    sled_dog 1 ton status

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    Re: rod length benefits? 5.7\", 6\", 6.2\" whats up?

    [ QUOTE ]
    Stroke it!!!!!!!!!!!!!!!!!!!!!!!!!!

    [/ QUOTE ] 358 cui limit. And yes I would like to replace rods anyway is my point.
     
  14. boz42

    boz42 1/2 ton status GMOTM Winner

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    Re: rod length benefits? 5.7\", 6\", 6.2\" whats up?

    a dyno test from comp cams says that in a 400 sbc, w/ everything the same a 6" rod over a 5.7"(factory 350, 400sbc came w/ a 5.65 rod) the 6" rod made 20 more horsepower & 20 more lbs of torque.

    given the engine they were testing was highly modified. a stocker would not make near that w/ just a rod change.
     
  15. m j

    m j 1/2 ton status

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  16. k30gambi

    k30gambi 1/2 ton status

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    Re: rod length benefits? 5.7\", 6\", 6.2\" whats up?

    I think if you read anything smokey yunich has published, you would see that he has invented, tested and proven everything Beast388 just posted. Sorry, I would have to side with pioneers of the sport like yunich, bill jenkins, randy dorton and ray quatrochi than, well, you. /forums/images/graemlins/grin.gif
     
  17. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    Re: rod length benefits? 5.7\", 6\", 6.2\" whats up?

    [ QUOTE ]
    (factory 350, 400sbc came w/ a 5.65 rod)

    [/ QUOTE ] 350 sbc uses a 5.7" rod and the 400 sbc uses A 5.65" rod.
     
  18. m j

    m j 1/2 ton status

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  19. SUBFAN

    SUBFAN 1/2 ton status

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    Re: rod length benefits? 5.7\", 6\", 6.2\" whats up?

    If building a 400 Chevy, I would at least put in 5.7" rods. This cuts down on the side loading and egging out of the cylinder wall....Getting redone 5.7" rods is just as cheap as redoing your 5.56" rods.

    There are plenty of pistons out there made for the 400 Chevy with the longer rod, that the extra cost is minimal....
     
  20. jeffro

    jeffro 1/2 ton status

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    Re: rod length benefits? 5.7\", 6\", 6.2\" whats up?

    [ QUOTE ]



    It is a MYTH that long rods are always better


    [/ QUOTE ]
    This is just what your girlfriend told you, MJ, and she lied so's not to hurt your feelings.
     

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