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R/s Question

Discussion in 'Engine Building' started by K2e2vin, Feb 1, 2003.

  1. K2e2vin

    K2e2vin Senior Member

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    ok i know the ideal r/s is 1.75, but what if the r/s is higher than 1.75? would it still work or be better or something?
     
  2. asmallsol

    asmallsol Super Moderator

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    it would work just the engine can not rev as high
     
  3. lsvtec

    lsvtec GNU/Linux Evangelist

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    Think about what R/S means for a second. Rod length / Stroke length. All it says is that ideally your rods are 1.75 times the length of you stroke. Now if you stop and consider why that ratio is so "good". The longer your rods are compared to you stroke the smaller the angles are between vertical and rod position as the pistons travels up and down. Envision a cross section of an inline reciprocating piston engine. As the piston starts to climb from BDC the rod forms an angle with vertical at the wrist pin. If you have taken any physics this is pretty basic. Think of a force diagram illustrating the forces acting on the wrist pin ( for simplicity sake assume that the piston and wristpin are one unit). The larger the angle between vertical and the rod, the larger the sine of that angle, and the larger the horizontal component of the force applied by the rod (meaning more side loading on the piston and cylinder wall causing ovaling of the cylinders). Now, at 1.75:1 the angle between horizontal and the rod is minimal.

    I would assume that if we were dealing with free, mass less engine components, that greater than 1.75:1 would better "better", I believe that engine designers do not attempt to achieve greater than 1.75:1 for several reasons:
    1. The longer the rod, the greater its mass. Adding mass is bad because it increases the amount of power generated by the engine that is consumed in keeping itself spinning.
    2. Cost, larger rods cost more to manufacture, and with longer rods you need to have longer blocks which also costs more. You get the idea.
    3. Diminishing returns. The relationship between side load and R/S ratio is not linear. Meaning that a .4 increase in the ration from 1.75 to 1.79 won't make as big a difference as from 1.71 to 1.75 did.

    Cliff Notes: A R/S ratio larger than 1.75:1 is unnecessary and the costs out weigh the benefits.
     
  4. BigJ

    BigJ I'm just about that action Boss. VIP

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    my god I learn something terribly valuable and new everyday I read the posts here.
     
  5. lsvtec

    lsvtec GNU/Linux Evangelist

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    :) Glad I could help.

    It's all in that class that everyone slept through in HS. :)
     
  6. K2e2vin

    K2e2vin Senior Member

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    Phew, thanks for the info. I asked that cuz i wanted to see if it is possible to run b16a cranks in b18 block and have a good r/s ratio. turns out its around 1.85. oh well.
     
  7. lsvtec

    lsvtec GNU/Linux Evangelist

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    It would work and you could spin the hell out of it, but you would lose the extra displacement.
     
  8. K2e2vin

    K2e2vin Senior Member

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    unless you rebore/resleeve it?
     
  9. lsvtec

    lsvtec GNU/Linux Evangelist

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    If you bore out to 84mm (the B20 bore) with a B16 crank that yeilds 1715.7cc's. (1.7 liters). Still 100 cc's less than the B18A/B.
     
  10. E_SolSi

    E_SolSi Member of the 20 nut club Moderator VIP

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    what kind of effect would this have on the compression ratio??
     
  11. lsvtec

    lsvtec GNU/Linux Evangelist

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    That is a good question, I haven't even stopped to think about it. It would lower the compression. Using the LS rods on a B16 crank with the pistons at TDC it will be sitting a little lower than it was using the LS crank.
     
  12. timEspeed

    timEspeed Senior Member

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    <-------- see my avatar



    Which isn't all that bad in certain applications where piston to head clearance is an issue.
     
  13. pissedoffsol

    pissedoffsol RETIRED

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    so check it out its like this:

    1.75:1 is the ideal ratio. why? because it is said to be by some mathematical formula that at this ratio, a motor will make a good amount of revs, while retaining some torque.

    the lower you go, the more torque you get, the less your engine can rev out safely.
    the higher you go, the less torque you get, the more your engine can rev out safely.

    so a motor with a 2.0:1 r/s ratio will scream easily to 15k rpms (ideal motor with head work to hold it) but it won't make dick for power until 12 grand. in other words, a b16a :)
    now, a motor with a lower r/s ratio lets say 1.5:1 will be a torque monster, but will not fair well to higher revs (read, V8)
     
  14. K2e2vin

    K2e2vin Senior Member

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    I know what you are saying, but its basically the stroke. shorter stroke=high rpm, but youll need a rod thats longer, or keep it short and have a very low compression ratio (the skyline has a stroke of somewhere around 72mm, and some skylines can rev to 15k, 9k stock though). A engine with a longer stroke would have greater torque but shorter rods, but that could cause the cylinder walls to flex and limit revs. also what lsvtec said made me relize the effect of a longer stroke. All of this is pretty much based on distance from the center of crank to the top of deck. I didnt relize the effect it would have on compression ratio till i used a cr calculator. i'v tried to figure out the best rod/stroke combo for the b18, it turns out it already comes with the best combo for the b18; 1.58 r/s found in b18c. ive also figured out the BEST ratio for an engine the size of the b18c but, the displacement went to 1.6; the cylinders can be bored to 84mm and there it is, a perfect r/s ratio and 1.8L, but requires custom rods and crank.
     
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