Connecting rod length discussion

Discussion in 'General Tech' started by El Diablo, Jun 9, 2018.

  1. El Diablo

    El Diablo Active Member

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    I know this topic has been brought up before, but just wanted to get your opinions and thoughts.

    On another forum pertaining to ATV drag racing, a few engine builders are claiming that stock or shorter length connecting rods will build more HP vs longer length rods due to the piston seeing less dwell time at TDC and BDC. In essence, the piston is working more and resting less.

    Other engine builders argue that the longer rod / stroke ratio (to an extent) builds more HP from higher combustion pressure, thus resulting in a quicker ET.

    Now keep in mind that this discussion was pertaining to 250-800 cc ATV engines. Is there any apples to apples comparisons that relate to the bigger V8 engines or are some of these guys full of BS?
     
  2. Darius

    Darius Well-Known Member

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    When is comes to engine powered vehicles, All of us guys are full of BS!

    Fun, ain't it!!

    d
     
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  3. El Diablo

    El Diablo Active Member

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    You ain't never lied Brother D
     
  4. 8ad-f85

    8ad-f85 Well-Known Member

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    Not enough info to say which ones might make more power but...
    like everything else in life, it's probably a matter of how any particular Kung Fu is applied to the overall strategy rather than there being a superior move above all.
    There's an aspect of this I've seen discussed that adds another dimension to the topic (there always is, right?)
    Mentioning some accepted and assumed trains of thought here...
    The longer rods claim certain frictional advantages as well as pressure applied against them is supposed to help acceleration from the slower dwell away from TDC.
    Commonly accepted small percentage gains there, and becomes a greater advantage after peak HP where things fall off fairly quickly.

    An advantage to shorter rods can be utilized by using the faster piston speeds at any rpm to utilize a larger induction tract through the entire power band and in a way that the longer rod equivalent might see as over-cammed at the lower end of the range, but at the expense of falling off faster after peak hp.
    Maybe a relatively substantial VE percentage increase could overcome the long rods mechanical ones?

    It seems like there's a point in which the advantages of each would trade places based on applications.
    Limited induction and more time near or beyond peak HP will utilize the small percentage gains of long rods, with the ability to use more numerical gear to overcome the torque loss over the rest of the powerband.
    Traction limited circle track coming off the corners onto a straight would do OK here and might offer some potential for a couple hundred more rpm up top if cammed right.

    Are these ATV engines at an extreme end of any relationships of proportions with geometry, architecture or induction?
    Would stretching these to the max starve out or be futile to hook up?
    Does a smaller displacement, shorter deck height rev up that much faster and run away despite lower hp?
    Stretching a big V8 to mountainous proportions with stroke often leaves a conventionally thought of as scary rod ratio and still shows advantages in certain ways.
    Would any design limitations or unfair ways of applying rules overcome known and tested aspects considered?


    So the question shifts away from the short block and leans towards the track, diver, chassis, etc..
     
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  5. Ted in Olympia

    Ted in Olympia Well-Known Member

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    The 500 has a pretty long rod already, 6.850 if I remember correctly. There have been 7.10 rods sold by vendors but it requires special pistons and in my opinion I cannot see where the advantage would outweigh the cost.

    Ted
     
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  6. Cadillac Kid 1

    Cadillac Kid 1 Active Member

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  7. 8ad-f85

    8ad-f85 Well-Known Member

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    Many times we find that a longer rod is needed to get the sides of the skirts away from hitting the crank C-weights.
    They don't swing quite the same from ban to bank, even belly-button brands have problems when you actually fit the parts together.
    Not as much as an issue with an ATV.
    Point being that there are enough challenges to building an engine let alone enjoying the good feels of a 2% gain ($$$ if you have to buy more sets of parts).
     
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  8. Cadillac Kid 1

    Cadillac Kid 1 Active Member

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    I don't remember if it was discussed in the previous post, but another thing a longer rod does is to maintain less of an angle (piston to crank throw) for a longer duration of the power stroke. since downward force is a function (cosine I believe) of the angle, the closer to straight, the closer to 1:1 as far as downward force is concerned. This carries through 1/2 stroke where the difference (mathematically) is just about 5% more force is operating 9in the direction of rotation.
    FWIW with a 472 the shorter stroke and the stpock rod length approaches the ideal length to stroke ratio.
    Be and a Be and a Be and that's all folks
    Greg Surfas
     
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  9. El Diablo

    El Diablo Active Member

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    To add more confusion to the mix, guys like me that run a 2 stroke setup have to take port timing into consideration when planning a potential build in respect to stroke and rod length.

    My particular combination has an 87.5mm bore, 86mm stroke with 143mm connecting rods inhaling through a 45mm carb and is good for 74hp @ 7,800 RPM on 110 Sunoco race fuel.

    One thing that can affect or influence any engine regardless of size is weight of the reciprocating assembly. I have the opportunity to go with a +2 (145mm) rod on my engine but my builder is hesitant due to the extra mass of the longer rod. His concern is that any potential gains may be negated by the extra weight.

    8ad, you bring up a good point. If shorter rods result in faster piston speed for any given RPM, is it safe to say that this also results in increased velocity through the ports (on a 4 stroke engine)?

    On a theoretical 400 small block Chevy with the short stock rods, it's optimal port volume along with cam duration and even carb size could be vastly different than the same engine with 6" rods because of the higher velocity, correct?
     
  10. 48Austin

    48Austin Active Member

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    So now it's just like in the old days. Build it with what you think should run with the least amount of bucks. IT GOES OR BLOWS! Maybe both and finger her out from there.
     
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  11. 8ad-f85

    8ad-f85 Well-Known Member

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    The difference in the way the engine breathes IME is easily noticeable when you street drive or transition from part to heavy throttle. They wake up sooner and remain responsive at the expense of tapping out the top end faster.
    Any of these attributes and 'known phenom's from testing' can be overcome because of how various points of diminishing returns and trade-offs work together or against each other (confusing, right? I need more coffee)
    Optimum port size for what and how? There's always trade offs.
    Maybe optimal relationships in gasoline engines have a much narrower powerband and than we demand of them...that's why we have the modern methods employed by the LS and equivalents.
    As long as the owner is happy. If the weaker range is shot past by converter or other aspects of the vehicle, does it matter?
    I could toss you my running shoes and leave 4 seconds sooner than you...as long as I get away from the bear :)

    As stated above, run it and find out what it does.
    What are the actual bob weight differences?
    Keep in mind that dyno and track testing might be different because of dyno inertia calcs when swept through a power band vs steady state and the phenomenon of an engine out accelerating a higher powered one because of lowered component weight.
    It still comes back to being all about the combo.
     
  12. 8ad-f85

    8ad-f85 Well-Known Member

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    I suppose when contemplating relationships of proportions and balancing out effects, there's always a need to consider what things the engine in question might need vs already having a strong point leaving little room for improvement.
    High and low rpm engines have different characteristics when using the ports to vaporize fuel, chamber shapes helping mixture motion and burning, etc..
    Thinking about what I read on Larry Widmer's head work with Nascar is that fairly profound improvements to BSFC were observed, but if the combo was changed after the fact much was lost or left to chance.
    There's very small gains until a paradigm shift happens it seems.
    Relating this to rod length...
    There's a difficult time for the hobbyist finding relevant math to predict combustion effects, esp. when you have to consider various throttle settings.
     
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  13. 48Austin

    48Austin Active Member

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    So now what you are saying is "Stab and Go". Who has the time OR MONEY to play games. I'm not John Force, Budweiser, or Team Strange.
    It still comes up with do some research, not alot. Build it the way you think and drive like a rentacar.
     
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